| Capt Crunch | 08-22-2006 05:03 PM |
Question on maximum braking
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I was explaining to my fiancee the point of heel and toe and showing her how jerky downshifts were if you didn't revmatch. I explained if you are threshold braking and don't engine brake, you can lock up the wheels (provided you don't have abs).
But then I got to thinking, is this really the case? The only limiting factor for most brakes are the tires, which basically can dissipate a certain maximum amount of energy per time via friction. If you try to exceed that amount, you lock up the wheels. But here is the thing, if you don't revmatch and just let out the clutch, energy goes into raising the rpm of the motor. The energy has to come from somewhere, so it must from from the energy of the moving car, thus slowing it down.
The only problem is the inertia of the motor, if you completely release the clutch, the wheels are going to want to spin slower (by the gear ratios) and so they will lock up. However if you slip the clutch slow enough, you could arguably soak up energy and slow the car down faster.
Which brings me to a couple questions. Am I right? If so, why is this technique not used? You can slow down a car a lot from engine braking, I remember it was a problem for racing automatic cars because they didn't have it.
Second, why don't race cars employ a spring mechanism? It seems it would be somewhat simple to rig a mechanical device that soaks up energy in the braking zone, and can be harnessed during the straightaways. Is it too complicated? Easy to break? Against the rules? Kind of like how rubber skirts and vacuum pumps on NASCAR is banned.
But then I got to thinking, is this really the case? The only limiting factor for most brakes are the tires, which basically can dissipate a certain maximum amount of energy per time via friction. If you try to exceed that amount, you lock up the wheels. But here is the thing, if you don't revmatch and just let out the clutch, energy goes into raising the rpm of the motor. The energy has to come from somewhere, so it must from from the energy of the moving car, thus slowing it down.
The only problem is the inertia of the motor, if you completely release the clutch, the wheels are going to want to spin slower (by the gear ratios) and so they will lock up. However if you slip the clutch slow enough, you could arguably soak up energy and slow the car down faster.
Which brings me to a couple questions. Am I right? If so, why is this technique not used? You can slow down a car a lot from engine braking, I remember it was a problem for racing automatic cars because they didn't have it.
Second, why don't race cars employ a spring mechanism? It seems it would be somewhat simple to rig a mechanical device that soaks up energy in the braking zone, and can be harnessed during the straightaways. Is it too complicated? Easy to break? Against the rules? Kind of like how rubber skirts and vacuum pumps on NASCAR is banned.
| brianmcd | 08-22-2006 05:13 PM |
The point of heel and toe'ing isn't about engine braking. The brakes can stop the car a lot better than the engine. The point is about car balance. If you're just slamming it into the lower gear and letting the clutch out, the car jerks and it affects the balance of the car. If you are heel and toeing correctly, the downshifting should not affect the balance of the car, so you'll have the car balanced the way you want it going into the turn.
| Scooby921 | 08-22-2006 05:17 PM |
That and brake pads are a lot cheaper than the clutch.
And with the tires, its not really dissipating energy. That is what the brakes are doing by creating heat. The tires work on the principle of slip angle and friction. By hitting the brakes you induce a slip angle and the tires go to work. Pending size and friction coefficient, certain tires can hold a larger slip angle and hence provide better performance.
And with the tires, its not really dissipating energy. That is what the brakes are doing by creating heat. The tires work on the principle of slip angle and friction. By hitting the brakes you induce a slip angle and the tires go to work. Pending size and friction coefficient, certain tires can hold a larger slip angle and hence provide better performance.
| Capt Crunch | 08-22-2006 05:29 PM |
[QUOTE=brianmcd]The point of heel and toe'ing isn't about engine braking. The brakes can stop the car a lot better than the engine. The point is about car balance. If you're just slamming it into the lower gear and letting the clutch out, the car jerks and it affects the balance of the car. If you are heel and toeing correctly, the downshifting should not affect the balance of the car, so you'll have the car balanced the way you want it going into the turn.[/QUOTE]
I can see how the jerk effects the balance of the car, but I think you misunderstand me when you say the brakes can stop the car a lot better than the engine. Yes it can, but the engine can provide additional slowing when the brakes are at their limit.
So the tires and brakes dissipate the energy of the car as heat, but other than upsetting the balance of the car, I don't see any reason why slipping the clutch on the downshifts wouldn't slow you down faster.
I can see how the jerk effects the balance of the car, but I think you misunderstand me when you say the brakes can stop the car a lot better than the engine. Yes it can, but the engine can provide additional slowing when the brakes are at their limit.
So the tires and brakes dissipate the energy of the car as heat, but other than upsetting the balance of the car, I don't see any reason why slipping the clutch on the downshifts wouldn't slow you down faster.
| cooleyjb | 08-22-2006 05:40 PM |
[QUOTE=Capt Crunch]I can see how the jerk effects the balance of the car, but I think you misunderstand me when you say the brakes can stop the car a lot better than the engine. Yes it can, but the engine can provide additional slowing when the brakes are at their limit.
So the tires and brakes dissipate the energy of the car as heat, but other than upsetting the balance of the car, I don't see any reason why slipping the clutch on the downshifts wouldn't slow you down faster.[/QUOTE]
If you are expecting to use your engine to slow you down because your brakes aren't gettign the job done, you should bring your car into the pits and get better brakes.
So the tires and brakes dissipate the energy of the car as heat, but other than upsetting the balance of the car, I don't see any reason why slipping the clutch on the downshifts wouldn't slow you down faster.[/QUOTE]
If you are expecting to use your engine to slow you down because your brakes aren't gettign the job done, you should bring your car into the pits and get better brakes.
| bjorn240 | 08-22-2006 05:45 PM |
[QUOTE=Capt Crunch]I can see how the jerk effects the balance of the car, but I think you misunderstand me when you say the brakes can stop the car a lot better than the engine. Yes it can, but the engine can provide additional slowing when the brakes are at their limit.
So the tires and brakes dissipate the energy of the car as heat, but other than upsetting the balance of the car, I don't see any reason why slipping the clutch on the downshifts wouldn't slow you down faster.[/QUOTE]
You misunderstand.
Threshold braking is not braking where your brakes are at their limit. It's braking where your tires are at their limit. If your tires are at their limit, any additive (and extremely marginal) slowing provided by engine braking will cause the tires to lock. If the additional slowing does not cause the tires to lock, you were (inherently) not at threshold.
I know this seems totally tautological to anyone who knows what threshold braking is...but I'm trying to help our Cereal-peddling friend.
- Christian
So the tires and brakes dissipate the energy of the car as heat, but other than upsetting the balance of the car, I don't see any reason why slipping the clutch on the downshifts wouldn't slow you down faster.[/QUOTE]
You misunderstand.
Threshold braking is not braking where your brakes are at their limit. It's braking where your tires are at their limit. If your tires are at their limit, any additive (and extremely marginal) slowing provided by engine braking will cause the tires to lock. If the additional slowing does not cause the tires to lock, you were (inherently) not at threshold.
I know this seems totally tautological to anyone who knows what threshold braking is...but I'm trying to help our Cereal-peddling friend.
- Christian
| solo-x | 08-22-2006 05:54 PM |
you are still limited by the tires. they can only provide a certain amount of rearward acceleration. excede that and you'll lock the tires.
| GarySheehan | 08-22-2006 07:18 PM |
[QUOTE=bjorn240]You misunderstand.
Threshold braking is not braking where your brakes are at their limit. It's braking where your tires are at their limit. If your tires are at their limit, any additive (and extremely marginal) slowing provided by engine braking will cause the tires to lock. If the additional slowing does not cause the tires to lock, you were (inherently) not at threshold.
I know this seems totally tautological to anyone who knows what threshold braking is...but I'm trying to help our Cereal-peddling friend.
- Christian[/QUOTE]
Christian,
I believe I understand where you are coming from. Here's the issue. When you are threshold braking and release the clutch suddenly without a rev-match, the lower engine speed drags down the wheelspeed of the car temporarily. It does not lock the wheels, it slows them down slower than roadspeed until the friction between the road and the tires rotating slower than roadspeed drags the engine back up to roadspeed.
During this time, the tires have exceeded their maximum grip at a given amount of slip. They have fallen down past the peak of the mu-slip curve and have become less efficient (meaning that the car is experiencing less negative acceleration). That means you've extended your braking distance.
If you let the clutch out slower, you still exceed the capability of the tires, just to a lesser extent. Note that this is true if you are really at the braking threshold of the tires. So even though you do it slower and upset the chassis less, the tires are no longer working at maximum adhesion and your braking distances increase.
If you feel a jerk of additional deceleratation when you let the clutch out, it means you were not truly threshold braking. You were just braking really hard, but the tires could still do more work.
Gary
Sheehan Motor Racing
[url]www.teamSMR.com[/url]
Threshold braking is not braking where your brakes are at their limit. It's braking where your tires are at their limit. If your tires are at their limit, any additive (and extremely marginal) slowing provided by engine braking will cause the tires to lock. If the additional slowing does not cause the tires to lock, you were (inherently) not at threshold.
I know this seems totally tautological to anyone who knows what threshold braking is...but I'm trying to help our Cereal-peddling friend.
- Christian[/QUOTE]
Christian,
I believe I understand where you are coming from. Here's the issue. When you are threshold braking and release the clutch suddenly without a rev-match, the lower engine speed drags down the wheelspeed of the car temporarily. It does not lock the wheels, it slows them down slower than roadspeed until the friction between the road and the tires rotating slower than roadspeed drags the engine back up to roadspeed.
During this time, the tires have exceeded their maximum grip at a given amount of slip. They have fallen down past the peak of the mu-slip curve and have become less efficient (meaning that the car is experiencing less negative acceleration). That means you've extended your braking distance.
If you let the clutch out slower, you still exceed the capability of the tires, just to a lesser extent. Note that this is true if you are really at the braking threshold of the tires. So even though you do it slower and upset the chassis less, the tires are no longer working at maximum adhesion and your braking distances increase.
If you feel a jerk of additional deceleratation when you let the clutch out, it means you were not truly threshold braking. You were just braking really hard, but the tires could still do more work.
Gary
Sheehan Motor Racing
[url]www.teamSMR.com[/url]
| bjorn240 | 08-22-2006 07:33 PM |
Gary, exactly. I was using "locked" in the vernacular; the correct phrasing, as you have it, is that you've now reduced, not increased, the tires efficiency, and thereby increased the stopping distance.
| 48mpg | 08-22-2006 07:39 PM |
basically you are asking, that if your brakes were unable to lock up the tires for whatever reason, (huge sticky tires, sub par brakes) if downshifting and letting out the clutch slowly would stop you faster as opposed to letting out the clutch quickly, balance being upset from weight transfer aside
i would think that letting it out quickly would stop you faster, no good explanation for it, sad really, since i'm an ME, maybe because when you let it out quickly it causes the engine to rise to a higher to help you slow down, engine braking is the effect of the work being done to turn the motor against the friction, compression, etc. so the more rpms the motor is forced to turn the more work is expended from your forward momentum, causing you to slow down/stop faster
i would think that letting it out quickly would stop you faster, no good explanation for it, sad really, since i'm an ME, maybe because when you let it out quickly it causes the engine to rise to a higher to help you slow down, engine braking is the effect of the work being done to turn the motor against the friction, compression, etc. so the more rpms the motor is forced to turn the more work is expended from your forward momentum, causing you to slow down/stop faster
| seattle944t | 08-22-2006 07:41 PM |
Also - easing out the clutch to bring the RPM's up is not engine braking - that is using the clutch as a brake. Engine braking is being in gear w/o throttle where the compression of the engine is acting to slow the car down.
The reason automatics don't engine brake well is because the slippage of the torque converter.
The reason automatics don't engine brake well is because the slippage of the torque converter.
| Capt Crunch | 08-23-2006 12:24 AM |
I got it guys, 15 minutes with some thought experiments and I figured it out. Thanks.
| REX8 | 08-23-2006 12:36 AM |
[QUOTE=Capt Crunch]I can see how the jerk effects the balance of the car, but I think you misunderstand me when you say the brakes can stop the car a lot better than the engine. Yes it can, but the engine can provide additional slowing when the brakes are at their limit.
So the tires and brakes dissipate the energy of the car as heat, but other than upsetting the balance of the car, I don't see any reason why slipping the clutch on the downshifts wouldn't slow you down faster.[/QUOTE]
Who doesn't use engine braking when downshifting in a performance situation?
Everyone does. That half the point of heel-toe. Ever see a guy brake form 6 to 2nd using all the gears on teh way down? He does that to help braking...
I think the bottom line is that you're truly not threshhold braking if you can add the extra braking force during the clutch engagement without breaking the tires loose.
So the tires and brakes dissipate the energy of the car as heat, but other than upsetting the balance of the car, I don't see any reason why slipping the clutch on the downshifts wouldn't slow you down faster.[/QUOTE]
Who doesn't use engine braking when downshifting in a performance situation?
Everyone does. That half the point of heel-toe. Ever see a guy brake form 6 to 2nd using all the gears on teh way down? He does that to help braking...
I think the bottom line is that you're truly not threshhold braking if you can add the extra braking force during the clutch engagement without breaking the tires loose.
| REX8 | 08-23-2006 12:40 AM |
[QUOTE=bjorn240]You misunderstand.
Threshold braking is not braking where your brakes are at their limit. It's braking where your tires are at their limit. If your tires are at their limit, any additive (and extremely marginal) slowing provided by engine braking will cause the tires to lock.
- Christian[/QUOTE]
WinnAR!
Threshold braking is not braking where your brakes are at their limit. It's braking where your tires are at their limit. If your tires are at their limit, any additive (and extremely marginal) slowing provided by engine braking will cause the tires to lock.
- Christian[/QUOTE]
WinnAR!
| jamal | 08-23-2006 01:04 AM |
[QUOTE=REX8]Who doesn't use engine braking when downshifting in a performance situation?
Everyone does. That half the point of heel-toe. Ever see a guy brake form 6 to 2nd using all the gears on teh way down? He does that to help braking...
[/QUOTE]
He does that so he can be in 2nd. And he'd probably slow down faster if he just went straight from 6 to 2. During the heel-toe downshifts people aren't usually on the braking threshold as well as they could be.
Or so some chart I saw said... Hey Gary do you have any braking telemetry we could see?
Everyone does. That half the point of heel-toe. Ever see a guy brake form 6 to 2nd using all the gears on teh way down? He does that to help braking...
[/QUOTE]
He does that so he can be in 2nd. And he'd probably slow down faster if he just went straight from 6 to 2. During the heel-toe downshifts people aren't usually on the braking threshold as well as they could be.
Or so some chart I saw said... Hey Gary do you have any braking telemetry we could see?
| seattle944t | 08-23-2006 02:31 AM |
[QUOTE=REX8]
<snip>
I think the bottom line is that you're truly not threshhold braking if you can add the extra braking force during the clutch engagement without breaking the tires loose.[/QUOTE]
This is the crux of it - if you have not exceeded your tire's traction threshold then you still have braking force you can apply, either from the brakes or the engine. But the brakes are [b]much[/b] more efficient at stopping the car.
The only situation where I can think of where engine braking could aid in stopping faster than brakes alone would be this:
Non ABS car
RWD
Brake proportioning too front biased.
In this circumstance the front tires will reach their traction threshold far sooner than the rears. Say the fronts are @ 99% (you are really good at threshold braking) and the rears are only at 80%. In this case the engine braking can aid the rear and maybe bring it up 5-10% nearer to the traction threshold.
But, assuming the above (poor) setup was the case, you would have to be a really freaking good driver (as in you are, or should be, making a 6 or 7+ figure income getting paid to drive) to be able to realize that extra traction from engine braking. And if you are really that good - your crew chief is going to get your car setup fixed... ;)
<snip>
I think the bottom line is that you're truly not threshhold braking if you can add the extra braking force during the clutch engagement without breaking the tires loose.[/QUOTE]
This is the crux of it - if you have not exceeded your tire's traction threshold then you still have braking force you can apply, either from the brakes or the engine. But the brakes are [b]much[/b] more efficient at stopping the car.
The only situation where I can think of where engine braking could aid in stopping faster than brakes alone would be this:
Non ABS car
RWD
Brake proportioning too front biased.
In this circumstance the front tires will reach their traction threshold far sooner than the rears. Say the fronts are @ 99% (you are really good at threshold braking) and the rears are only at 80%. In this case the engine braking can aid the rear and maybe bring it up 5-10% nearer to the traction threshold.
But, assuming the above (poor) setup was the case, you would have to be a really freaking good driver (as in you are, or should be, making a 6 or 7+ figure income getting paid to drive) to be able to realize that extra traction from engine braking. And if you are really that good - your crew chief is going to get your car setup fixed... ;)
| hotrod | 08-23-2006 03:21 AM |
[quote]That and brake pads are a lot cheaper than the clutch.[/quote]
[quote]Also - easing out the clutch to bring the RPM's up is not engine braking - that is using the clutch as a brake. Engine braking is being in gear w/o throttle where the compression of the engine is acting to slow the car down.
[/quote]
Down shifting and using engine compression for braking has very little impact on the clutch. First it has maybe 10x the friction material surface area as your brake pads do, and it spends very little time slipping, the vast majority of the energy dissipated in engine braking is not dissapated in the clutch material as it quickly locks up with the fly wheel, the energy is dissipated in the work done trying to suck air through a closed throttle plate, and compression on the air going through the engine.
Engine braking is a very good way to give the brakes a break (pun intended) --- if you are pushing very hard and beginning to see early symptoms of brake fade or soft brake peddle, using the engine is both a safe and economical way to take the load off the brakes for abit so they can cool down.
In short it is a safety device (additional braking power) that is available if you need it.
The reason your brakes might be over heated or not functioning at 100% is a different issue. Engine braking -- works, it is safe, it is not hard on the equipment or abuse the clutch unless your a moron. Use it when it is appropriate, and learn it if you want to be a fully developed driver.
Larry
[quote]Also - easing out the clutch to bring the RPM's up is not engine braking - that is using the clutch as a brake. Engine braking is being in gear w/o throttle where the compression of the engine is acting to slow the car down.
[/quote]
Down shifting and using engine compression for braking has very little impact on the clutch. First it has maybe 10x the friction material surface area as your brake pads do, and it spends very little time slipping, the vast majority of the energy dissipated in engine braking is not dissapated in the clutch material as it quickly locks up with the fly wheel, the energy is dissipated in the work done trying to suck air through a closed throttle plate, and compression on the air going through the engine.
Engine braking is a very good way to give the brakes a break (pun intended) --- if you are pushing very hard and beginning to see early symptoms of brake fade or soft brake peddle, using the engine is both a safe and economical way to take the load off the brakes for abit so they can cool down.
In short it is a safety device (additional braking power) that is available if you need it.
The reason your brakes might be over heated or not functioning at 100% is a different issue. Engine braking -- works, it is safe, it is not hard on the equipment or abuse the clutch unless your a moron. Use it when it is appropriate, and learn it if you want to be a fully developed driver.
Larry
| seattle944t | 08-23-2006 03:56 AM |
[QUOTE=hotrod]Down shifting and using engine compression for braking has very little impact on the clutch. First it has maybe 10x the friction material surface area as your brake pads do, and it spends very little time slipping, the vast majority of the energy dissipated in engine braking is not dissapated in the clutch material as it quickly locks up with the fly wheel, the energy is dissipated in the work done trying to suck air through a closed throttle plate, and compression on the air going through the engine.
<snip>
Larry[/QUOTE]
You are misunderstanding what I said - I thought it was clear enough but I guess not.
So, more detailed...
You are at 6000RPM's WOT in third and you roll off the gas. You are now engine braking. [b]This is fine.[/b]
You push in the clutch, and the engine drops to idle.
You downshift to 2nd.
You [b]slowly[/b] or worse, quickly, let out the clutch to bring the RPMs up from 750 to ~6800. [b]This is not fine. This is using the clutch as a brake.[/b]
If you are going to downshift like this you need to rev match. Otherwise you are burning the clutch and upsetting the car's balance.
Proper sequence (ignoring braking for simplicity and a bad example since I usually let the RPMS drop much lower before downshifts, but what ever...)
You are at 6000RPM's in third, you clutch in.
You blip the throttle to bring the revs up to ~6500
You shift to 2nd
You release the clutch.
You are now engine braking in 2nd gear.
<snip>
Larry[/QUOTE]
You are misunderstanding what I said - I thought it was clear enough but I guess not.
So, more detailed...
You are at 6000RPM's WOT in third and you roll off the gas. You are now engine braking. [b]This is fine.[/b]
You push in the clutch, and the engine drops to idle.
You downshift to 2nd.
You [b]slowly[/b] or worse, quickly, let out the clutch to bring the RPMs up from 750 to ~6800. [b]This is not fine. This is using the clutch as a brake.[/b]
If you are going to downshift like this you need to rev match. Otherwise you are burning the clutch and upsetting the car's balance.
Proper sequence (ignoring braking for simplicity and a bad example since I usually let the RPMS drop much lower before downshifts, but what ever...)
You are at 6000RPM's in third, you clutch in.
You blip the throttle to bring the revs up to ~6500
You shift to 2nd
You release the clutch.
You are now engine braking in 2nd gear.
| kellygnsd | 08-23-2006 05:14 AM |
[QUOTE=REX8]Who doesn't use engine braking when downshifting in a performance situation?
Everyone does. That half the point of heel-toe. Ever see a guy brake form 6 to 2nd using all the gears on teh way down? He does that to help braking...
I think the bottom line is that you're truly not threshold braking if you can add the extra braking force during the clutch engagement without breaking the tires loose.[/QUOTE]
I see guys do that but that's because they're using sequential gearboxes. If I'm coming down the back straight and have a second gear corner at the end I'm not going to downshift through all of the gears to get to second stopping at each to engine brake I'm going to accelerate all the way up to my braking point. Get on the brakes hard, blip throttle, throw it in second and get back on the gas. It quicker than stopping at each gear and brakes are good for stuff like that.
Everyone does. That half the point of heel-toe. Ever see a guy brake form 6 to 2nd using all the gears on teh way down? He does that to help braking...
I think the bottom line is that you're truly not threshold braking if you can add the extra braking force during the clutch engagement without breaking the tires loose.[/QUOTE]
I see guys do that but that's because they're using sequential gearboxes. If I'm coming down the back straight and have a second gear corner at the end I'm not going to downshift through all of the gears to get to second stopping at each to engine brake I'm going to accelerate all the way up to my braking point. Get on the brakes hard, blip throttle, throw it in second and get back on the gas. It quicker than stopping at each gear and brakes are good for stuff like that.
| cowapult | 08-23-2006 08:06 AM |
[QUOTE=Capt Crunch]
I explained if you are threshold braking and don't engine brake, you can lock up the wheels (provided you don't have abs). But then I got to thinking, is this really the case? The only limiting factor for most brakes are the tires, which basically can dissipate a certain maximum amount of energy per time via friction. If you try to exceed that amount, you lock up the wheels.
[/quote]
Yeah, you are right in the second half. The simplist explanation is that the tires don't care how they are being stopped. Engine braking is no different than brake pads - in both cases the limiting factor is the tire friction and the tires don't care how they are being stopped. But, one difference is that your brake system has an important front bias whereas your engine braking is going to have the bias of your center diff. Even if your diff is 50/50 split, that favors oversteer because the rear of the car will have little weight on it. On the flip side, one of the advantages of engine braking is that it doesn't heat up your brake pads.
[QUOTE=Capt Crunch]
But here is the thing, if you don't revmatch and just let out the clutch, energy goes into raising the rpm of the motor. The energy has to come from somewhere, so it must from from the energy of the moving car, thus slowing it down.
[/quote]
Yes, this is one of the few advantages of using engine braking to help - the energy goes into pumping losses in the engine instead of your brake pads so you will not fade your brakes as soon.
[QUOTE=Capt Crunch]
Which brings me to a couple questions. Am I right? If so, why is this technique not used? You can slow down a car a lot from engine braking, I remember it was a problem for racing automatic cars because they didn't have it.
[/quote]
Who said it's not used? When you want to slow down, you lift the throttle and it helps you slow down. Although, for a real braking zone you have to stomp on the brake too to get your tires to the limit.
[QUOTE=Capt Crunch]
Second, why don't race cars employ a spring mechanism? It seems it would be somewhat simple to rig a mechanical device that soaks up energy in the braking zone, and can be harnessed during the straightaways. Is it too complicated? Easy to break? Against the rules? Kind of like how rubber skirts and vacuum pumps on NASCAR is banned.[/QUOTE]
Just not practical. You would need to store an insane amount of energy, so the spring would be huge and heavy.
But, what you describe is the entire principle of a hybrid electric. They just store the energy in a battery or capacitor when you brake instead of a spring.
I explained if you are threshold braking and don't engine brake, you can lock up the wheels (provided you don't have abs). But then I got to thinking, is this really the case? The only limiting factor for most brakes are the tires, which basically can dissipate a certain maximum amount of energy per time via friction. If you try to exceed that amount, you lock up the wheels.
[/quote]
Yeah, you are right in the second half. The simplist explanation is that the tires don't care how they are being stopped. Engine braking is no different than brake pads - in both cases the limiting factor is the tire friction and the tires don't care how they are being stopped. But, one difference is that your brake system has an important front bias whereas your engine braking is going to have the bias of your center diff. Even if your diff is 50/50 split, that favors oversteer because the rear of the car will have little weight on it. On the flip side, one of the advantages of engine braking is that it doesn't heat up your brake pads.
[QUOTE=Capt Crunch]
But here is the thing, if you don't revmatch and just let out the clutch, energy goes into raising the rpm of the motor. The energy has to come from somewhere, so it must from from the energy of the moving car, thus slowing it down.
[/quote]
Yes, this is one of the few advantages of using engine braking to help - the energy goes into pumping losses in the engine instead of your brake pads so you will not fade your brakes as soon.
[QUOTE=Capt Crunch]
Which brings me to a couple questions. Am I right? If so, why is this technique not used? You can slow down a car a lot from engine braking, I remember it was a problem for racing automatic cars because they didn't have it.
[/quote]
Who said it's not used? When you want to slow down, you lift the throttle and it helps you slow down. Although, for a real braking zone you have to stomp on the brake too to get your tires to the limit.
[QUOTE=Capt Crunch]
Second, why don't race cars employ a spring mechanism? It seems it would be somewhat simple to rig a mechanical device that soaks up energy in the braking zone, and can be harnessed during the straightaways. Is it too complicated? Easy to break? Against the rules? Kind of like how rubber skirts and vacuum pumps on NASCAR is banned.[/QUOTE]
Just not practical. You would need to store an insane amount of energy, so the spring would be huge and heavy.
But, what you describe is the entire principle of a hybrid electric. They just store the energy in a battery or capacitor when you brake instead of a spring.
| 555ideways | 08-23-2006 11:21 AM |
The brakes are for braking. The engine is for accelerating. Try not to get them mixed up. 5 min on a race track and it's obvious. Sometimes the simplest way is also the fastest and most efficient.
| grippgoat | 08-23-2006 12:12 PM |
[QUOTE=kellygnsd]I see guys do that but that's because they're using sequential gearboxes. If I'm coming down the back straight and have a second gear corner at the end I'm not going to downshift through all of the gears to get to second stopping at each to engine brake I'm going to accelerate all the way up to my braking point. Get on the brakes hard, blip throttle, throw it in second and get back on the gas. It quicker than stopping at each gear and brakes are good for stuff like that.[/QUOTE]
Are you sure you're not exaggerating? In my car (05 STI), I'd have to be hitting at least 135mph on the straight to get into 6th gear, and then I'd have to be going like 40mph to want 2nd gear.... I think you'd want to do at least one extra downshift when dropping 95mph. Otherwise you'd be losing a lot of extra engine braking, and your brain would have to think harder about where to actually put the shifter. Skipping 4 gears with such a big RPM change would work your synchros harder (and increase chance of miss-shift), and also make your last corner-entry downshift a lot more complicated, and probably slow you down. Of course, maybe this is just a hypothetical situation. :) Something about the way you worded your post just made me think you were forgetting how long it takes to slow a car down from 6th-gear speeds to 2nd-gear speeds, and how long it takes to rev the engine up to cover a 4-gear gap. Of course with weight reduction, race tires, big brakes, and a light flywheel, maybe it works better your way.
-Mike
Are you sure you're not exaggerating? In my car (05 STI), I'd have to be hitting at least 135mph on the straight to get into 6th gear, and then I'd have to be going like 40mph to want 2nd gear.... I think you'd want to do at least one extra downshift when dropping 95mph. Otherwise you'd be losing a lot of extra engine braking, and your brain would have to think harder about where to actually put the shifter. Skipping 4 gears with such a big RPM change would work your synchros harder (and increase chance of miss-shift), and also make your last corner-entry downshift a lot more complicated, and probably slow you down. Of course, maybe this is just a hypothetical situation. :) Something about the way you worded your post just made me think you were forgetting how long it takes to slow a car down from 6th-gear speeds to 2nd-gear speeds, and how long it takes to rev the engine up to cover a 4-gear gap. Of course with weight reduction, race tires, big brakes, and a light flywheel, maybe it works better your way.
-Mike
| Homemade WRX | 08-23-2006 01:16 PM |
[QUOTE=cowapult]Just not practical. You would need to store an insane amount of energy, so the spring would be huge and heavy.
But, what you describe is the entire principle of a hybrid electric. They just store the energy in a battery or capacitor when you brake instead of a spring.[/QUOTE]
tpo add to that there are diesel trucks that are using hydraulic pressure to stop...it is stored and once the light turns green the hydraulic pressure gets them moving again...
But, what you describe is the entire principle of a hybrid electric. They just store the energy in a battery or capacitor when you brake instead of a spring.[/QUOTE]
tpo add to that there are diesel trucks that are using hydraulic pressure to stop...it is stored and once the light turns green the hydraulic pressure gets them moving again...
| kellygnsd | 08-23-2006 03:18 PM |
Grippgoat, I'm rocking a Rex with the 5 speed still and the fastest track I've been on so far has been road America in Wisconsin. I've hit close to 140mph coming down the back straight (over 1/4 mile) into turn 1 which was about a 60 mph turn. I usually dropped from 5th to 3rd here but if there was traffic I'd go to second, slow more and get the drive on the exit. For me, most tracks I've run on had no use for 5th so on tracks like Blackhawk Farms I would consistently go from 4th to 2nd. If you're familiar with the track and your car and you know what gears are safe for what speeds it is definitely the fasted way to slow down and then get back on the power to get around the track the fastest. Stopping at every gear on the way down is time lost either braking at the limit or accelerating and if your not doing one of those two things you're going to get passed.
| Capt Crunch | 08-23-2006 03:41 PM |
[QUOTE=555ideways]The brakes are for braking. The engine is for accelerating. Try not to get them mixed up. 5 min on a race track and it's obvious. Sometimes the simplest way is also the fastest and most efficient.[/QUOTE]
Thanks for the condecending tone, jerk. I've been to a track day. What prompted me to think about it more was that 2 out of the 3 instructors I rode with did not heel and toe like I did.
Thanks for the condecending tone, jerk. I've been to a track day. What prompted me to think about it more was that 2 out of the 3 instructors I rode with did not heel and toe like I did.
| RyanC | 08-23-2006 03:44 PM |
[QUOTE=Capt Crunch]Thanks for the condecending tone, jerk. I've been to a track day. What prompted me to think about it more was that 2 out of the 3 instructors I rode with did not heel and toe like I did.[/QUOTE]
Hey there Count Chocula, if your instructor used the armco as a braking aid would you do that too?
Hey there Count Chocula, if your instructor used the armco as a braking aid would you do that too?
| Capt Crunch | 08-23-2006 04:39 PM |
[QUOTE=RyanC]Hey there Count Chocula, if your instructor used the armco as a braking aid would you do that too?[/QUOTE]
Well, [i]I[/i] heel-toed every downshift. The instructors arguably know more than I do, so it doesn't seem completely retarded of me to ask the question.
Well, [i]I[/i] heel-toed every downshift. The instructors arguably know more than I do, so it doesn't seem completely retarded of me to ask the question.
| hotrod | 08-23-2006 04:50 PM |
[quote]You are misunderstanding what I said - I thought it was clear enough but I guess not.[/quote]
Nope understood just fine -- only you are wrong, in both cases you are using engine braking.
[quote]You slowly or worse, quickly, let out the clutch to bring the RPMs up from 750 to ~6800. This is not fine. This is using the clutch as a brake.
If you are going to downshift like this you need to rev match. Otherwise you are burning the clutch and upsetting the car's balance.[/quote]
If you rev match you are by definition not using engine braking and defeating the whole purpose of the process. The engine uses up a lot of energy when it spins up from a lower rpm.
You are not using the clutch for a brake, because no significant clutch slippage occurs, as soon as you let out the clutch it engages (the same as it does when you accelerate) the braking action is due to both the enertia effects of spinning up the engine, and the work it has to due performing useless, compression.
yes it "[b]can[/b] upset the cars balance" but this is not always the case, for instance in a RWD car the compressional braking of the rear wheels can pull the back end back into line. It also applies a smoother application of resistance than any driver and even top of the line ABS systems can apply and is superior to using the brakes on slippery / icy surfaces as you can modulate the resistance very easily with the throttle.
It certainly upsets the car a lot less than running into a bridge abutment or tree because you cooked the brakes or as in my case a brake line blew off as I was coming up to a red light. (In my situation I was able to get the car slowed down enough to do a max effort right tune through the red light in a gap in the traffic and get it off on the shoulder without hitting anything.)
The point I was making-- which you missed -- is that using the engine to slow the car by down shifting is a [b][i]valid and necessary technique [/b][/i] every driver should learn, and if you don't use it and or understand how to use it, you are not a complete driver and are depriving your self of a valuable tool and safety technique. It may have limited applications, but but it is an important technique to know.
Here in Mountain country, it can double you brake life if you routinely drive mountain roads with steep grades and leaves your brakes cool so their full braking ability is available if you need it. On the Back side (west approach) of Eisenhower tunnel on I-70 you have about 12 miles of steep grade (some as high as 7%). If left unrestricted cars will reach speeds over 100 mph. The bad news is some of these corners are 60 -80 mph corners under ideal conditions. Lots of folks repeatedly apply the brakes to maintain either a safe or legal speed. By the time they near the bottom they are going 70 odd mph and have hot brakes. Someone who selects the proper gear and uses engine braking, can cruise down the mountain at 70 mph foot completely off the throttle and never touch the brakes for miles at a time all while maintaining a very balanced car (more so than they would have on repeated brake applications at each tight corner when they realize they are over cooking the corner).
This and other similar sections of down grade here in Colorado have mulitiple emergency run out ramps for truckers and cars that burn up their brakes because they think "brakes are for stopping and clutches are for shifting".
I repeat again, even if you are "jambing gears" down shifting like a fiend, you are not "burning the clutch"! It is no different than accelerating hard, the clutch engages immediately, and the braking effort is due to the engine and the energy it dissipates due to friction losses working as a very in-effecient air pump. That is preciesly why diesel trucks use engine braking with their "Jake brakes" it does not "burn up their brakes or their clutches". They use it because it reduces wear and tear on their trucks!
Larry
Nope understood just fine -- only you are wrong, in both cases you are using engine braking.
[quote]You slowly or worse, quickly, let out the clutch to bring the RPMs up from 750 to ~6800. This is not fine. This is using the clutch as a brake.
If you are going to downshift like this you need to rev match. Otherwise you are burning the clutch and upsetting the car's balance.[/quote]
If you rev match you are by definition not using engine braking and defeating the whole purpose of the process. The engine uses up a lot of energy when it spins up from a lower rpm.
You are not using the clutch for a brake, because no significant clutch slippage occurs, as soon as you let out the clutch it engages (the same as it does when you accelerate) the braking action is due to both the enertia effects of spinning up the engine, and the work it has to due performing useless, compression.
yes it "[b]can[/b] upset the cars balance" but this is not always the case, for instance in a RWD car the compressional braking of the rear wheels can pull the back end back into line. It also applies a smoother application of resistance than any driver and even top of the line ABS systems can apply and is superior to using the brakes on slippery / icy surfaces as you can modulate the resistance very easily with the throttle.
It certainly upsets the car a lot less than running into a bridge abutment or tree because you cooked the brakes or as in my case a brake line blew off as I was coming up to a red light. (In my situation I was able to get the car slowed down enough to do a max effort right tune through the red light in a gap in the traffic and get it off on the shoulder without hitting anything.)
The point I was making-- which you missed -- is that using the engine to slow the car by down shifting is a [b][i]valid and necessary technique [/b][/i] every driver should learn, and if you don't use it and or understand how to use it, you are not a complete driver and are depriving your self of a valuable tool and safety technique. It may have limited applications, but but it is an important technique to know.
Here in Mountain country, it can double you brake life if you routinely drive mountain roads with steep grades and leaves your brakes cool so their full braking ability is available if you need it. On the Back side (west approach) of Eisenhower tunnel on I-70 you have about 12 miles of steep grade (some as high as 7%). If left unrestricted cars will reach speeds over 100 mph. The bad news is some of these corners are 60 -80 mph corners under ideal conditions. Lots of folks repeatedly apply the brakes to maintain either a safe or legal speed. By the time they near the bottom they are going 70 odd mph and have hot brakes. Someone who selects the proper gear and uses engine braking, can cruise down the mountain at 70 mph foot completely off the throttle and never touch the brakes for miles at a time all while maintaining a very balanced car (more so than they would have on repeated brake applications at each tight corner when they realize they are over cooking the corner).
This and other similar sections of down grade here in Colorado have mulitiple emergency run out ramps for truckers and cars that burn up their brakes because they think "brakes are for stopping and clutches are for shifting".
I repeat again, even if you are "jambing gears" down shifting like a fiend, you are not "burning the clutch"! It is no different than accelerating hard, the clutch engages immediately, and the braking effort is due to the engine and the energy it dissipates due to friction losses working as a very in-effecient air pump. That is preciesly why diesel trucks use engine braking with their "Jake brakes" it does not "burn up their brakes or their clutches". They use it because it reduces wear and tear on their trucks!
Larry
| REX8 | 08-23-2006 04:54 PM |
[QUOTE=kellygnsd]I see guys do that but that's because they're using sequential gearboxes. If I'm coming down the back straight and have a second gear corner at the end I'm not going to downshift through all of the gears to get to second stopping at each to engine brake I'm going to accelerate all the way up to my braking point. Get on the brakes hard, blip throttle, throw it in second and get back on the gas. It quicker than stopping at each gear and brakes are good for stuff like that.[/QUOTE]
No, not just for sequential gearboxes...at all. You should watch some more in car videos or some more ride alongs.
How is it any slower downshifting? Your foot never leaves the brake pedal. Theoretically, you achive the same maximum braking, while having you brakes do less work. That can be a BIG deal on a lot of heavier cars.
Again, how is this any slower? You are constantly braking....just adding in the engine braking as well.
5th or 6th to 2nd without at least 3rd gear in there? :confused:
No, not just for sequential gearboxes...at all. You should watch some more in car videos or some more ride alongs.
How is it any slower downshifting? Your foot never leaves the brake pedal. Theoretically, you achive the same maximum braking, while having you brakes do less work. That can be a BIG deal on a lot of heavier cars.
Again, how is this any slower? You are constantly braking....just adding in the engine braking as well.
5th or 6th to 2nd without at least 3rd gear in there? :confused:
| hotrod | 08-23-2006 05:09 PM |
[quote]I'm going to accelerate all the way up to my braking point. Get on the brakes hard, blip throttle, throw it in second and get back on the gas. It quicker than stopping at each gear and brakes are good for stuff like that.[/quote]
And late in the race the other guy that was using engine braking will out brake you on the final turn and win the race because he was not using up as much of his braking system.
Larry
And late in the race the other guy that was using engine braking will out brake you on the final turn and win the race because he was not using up as much of his braking system.
Larry
| 555ideways | 08-23-2006 06:00 PM |
[QUOTE=Capt Crunch]Thanks for the condecending tone, jerk. I've been to a track day. What prompted me to think about it more was that 2 out of the 3 instructors I rode with did not heel and toe like I did.[/QUOTE]
I hear yah. It's worth thinking about. I think i forgot a smiley or 2. :p
Thats interesting that 2 out of the 3 didn't heel toe. In a normal production car with a normal synchro gearbox and clutch i was taught a right way and a slow way.
The only people i've met at the track who don't heel toe are those who struggled with it, then chose to forget about it and concentrate on other aspects. When they finally fell into a routine and a large part of their learning curve was completed they could make it around the track at a decent pace without the maneuver.
I think it's like left foot braking. If you learn it early in your life/track career it can be used effectively to shave off 10ths and gain a consistent edge over a competitor who chose to forget about it. (i don't left foot brake and wish i had more motivation to learn)
-Paddy
I hear yah. It's worth thinking about. I think i forgot a smiley or 2. :p
Thats interesting that 2 out of the 3 didn't heel toe. In a normal production car with a normal synchro gearbox and clutch i was taught a right way and a slow way.
The only people i've met at the track who don't heel toe are those who struggled with it, then chose to forget about it and concentrate on other aspects. When they finally fell into a routine and a large part of their learning curve was completed they could make it around the track at a decent pace without the maneuver.
I think it's like left foot braking. If you learn it early in your life/track career it can be used effectively to shave off 10ths and gain a consistent edge over a competitor who chose to forget about it. (i don't left foot brake and wish i had more motivation to learn)
-Paddy
| GarySheehan | 08-23-2006 06:23 PM |
I never use the engine to help slow the car unless I'm having braking issues.
I tend to downshift through all the gears for the rhythm and to be at the right rpm for that lowest gear. I do skip gears in some braking zones that are particularly busy. However, rowing down through the gears does nothing to help slow the car down. As a matter of fact, with a stock flywheel, the brakes are dragging engine speed down faster than it would have dropped in neutral, which actually ADDS a slight bit of work to the braking system rather than relieving it of work.
Here's that old video again. Watch how fast engine rpms drop under braking vs. how fast they drop during an upshift (prior to letting the clutch out). The engine rpm just tends to float when the clutch is in. Not much braking force generated there.
[url]http://www.teamsmr.com/movies/Footbox%20Small.wmv[/url]
Gary
Sheehan Motor Racing
[url]www.teamSMR.com[/url]
I tend to downshift through all the gears for the rhythm and to be at the right rpm for that lowest gear. I do skip gears in some braking zones that are particularly busy. However, rowing down through the gears does nothing to help slow the car down. As a matter of fact, with a stock flywheel, the brakes are dragging engine speed down faster than it would have dropped in neutral, which actually ADDS a slight bit of work to the braking system rather than relieving it of work.
Here's that old video again. Watch how fast engine rpms drop under braking vs. how fast they drop during an upshift (prior to letting the clutch out). The engine rpm just tends to float when the clutch is in. Not much braking force generated there.
[url]http://www.teamsmr.com/movies/Footbox%20Small.wmv[/url]
Gary
Sheehan Motor Racing
[url]www.teamSMR.com[/url]
| waktasz | 08-23-2006 06:29 PM |
[QUOTE=Going Faster (Skip Barber Racing School textbook)]
What is downshifting really for?
We ask this basic question of every racing school class. The most frequent (and incorrect) answer is 'to slow the car down.' /..../ You downshift to get the car in the proper gear to exit the corner
[/QUOTE]
^^^^^
What is downshifting really for?
We ask this basic question of every racing school class. The most frequent (and incorrect) answer is 'to slow the car down.' /..../ You downshift to get the car in the proper gear to exit the corner
[/QUOTE]
^^^^^
| waktasz | 08-23-2006 06:34 PM |
[QUOTE=hotrod]And late in the race the other guy that was using engine braking will out brake you on the final turn and win the race because he was not using up as much of his braking system.
Larry[/QUOTE]
Meanwhile you're in the pit because your clutch is gone.
Larry[/QUOTE]
Meanwhile you're in the pit because your clutch is gone.
| seattle944t | 08-23-2006 07:41 PM |
[QUOTE=hotrod]Nope understood just fine -- only you are wrong, in both cases you are using engine braking.[/QUOTE]
No, I am not wrong it is you that is mistaken. Engine Braking is using the compression of the engine to slow the car down [b]while in gear[/b] that is why it is also called compression braking. Slipping the clutch to slow the car down is using the clutch as a brake.
[QUOTE=hotrod]
If you rev match you are by definition not using engine braking and defeating the whole purpose of the process. The engine uses up a lot of energy when it spins up from a lower rpm.[/QUOTE]
Again, wrong, that is using the friction of the clutch to slow the car down. Accelerate to 6000 RPM in any gear and get off the gas - that is engine (compression) braking. Push in the clutch and let the engine drop down to idle and ease it out and you are [b]slipping the clutch[/b] to generate braking.
[QUOTE=hotrod]
You are not using the clutch for a brake, because no significant clutch slippage occurs, as soon as you let out the clutch it engages (the same as it does when you accelerate) the braking action is due to both the enertia effects of spinning up the engine, and the work it has to due performing useless, compression.
[/QUOTE]
Way wrong, addressed below.
[QUOTE=hotrod]
yes it "[b]can[/b] upset the cars balance" but this is not always the case, for instance in a RWD car the compressional braking of the rear wheels can pull the back end back into line. It also applies a smoother application of resistance than any driver and even top of the line ABS systems can apply and is superior to using the brakes on slippery / icy surfaces as you can modulate the resistance very easily with the throttle.
[/QUOTE]
Have you ever tracked a RWD race car before? Obvioulsy not from that response. If you use the clutch to bring up the RPMs on a RWD car on the track you are going to be in the dirt because you are going to spin it.
[QUOTE=hotrod]
It certainly upsets the car a lot less than running into a bridge abutment or tree because you cooked the brakes or as in my case a brake line blew off as I was coming up to a red light. (In my situation I was able to get the car slowed down enough to do a max effort right tune through the red light in a gap in the traffic and get it off on the shoulder without hitting anything.)
[/QUOTE]
We are not talking about emergency proceedures when your brakes fail - we are talking about [b]how to stop the car the fastest in the motorsports forum [/b]and that means brakes. By all means if my brakes are out I am going to be using the clutch, the e-brake, the steering wheel, my feet, and any thing else I can think of to slow the car down.
[QUOTE=hotrod]
The point I was making-- which you missed -- is that using the engine to slow the car by down shifting is a [b][i]valid and necessary technique [/b][/i] every driver should learn, and if you don't use it and or understand how to use it, you are not a complete driver and are depriving your self of a valuable tool and safety technique. It may have limited applications, but but it is an important technique to know.
[/QUOTE]
I didn't miss it - you did. For an emergency absolutly use every means to stop the car.
[QUOTE=hotrod]
Here in Mountain country, it can double you brake life if you routinely drive mountain roads with steep grades and leaves your brakes cool so their full braking ability is available if you need it. On the Back side (west approach) of Eisenhower tunnel on I-70 you have about 12 miles of steep grade (some as high as 7%). If left unrestricted cars will reach speeds over 100 mph. The bad news is some of these corners are 60 -80 mph corners under ideal conditions. Lots of folks repeatedly apply the brakes to maintain either a safe or legal speed. By the time they near the bottom they are going 70 odd mph and have hot brakes. Someone who selects the proper gear and uses engine braking, can cruise down the mountain at 70 mph foot completely off the throttle and never touch the brakes for miles at a time all while maintaining a very balanced car (more so than they would have on repeated brake applications at each tight corner when they realize they are over cooking the corner).
[/QUOTE]
No argument as you have described it here - engine braking is good and you are a lousy and poorly trained driver if you dont do it on grades. But Engine braking (in gear) is being in a lower gear and off the gas, not slipping your clutch to bring the RPM's up from idle. According to your idea of engine braking you are slipping your clutch all the way down the hill to get the 'most effective' use of engine braking, and that is just plain stupid.
[QUOTE=hotrod]
This and other similar sections of down grade here in Colorado have mulitiple emergency run out ramps for truckers and cars that burn up their brakes because they think "brakes are for stopping and clutches are for shifting".
[/QUOTE]
Um, thats because BRAKES ARE FOR STOPPING AND CLUTCHES ARE FOR SHIFTING. That has [b]nothing, I say again nothing[/b] to do with engine braking. Using the clutch as a brake going down a grade is idiocy. Using engine braking is the right way to get down a grade.
[QUOTE=hotrod]
I repeat again, even if you are "jambing gears" down shifting like a fiend, you are not "burning the clutch"! It is no different than accelerating hard, the clutch engages immediately, and the braking effort is due to the engine and the energy it dissipates due to friction losses working as a very in-effecient air pump. That is preciesly why diesel trucks use engine braking with their "Jake brakes" it does not "burn up their brakes or their clutches". They use it because it reduces wear and tear on their trucks!
Larry[/QUOTE]
You still don't get it. Using your clutch to bring the motor from 750RPM to 6000 RPM's is [b]vastly different[/b] than a WOT up shift - by about 4000RPMs of speed differential. And I am pretty sure that 4000RPM's of speed differential generates, oh I dont know, about [b]4 times[/b] the friction of a 1000RPM speed differential on an upshift.
Jake brakes are used [b]in gear[/b] they are not slipping thier clutches to bring RPMs up. They are using the jake brake to attempt to keep the motor at a specific RPM. They are [b]always in gear[/b]. They are not using the clutch as a brake. Period.
Here is a wiki for you to read...
[url]http://en.wikipedia.org/wiki/Engine_braking[/url]
[QUOTE]
Engine braking is always active in all non-hybrid cars with an internal combustion engine, regardless of transmission type. Engine braking passively reduces wear on brakes and helps a driver maintain control of the car. It is always active when the foot is lifted off the accelerator, [B]the transmission is not in neutral, the clutch is engaged and a freewheel is not engaged[/B]. This is often called engine drag.
[/QUOTE]
Engine braking = in gear, compression of motor slowing car down.
Clutch braking = using the clutch to raise the RPM's of the motor.
No, I am not wrong it is you that is mistaken. Engine Braking is using the compression of the engine to slow the car down [b]while in gear[/b] that is why it is also called compression braking. Slipping the clutch to slow the car down is using the clutch as a brake.
[QUOTE=hotrod]
If you rev match you are by definition not using engine braking and defeating the whole purpose of the process. The engine uses up a lot of energy when it spins up from a lower rpm.[/QUOTE]
Again, wrong, that is using the friction of the clutch to slow the car down. Accelerate to 6000 RPM in any gear and get off the gas - that is engine (compression) braking. Push in the clutch and let the engine drop down to idle and ease it out and you are [b]slipping the clutch[/b] to generate braking.
[QUOTE=hotrod]
You are not using the clutch for a brake, because no significant clutch slippage occurs, as soon as you let out the clutch it engages (the same as it does when you accelerate) the braking action is due to both the enertia effects of spinning up the engine, and the work it has to due performing useless, compression.
[/QUOTE]
Way wrong, addressed below.
[QUOTE=hotrod]
yes it "[b]can[/b] upset the cars balance" but this is not always the case, for instance in a RWD car the compressional braking of the rear wheels can pull the back end back into line. It also applies a smoother application of resistance than any driver and even top of the line ABS systems can apply and is superior to using the brakes on slippery / icy surfaces as you can modulate the resistance very easily with the throttle.
[/QUOTE]
Have you ever tracked a RWD race car before? Obvioulsy not from that response. If you use the clutch to bring up the RPMs on a RWD car on the track you are going to be in the dirt because you are going to spin it.
[QUOTE=hotrod]
It certainly upsets the car a lot less than running into a bridge abutment or tree because you cooked the brakes or as in my case a brake line blew off as I was coming up to a red light. (In my situation I was able to get the car slowed down enough to do a max effort right tune through the red light in a gap in the traffic and get it off on the shoulder without hitting anything.)
[/QUOTE]
We are not talking about emergency proceedures when your brakes fail - we are talking about [b]how to stop the car the fastest in the motorsports forum [/b]and that means brakes. By all means if my brakes are out I am going to be using the clutch, the e-brake, the steering wheel, my feet, and any thing else I can think of to slow the car down.
[QUOTE=hotrod]
The point I was making-- which you missed -- is that using the engine to slow the car by down shifting is a [b][i]valid and necessary technique [/b][/i] every driver should learn, and if you don't use it and or understand how to use it, you are not a complete driver and are depriving your self of a valuable tool and safety technique. It may have limited applications, but but it is an important technique to know.
[/QUOTE]
I didn't miss it - you did. For an emergency absolutly use every means to stop the car.
[QUOTE=hotrod]
Here in Mountain country, it can double you brake life if you routinely drive mountain roads with steep grades and leaves your brakes cool so their full braking ability is available if you need it. On the Back side (west approach) of Eisenhower tunnel on I-70 you have about 12 miles of steep grade (some as high as 7%). If left unrestricted cars will reach speeds over 100 mph. The bad news is some of these corners are 60 -80 mph corners under ideal conditions. Lots of folks repeatedly apply the brakes to maintain either a safe or legal speed. By the time they near the bottom they are going 70 odd mph and have hot brakes. Someone who selects the proper gear and uses engine braking, can cruise down the mountain at 70 mph foot completely off the throttle and never touch the brakes for miles at a time all while maintaining a very balanced car (more so than they would have on repeated brake applications at each tight corner when they realize they are over cooking the corner).
[/QUOTE]
No argument as you have described it here - engine braking is good and you are a lousy and poorly trained driver if you dont do it on grades. But Engine braking (in gear) is being in a lower gear and off the gas, not slipping your clutch to bring the RPM's up from idle. According to your idea of engine braking you are slipping your clutch all the way down the hill to get the 'most effective' use of engine braking, and that is just plain stupid.
[QUOTE=hotrod]
This and other similar sections of down grade here in Colorado have mulitiple emergency run out ramps for truckers and cars that burn up their brakes because they think "brakes are for stopping and clutches are for shifting".
[/QUOTE]
Um, thats because BRAKES ARE FOR STOPPING AND CLUTCHES ARE FOR SHIFTING. That has [b]nothing, I say again nothing[/b] to do with engine braking. Using the clutch as a brake going down a grade is idiocy. Using engine braking is the right way to get down a grade.
[QUOTE=hotrod]
I repeat again, even if you are "jambing gears" down shifting like a fiend, you are not "burning the clutch"! It is no different than accelerating hard, the clutch engages immediately, and the braking effort is due to the engine and the energy it dissipates due to friction losses working as a very in-effecient air pump. That is preciesly why diesel trucks use engine braking with their "Jake brakes" it does not "burn up their brakes or their clutches". They use it because it reduces wear and tear on their trucks!
Larry[/QUOTE]
You still don't get it. Using your clutch to bring the motor from 750RPM to 6000 RPM's is [b]vastly different[/b] than a WOT up shift - by about 4000RPMs of speed differential. And I am pretty sure that 4000RPM's of speed differential generates, oh I dont know, about [b]4 times[/b] the friction of a 1000RPM speed differential on an upshift.
Jake brakes are used [b]in gear[/b] they are not slipping thier clutches to bring RPMs up. They are using the jake brake to attempt to keep the motor at a specific RPM. They are [b]always in gear[/b]. They are not using the clutch as a brake. Period.
Here is a wiki for you to read...
[url]http://en.wikipedia.org/wiki/Engine_braking[/url]
[QUOTE]
Engine braking is always active in all non-hybrid cars with an internal combustion engine, regardless of transmission type. Engine braking passively reduces wear on brakes and helps a driver maintain control of the car. It is always active when the foot is lifted off the accelerator, [B]the transmission is not in neutral, the clutch is engaged and a freewheel is not engaged[/B]. This is often called engine drag.
[/QUOTE]
Engine braking = in gear, compression of motor slowing car down.
Clutch braking = using the clutch to raise the RPM's of the motor.
| REX8 | 08-23-2006 08:32 PM |
[QUOTE=waktasz]Meanwhile you're in the pit because your clutch is gone.[/QUOTE]
There would be no real clutch wear with a proper rev matched downshift...I think hotrod is assuming the OP means downshifting to use engine braking adn heal-toeing...not the burn your clutch style (who even does this?) talked about above.
In any case, its very much more common to see someone row the gearbox down from high speeds than go from like 6th to 2nd....
There would be no real clutch wear with a proper rev matched downshift...I think hotrod is assuming the OP means downshifting to use engine braking adn heal-toeing...not the burn your clutch style (who even does this?) talked about above.
In any case, its very much more common to see someone row the gearbox down from high speeds than go from like 6th to 2nd....
| hotrod | 08-23-2006 08:47 PM |
[quote]Engine Braking is using the compression of the engine to slow the car down while in gear that is why it is also called compression braking. Slipping the clutch to slow the car down is using the clutch as a brake.[/quote]
Good we agree, if your slipping the clutch your an idiot and doing it wrong!!
The problem is you keep insisting that the driver is "slipping the clutch to slow the car down" --- that is not what I am talking about! Nor have I ever suggested you do that.
You let the clutch out -- it engages, the engine provides the compression braking when you are fully off the throttle and in a low enough gear to get good compression braking.
Your making assumptions about how to use the technique that are not valid. --- it has its place, but it is also something that will get you in trouble -- like spin you out if done at the wrong time and place.
The point I'm trying to make is some of you are making absolute statements that it is always bad without any qualification. That is incorrect!!!
There is a place for it, it does not hurt the clutch, and you should understand how to do it and its limits.
Years ago when brakes were not as good as today (ie drum brakes) it was an absolute necessitity to use it to keep your brakes alive for the duration of a race, or in the case of severe mountain driving if you wanted to not totally ruin your brakes by the time you descended a mountain pass.
Although disk brakes are much better than those days, they still have limits, and a good driver understands that he can extend those limits and or rest his brakes a bit by using compression braking so at a later time he can "out brake an opponet who has hotter brakes". <--- see maximum braking in motor sport situation.
Just like hard braking, you don't do it while entering a corner (unless you like going off course) but on the straight just before you turn in. If you come off the throttle while still in gear the engine will start to slow you down. That uses up energy that otherwise would go into the brakes, so the brakes have less work to do overall, stay cooler and last longer.
[quote]Um, thats because BRAKES ARE FOR STOPPING AND CLUTCHES ARE FOR SHIFTING. That has nothing, I say again nothing to do with engine braking. Using the clutch as a brake going down a grade is idiocy. Using engine braking is the right way to get down a grade.[/quote]
[quote]According to your idea of engine braking you are slipping your clutch all the way down the hill to get the 'most effective' use of engine braking, and that is just plain stupid.[/quote]
Never said that, never suggested that, never do that, I can't believe you think anyone would even try that ---- for the umpteenth time --- [B]you do not slip the clutch!!!!!!!![/b] you simply let it out and modulate engine rpm with the throttle so there is smooth engagement, then let off the throttle to develop compression braking.
Exactly right --- again I am not and have never recommended using the clutch as a brake, or "slipping it" --- you are using the engine compression. In fact I cannot imagine how you believe it is possible to "use your clutch for braking" the physical design of the the car simply does not allow it. You put it in a lower gear you let out the clutch, the clutch engages (with no significant slippage) locking transmission input shaft up to engine crank shaft and [b]then[/b] the engine begins to provide compressional braking by resisting acceleration by the drive train. Your definition of "clutch braking" is simply not sensible as the clutch never slips significantly (as anyone who has accidently over reved an engine by down shifting in to the wrong gear knows very well --- even in that extreme case, the clutch simply locks up and buzzes the crap out of the engine while slowing you down violently.) The clutch never has any load placed on it that it does not see every day at every stop light. The only difference is the direction of energy transfer. At the stop light, the engine accelerates the drive train, in compression braking the drive train attempts to accelerate the engine, but is resisted by the compression braking as the engine does negative work trying to move air.
Even the quote you posted from wikipedia agrees with what I have been saying!
[quote]Engine braking passively reduces wear on brakes and helps a driver maintain control of the car. [/quote]
Larry
Good we agree, if your slipping the clutch your an idiot and doing it wrong!!
The problem is you keep insisting that the driver is "slipping the clutch to slow the car down" --- that is not what I am talking about! Nor have I ever suggested you do that.
You let the clutch out -- it engages, the engine provides the compression braking when you are fully off the throttle and in a low enough gear to get good compression braking.
Your making assumptions about how to use the technique that are not valid. --- it has its place, but it is also something that will get you in trouble -- like spin you out if done at the wrong time and place.
The point I'm trying to make is some of you are making absolute statements that it is always bad without any qualification. That is incorrect!!!
There is a place for it, it does not hurt the clutch, and you should understand how to do it and its limits.
Years ago when brakes were not as good as today (ie drum brakes) it was an absolute necessitity to use it to keep your brakes alive for the duration of a race, or in the case of severe mountain driving if you wanted to not totally ruin your brakes by the time you descended a mountain pass.
Although disk brakes are much better than those days, they still have limits, and a good driver understands that he can extend those limits and or rest his brakes a bit by using compression braking so at a later time he can "out brake an opponet who has hotter brakes". <--- see maximum braking in motor sport situation.
Just like hard braking, you don't do it while entering a corner (unless you like going off course) but on the straight just before you turn in. If you come off the throttle while still in gear the engine will start to slow you down. That uses up energy that otherwise would go into the brakes, so the brakes have less work to do overall, stay cooler and last longer.
[quote]Um, thats because BRAKES ARE FOR STOPPING AND CLUTCHES ARE FOR SHIFTING. That has nothing, I say again nothing to do with engine braking. Using the clutch as a brake going down a grade is idiocy. Using engine braking is the right way to get down a grade.[/quote]
[quote]According to your idea of engine braking you are slipping your clutch all the way down the hill to get the 'most effective' use of engine braking, and that is just plain stupid.[/quote]
Never said that, never suggested that, never do that, I can't believe you think anyone would even try that ---- for the umpteenth time --- [B]you do not slip the clutch!!!!!!!![/b] you simply let it out and modulate engine rpm with the throttle so there is smooth engagement, then let off the throttle to develop compression braking.
Exactly right --- again I am not and have never recommended using the clutch as a brake, or "slipping it" --- you are using the engine compression. In fact I cannot imagine how you believe it is possible to "use your clutch for braking" the physical design of the the car simply does not allow it. You put it in a lower gear you let out the clutch, the clutch engages (with no significant slippage) locking transmission input shaft up to engine crank shaft and [b]then[/b] the engine begins to provide compressional braking by resisting acceleration by the drive train. Your definition of "clutch braking" is simply not sensible as the clutch never slips significantly (as anyone who has accidently over reved an engine by down shifting in to the wrong gear knows very well --- even in that extreme case, the clutch simply locks up and buzzes the crap out of the engine while slowing you down violently.) The clutch never has any load placed on it that it does not see every day at every stop light. The only difference is the direction of energy transfer. At the stop light, the engine accelerates the drive train, in compression braking the drive train attempts to accelerate the engine, but is resisted by the compression braking as the engine does negative work trying to move air.
Even the quote you posted from wikipedia agrees with what I have been saying!
[quote]Engine braking passively reduces wear on brakes and helps a driver maintain control of the car. [/quote]
Larry
| RyanC | 08-23-2006 09:27 PM |
[QUOTE=GarySheehan]I never use the engine to help slow the car unless I'm having braking issues.
I tend to downshift through all the gears for the rhythm and to be at the right rpm for that lowest gear. I do skip gears in some braking zones that are particularly busy. However, rowing down through the gears does nothing to help slow the car down. As a matter of fact, with a stock flywheel, the brakes are dragging engine speed down faster than it would have dropped in neutral, which actually ADDS a slight bit of work to the braking system rather than relieving it of work.
Here's that old video again. Watch how fast engine rpms drop under braking vs. how fast they drop during an upshift (prior to letting the clutch out). The engine rpm just tends to float when the clutch is in. Not much braking force generated there.
[url]http://www.teamsmr.com/movies/Footbox%20Small.wmv[/url]
Gary
Sheehan Motor Racing
[url]www.teamSMR.com[/url][/QUOTE]
I'm not nearly as awesome as Gary, but I do exactly the same thing. Good to see it's also the right thing :)
Plus look at it this way, no matter what is slowing you down (outside of Gary, who is the ONLY mofo here who actually gets paid to do this for a living), brakes are a hell of a lot cheaper to replace than a motor.
Ryan
I tend to downshift through all the gears for the rhythm and to be at the right rpm for that lowest gear. I do skip gears in some braking zones that are particularly busy. However, rowing down through the gears does nothing to help slow the car down. As a matter of fact, with a stock flywheel, the brakes are dragging engine speed down faster than it would have dropped in neutral, which actually ADDS a slight bit of work to the braking system rather than relieving it of work.
Here's that old video again. Watch how fast engine rpms drop under braking vs. how fast they drop during an upshift (prior to letting the clutch out). The engine rpm just tends to float when the clutch is in. Not much braking force generated there.
[url]http://www.teamsmr.com/movies/Footbox%20Small.wmv[/url]
Gary
Sheehan Motor Racing
[url]www.teamSMR.com[/url][/QUOTE]
I'm not nearly as awesome as Gary, but I do exactly the same thing. Good to see it's also the right thing :)
Plus look at it this way, no matter what is slowing you down (outside of Gary, who is the ONLY mofo here who actually gets paid to do this for a living), brakes are a hell of a lot cheaper to replace than a motor.
Ryan
| waktasz | 08-23-2006 09:54 PM |
[QUOTE=REX8]There would be no real clutch wear with a proper rev matched downshift...I think hotrod is assuming the OP means downshifting to use engine braking adn heal-toeing...not the burn your clutch style (who even does this?) talked about above.
In any case, its very much more common to see someone row the gearbox down from high speeds than go from like 6th to 2nd....[/QUOTE]
You're right. Hotrod was referring to downshifting without rev matching and using the clutch to pull the engine up to the right RPMs...bad idea.
Or maybe he just can't write well enough to get his ideas out properly, because most of us understood his post the way I did.
In any case, its very much more common to see someone row the gearbox down from high speeds than go from like 6th to 2nd....[/QUOTE]
You're right. Hotrod was referring to downshifting without rev matching and using the clutch to pull the engine up to the right RPMs...bad idea.
Or maybe he just can't write well enough to get his ideas out properly, because most of us understood his post the way I did.
| seattle944t | 08-23-2006 10:57 PM |
[QUOTE=hotrod]Good we agree, if your slipping the clutch your an idiot and doing it wrong!!
The problem is you keep insisting that the driver is "slipping the clutch to slow the car down" --- that is not what I am talking about! Nor have I ever suggested you do that.[/QUOTE]
Well, at least we are getting closer - but if you are not rev-matching then you *are* slipping the clutch. Because of speed differential between the flywheel and the drive shaft you are generating friction (slip) and the clutch is absorbing all of that heat and wear. Either you do it fast by dumping the clutch, which puts all the force on it at once and completely upsets the car, or you do it slower to prevent lockup which creates even more friction, heat, and wear. In either case it is clutch slip that absorbs the speed differential.
[QUOTE=hotrod]
The point I'm trying to make is some of you are making absolute statements that it is always bad without any qualification. That is incorrect!!!
[/QUOTE]
The only absolute I said was bad is an inproper downshift, not rev matching and slipping the clutch instead. Maybe you dont see it that way, but in fact you are creating 4 to 6 times as much wear on the clutch by not rev matching.
The track I spend most of my time on involves 7 downshifts per lap. Putting 30 times more wear on the clutch, per lap, is going to burn it out fast.
[QUOTE=hotrod]
There is a place for it, it does not hurt the clutch, and you should understand how to do it and its limits.
[/QUOTE]
Again - we are not talking a 2000RPM downshift from 3rd to second - we are talking threshold braking and heel-toe downshifting in a motorsports form.
Do you need to rev match a 2000RPM 3rd to 2nd downshift? Hardly. So yes, not rev-matching does have its place and on the street in most circumstances not doing a rev match is fine.
[QUOTE=hotrod]
Years ago when brakes were not as good as today (ie drum brakes) it was an absolute necessitity to use it to keep your brakes alive for the duration of a race, or in the case of severe mountain driving if you wanted to not totally ruin your brakes by the time you descended a mountain pass.
Although disk brakes are much better than those days, they still have limits, and a good driver understands that he can extend those limits and or rest his brakes a bit by using compression braking so at a later time he can "out brake an opponet who has hotter brakes". <--- see maximum braking in motor sport situation.
[/QUOTE]
And up until that last sentance you were fine - you are doing more harm than good by not matching the revs on the downshifts by slipping the clutch to match the revs and if you are anywhere near your brake threshold the [b]only[/b] way you can do this is by a slow, prolonged release of the clutch or you will totally upset the car and lose traction.
[QUOTE=hotrod]
Just like hard braking, you don't do it while entering a corner (unless you like going off course) but on the straight just before you turn in. If you come off the throttle while still in gear the engine will start to slow you down. That uses up energy that otherwise would go into the brakes, so the brakes have less work to do overall, stay cooler and last longer.
[/QUOTE]
Here is the difference - brakes are designed to get hot. They are also designed to get fresh cooling air to dissipate that heat. Your clutch has neither of these design criteria, when it gets hot it *stays* hot, and if you keep adding heat it is going to fail or begin to slip on its own which adds further heat.
[QUOTE=seattle944t]
According to your idea of engine braking you are slipping your clutch all the way down the hill to get the 'most effective' use of engine braking, and that is just plain stupid.
[/quote]
[QUOTE=hotrod]
Never said that, never suggested that, never do that, I can't believe you think anyone would even try that ---- for the umpteenth time --- [B]you do not slip the clutch!!!!!!!![/b] you simply let it out and modulate engine rpm with the throttle so there is smooth engagement, then let off the throttle to develop compression braking.
[/quote]
Yes you did:
[QUOTE=hotrod]
If you rev match you are by definition not using engine braking and defeating the whole purpose of the process. The engine uses up a lot of energy when it spins up from a lower rpm.
[/Quote]
If you are using the throttle to modulate the RPM's then you are attempting to rev match.
The engine uses a lot more energy coming down from high RPM's in gear than when it does spinning up from a lower RPM. Why? In gear coming down from high RPM there is [b]no slip[/b] it is a direct connnection between the drive wheels and the motor. Bringing the motor up from lower RPM's much of the energy is wasted in friction (heat) in the clutch and not used for slowing the car down.
[QUOTE=hotrod]
Exactly right --- again I am not and have never recommended using the clutch as a brake, or "slipping it" --- you are using the engine compression. In fact I cannot imagine how you believe it is possible to "use your clutch for braking" the physical design of the the car simply does not allow it. You put it in a lower gear you let out the clutch, the clutch engages (with no significant slippage) locking transmission input shaft up to engine crank shaft and [b]then[/b] the engine begins to provide compressional braking by resisting acceleration by the drive train. Your definition of "clutch braking" is simply not sensible as the clutch never slips significantly (as anyone who has accidently over reved an engine by down shifting in to the wrong gear knows very well --- even in that extreme case, the clutch simply locks up and buzzes the crap out of the engine while slowing you down violently.) The clutch never has any load placed on it that it does not see every day at every stop light. The only difference is the direction of energy transfer. At the stop light, the engine accelerates the drive train, in compression braking the drive train attempts to accelerate the engine, but is resisted by the compression braking as the engine does negative work trying to move air.
Even the quote you posted from wikipedia agrees with what I have been saying!
Larry[/QUOTE]
What you descibe here and above involves clutch slip as engine braking. And as I have been trying to point out - using the clutch to match revs between the flywheel and driveshaft is using the friction of the clutch to slow the car down. Nothing is free - the energy to dissapate the differential in speed needs to come from somewhere. Doing what you descibe means the energy comes from friction of the clutch plate. This is using the friction surface of the clutch as a brake. This does in fact create more wear than the clutch sees in day to day driving.
But again - street driving is completely different than on the track. When you don't have a big RPM difference between gears and are downshifting it is fine to not match RPMS.
But letting the motor drop to idle and then using the clutch to bring the RPM's back up is a bad thing to do on a regular basis when there is a large RPM differetial.
Cheers :D
The problem is you keep insisting that the driver is "slipping the clutch to slow the car down" --- that is not what I am talking about! Nor have I ever suggested you do that.[/QUOTE]
Well, at least we are getting closer - but if you are not rev-matching then you *are* slipping the clutch. Because of speed differential between the flywheel and the drive shaft you are generating friction (slip) and the clutch is absorbing all of that heat and wear. Either you do it fast by dumping the clutch, which puts all the force on it at once and completely upsets the car, or you do it slower to prevent lockup which creates even more friction, heat, and wear. In either case it is clutch slip that absorbs the speed differential.
[QUOTE=hotrod]
The point I'm trying to make is some of you are making absolute statements that it is always bad without any qualification. That is incorrect!!!
[/QUOTE]
The only absolute I said was bad is an inproper downshift, not rev matching and slipping the clutch instead. Maybe you dont see it that way, but in fact you are creating 4 to 6 times as much wear on the clutch by not rev matching.
The track I spend most of my time on involves 7 downshifts per lap. Putting 30 times more wear on the clutch, per lap, is going to burn it out fast.
[QUOTE=hotrod]
There is a place for it, it does not hurt the clutch, and you should understand how to do it and its limits.
[/QUOTE]
Again - we are not talking a 2000RPM downshift from 3rd to second - we are talking threshold braking and heel-toe downshifting in a motorsports form.
Do you need to rev match a 2000RPM 3rd to 2nd downshift? Hardly. So yes, not rev-matching does have its place and on the street in most circumstances not doing a rev match is fine.
[QUOTE=hotrod]
Years ago when brakes were not as good as today (ie drum brakes) it was an absolute necessitity to use it to keep your brakes alive for the duration of a race, or in the case of severe mountain driving if you wanted to not totally ruin your brakes by the time you descended a mountain pass.
Although disk brakes are much better than those days, they still have limits, and a good driver understands that he can extend those limits and or rest his brakes a bit by using compression braking so at a later time he can "out brake an opponet who has hotter brakes". <--- see maximum braking in motor sport situation.
[/QUOTE]
And up until that last sentance you were fine - you are doing more harm than good by not matching the revs on the downshifts by slipping the clutch to match the revs and if you are anywhere near your brake threshold the [b]only[/b] way you can do this is by a slow, prolonged release of the clutch or you will totally upset the car and lose traction.
[QUOTE=hotrod]
Just like hard braking, you don't do it while entering a corner (unless you like going off course) but on the straight just before you turn in. If you come off the throttle while still in gear the engine will start to slow you down. That uses up energy that otherwise would go into the brakes, so the brakes have less work to do overall, stay cooler and last longer.
[/QUOTE]
Here is the difference - brakes are designed to get hot. They are also designed to get fresh cooling air to dissipate that heat. Your clutch has neither of these design criteria, when it gets hot it *stays* hot, and if you keep adding heat it is going to fail or begin to slip on its own which adds further heat.
[QUOTE=seattle944t]
According to your idea of engine braking you are slipping your clutch all the way down the hill to get the 'most effective' use of engine braking, and that is just plain stupid.
[/quote]
[QUOTE=hotrod]
Never said that, never suggested that, never do that, I can't believe you think anyone would even try that ---- for the umpteenth time --- [B]you do not slip the clutch!!!!!!!![/b] you simply let it out and modulate engine rpm with the throttle so there is smooth engagement, then let off the throttle to develop compression braking.
[/quote]
Yes you did:
[QUOTE=hotrod]
If you rev match you are by definition not using engine braking and defeating the whole purpose of the process. The engine uses up a lot of energy when it spins up from a lower rpm.
[/Quote]
If you are using the throttle to modulate the RPM's then you are attempting to rev match.
The engine uses a lot more energy coming down from high RPM's in gear than when it does spinning up from a lower RPM. Why? In gear coming down from high RPM there is [b]no slip[/b] it is a direct connnection between the drive wheels and the motor. Bringing the motor up from lower RPM's much of the energy is wasted in friction (heat) in the clutch and not used for slowing the car down.
[QUOTE=hotrod]
Exactly right --- again I am not and have never recommended using the clutch as a brake, or "slipping it" --- you are using the engine compression. In fact I cannot imagine how you believe it is possible to "use your clutch for braking" the physical design of the the car simply does not allow it. You put it in a lower gear you let out the clutch, the clutch engages (with no significant slippage) locking transmission input shaft up to engine crank shaft and [b]then[/b] the engine begins to provide compressional braking by resisting acceleration by the drive train. Your definition of "clutch braking" is simply not sensible as the clutch never slips significantly (as anyone who has accidently over reved an engine by down shifting in to the wrong gear knows very well --- even in that extreme case, the clutch simply locks up and buzzes the crap out of the engine while slowing you down violently.) The clutch never has any load placed on it that it does not see every day at every stop light. The only difference is the direction of energy transfer. At the stop light, the engine accelerates the drive train, in compression braking the drive train attempts to accelerate the engine, but is resisted by the compression braking as the engine does negative work trying to move air.
Even the quote you posted from wikipedia agrees with what I have been saying!
Larry[/QUOTE]
What you descibe here and above involves clutch slip as engine braking. And as I have been trying to point out - using the clutch to match revs between the flywheel and driveshaft is using the friction of the clutch to slow the car down. Nothing is free - the energy to dissapate the differential in speed needs to come from somewhere. Doing what you descibe means the energy comes from friction of the clutch plate. This is using the friction surface of the clutch as a brake. This does in fact create more wear than the clutch sees in day to day driving.
But again - street driving is completely different than on the track. When you don't have a big RPM difference between gears and are downshifting it is fine to not match RPMS.
But letting the motor drop to idle and then using the clutch to bring the RPM's back up is a bad thing to do on a regular basis when there is a large RPM differetial.
Cheers :D
| hotrod | 08-23-2006 10:58 PM |
[quote]You're right. Hotrod was referring to downshifting without rev matching and using the clutch to pull the engine up to the right RPMs...bad idea.[/quote]
My first post ---
[quote]Down shifting and using engine compression for braking has very little impact on the clutch. First it has maybe 10x the friction material surface area as your brake pads do, and it [b][i]spends very little time slipping,[/i][/b] the vast majority of the energy dissipated in engine braking is not dissapated in the clutch material as [b][i]it quickly locks up with the fly wheel,[/b][/i] the energy is dissipated in the work done trying to suck air through a closed throttle plate, and compression on the air going through the engine.[/quote]
Just where in there does it say anything about intentionally slipping the clutch ?
In fact I'm pointing out that almost no clutch slippage takes place during a normal down shift --- exactly the opposite of what some of you keep insisting is happening. It was in direct reponse to seattle944t's comment that actually suggested clutch slipping by letting out the clutch slowly was a problem.
[quote="seattle944t"]Also - easing out the clutch to bring the RPM's up is not engine braking - that is using the clutch as a brake. Engine braking is being in gear w/o throttle where the compression of the engine is acting to slow the car down.[/quote]
[quote="seattle944t"] If you are going to downshift like this you need to rev match. Otherwise you are burning the clutch and upsetting the car's balance.[/quote]
I was pointing out that that is not the way to do it, there is very little if any clutch slippage when you aggressively down shift, and are trying to slow the car with engine braking, and it is not a significant wear factor on the clutch. Flywheel and clutch disk speeds equalize almost instantly as you release the clutch.
I did mention in my second post that if you fully rev matched your down shift, you would have little or no engine compression when you let out the clutch and thus by definition would not have any significant engine braking. [b]But[/b] if there is a slight miss match in engine rpm vs road speed you will get instant compression braking as you instantly bring manifold vacuum up to maximum levels as the clutch engages and the engine speed equalizes with the drive train speed..
[quote="hotrod"]If you rev match you are by definition not using engine braking and defeating the whole purpose of the process. The engine uses up a lot of energy when it spins up from a lower rpm.[/quote]
Note I did not suggest anything stupid like bringing the engine rpm up from idle speed to 6000 grand that is an assumption you guys keep sticking in there for no reason.
The clutch slippage you guys are fixated on is trivial and absolutly identical to what you get every time you leave a stop light --- it is not a significant wear issue with the clutch, it is not abuse of the clutch, it does not cook the clutch --- it is normal wear and tear.
The spinning up of the engine I mention happens in every down shift, it is only a matter of degree and timing determined by how bright the driver is. If you are creating such a miss match that you are unsettling the car, or have to extensively slip the clutch to keep from tossing yourself through the windshied you are a moron, and deserve to kiss a tree on turn exit. If you rev match just enough to get the thing in the lower gear and back off the throttle as you release the clutch, you will and must spin up the engine as the drive train is trying to spin the engine faster than it wants to go with the throttle closed. I think some of you are making an unwarranted assumption that I am suggesting you buzz the crap out of the engine as you let the clutch out.
Not the case --- never said it, don't want to do it. I am simply saying that the energy of forward motion is being fed into the engine by the drive train and attempting to spin it faster than it would go if you were out of gear or had the clutch in. That stores energy in the rotating mass of the engine (also keeps engine rpm up where when you go back on throttle its ready to make power), and you are dissipating energy pumping air and throwing oil around inside the engine and gear box.
Quit thinking in terms of absolutes (which is what my original post was about). Every driving technique can be over done and become abusive, but to say any driving technique is always bad displays a lack of understanding of vehicle dynamics and the creativity and judgment a driver must use to push the limits.
There is a time and a place for every technique --- even intentionally spinning out has its place.
Your clutch is no where near as fragile as some of you seem to think.
If you are off throttle in gear and on the brakes you have more braking power than you would if you were only on the brakes (assuming you have enough traction to use it all).
If you are off throttle in gear and on the brakes but must ease off the brakes to avoid wheel lock, you still have the same braking power you would have had with brakes only but you are putting less heat into your brake pads and disks for the same rate of deceleration, that by definition gives you more reserve and less stress on the brakes.
One of the beauties of AWD is that compressional braking is shared by all the wheels so it is no where near as upsetting to the vehicle as it would be on a 2wd.
Larry
My first post ---
[quote]Down shifting and using engine compression for braking has very little impact on the clutch. First it has maybe 10x the friction material surface area as your brake pads do, and it [b][i]spends very little time slipping,[/i][/b] the vast majority of the energy dissipated in engine braking is not dissapated in the clutch material as [b][i]it quickly locks up with the fly wheel,[/b][/i] the energy is dissipated in the work done trying to suck air through a closed throttle plate, and compression on the air going through the engine.[/quote]
Just where in there does it say anything about intentionally slipping the clutch ?
In fact I'm pointing out that almost no clutch slippage takes place during a normal down shift --- exactly the opposite of what some of you keep insisting is happening. It was in direct reponse to seattle944t's comment that actually suggested clutch slipping by letting out the clutch slowly was a problem.
[quote="seattle944t"]Also - easing out the clutch to bring the RPM's up is not engine braking - that is using the clutch as a brake. Engine braking is being in gear w/o throttle where the compression of the engine is acting to slow the car down.[/quote]
[quote="seattle944t"] If you are going to downshift like this you need to rev match. Otherwise you are burning the clutch and upsetting the car's balance.[/quote]
I was pointing out that that is not the way to do it, there is very little if any clutch slippage when you aggressively down shift, and are trying to slow the car with engine braking, and it is not a significant wear factor on the clutch. Flywheel and clutch disk speeds equalize almost instantly as you release the clutch.
I did mention in my second post that if you fully rev matched your down shift, you would have little or no engine compression when you let out the clutch and thus by definition would not have any significant engine braking. [b]But[/b] if there is a slight miss match in engine rpm vs road speed you will get instant compression braking as you instantly bring manifold vacuum up to maximum levels as the clutch engages and the engine speed equalizes with the drive train speed..
[quote="hotrod"]If you rev match you are by definition not using engine braking and defeating the whole purpose of the process. The engine uses up a lot of energy when it spins up from a lower rpm.[/quote]
Note I did not suggest anything stupid like bringing the engine rpm up from idle speed to 6000 grand that is an assumption you guys keep sticking in there for no reason.
The clutch slippage you guys are fixated on is trivial and absolutly identical to what you get every time you leave a stop light --- it is not a significant wear issue with the clutch, it is not abuse of the clutch, it does not cook the clutch --- it is normal wear and tear.
The spinning up of the engine I mention happens in every down shift, it is only a matter of degree and timing determined by how bright the driver is. If you are creating such a miss match that you are unsettling the car, or have to extensively slip the clutch to keep from tossing yourself through the windshied you are a moron, and deserve to kiss a tree on turn exit. If you rev match just enough to get the thing in the lower gear and back off the throttle as you release the clutch, you will and must spin up the engine as the drive train is trying to spin the engine faster than it wants to go with the throttle closed. I think some of you are making an unwarranted assumption that I am suggesting you buzz the crap out of the engine as you let the clutch out.
Not the case --- never said it, don't want to do it. I am simply saying that the energy of forward motion is being fed into the engine by the drive train and attempting to spin it faster than it would go if you were out of gear or had the clutch in. That stores energy in the rotating mass of the engine (also keeps engine rpm up where when you go back on throttle its ready to make power), and you are dissipating energy pumping air and throwing oil around inside the engine and gear box.
Quit thinking in terms of absolutes (which is what my original post was about). Every driving technique can be over done and become abusive, but to say any driving technique is always bad displays a lack of understanding of vehicle dynamics and the creativity and judgment a driver must use to push the limits.
There is a time and a place for every technique --- even intentionally spinning out has its place.
Your clutch is no where near as fragile as some of you seem to think.
If you are off throttle in gear and on the brakes you have more braking power than you would if you were only on the brakes (assuming you have enough traction to use it all).
If you are off throttle in gear and on the brakes but must ease off the brakes to avoid wheel lock, you still have the same braking power you would have had with brakes only but you are putting less heat into your brake pads and disks for the same rate of deceleration, that by definition gives you more reserve and less stress on the brakes.
One of the beauties of AWD is that compressional braking is shared by all the wheels so it is no where near as upsetting to the vehicle as it would be on a 2wd.
Larry
| kellygnsd | 08-23-2006 11:46 PM |
[QUOTE=REX8]No, not just for sequential gearboxes...at all. You should watch some more in car videos or some more ride alongs.
How is it any slower downshifting? Your foot never leaves the brake pedal. Theoretically, you achive the same maximum braking, while having you brakes do less work. That can be a BIG deal on a lot of heavier cars.
Again, how is this any slower? You are constantly braking....just adding in the engine braking as well.
5th or 6th to 2nd without at least 3rd gear in there? :confused:[/QUOTE]
In no way I'm I a proffesional driver nor do I claim to be but I've found that for me, this is the fastest way. I do sometimes walk the gears down through the shift pattern but while holding in the clutch because I find it easier to find the gear sometimes. I guess I can agree with you that if you have your heel toe-ing down it may not be much slower but again for me, I'm no pro and I have enough to concentrate on with braking points, turn in points, racing line, and other stupid fools on the track that I find that rev matching once as opposed to 3 times on a 5th to 2nd gear transition is faster and smoother for me. Does that mean you have to do it that way... NO. I'm just saying it works for me. It works and it gets the job done, what's wrong with that?
How is it any slower downshifting? Your foot never leaves the brake pedal. Theoretically, you achive the same maximum braking, while having you brakes do less work. That can be a BIG deal on a lot of heavier cars.
Again, how is this any slower? You are constantly braking....just adding in the engine braking as well.
5th or 6th to 2nd without at least 3rd gear in there? :confused:[/QUOTE]
In no way I'm I a proffesional driver nor do I claim to be but I've found that for me, this is the fastest way. I do sometimes walk the gears down through the shift pattern but while holding in the clutch because I find it easier to find the gear sometimes. I guess I can agree with you that if you have your heel toe-ing down it may not be much slower but again for me, I'm no pro and I have enough to concentrate on with braking points, turn in points, racing line, and other stupid fools on the track that I find that rev matching once as opposed to 3 times on a 5th to 2nd gear transition is faster and smoother for me. Does that mean you have to do it that way... NO. I'm just saying it works for me. It works and it gets the job done, what's wrong with that?
| kellygnsd | 08-23-2006 11:49 PM |
Gary, are you running a dogbox in the race car? Is rowing through the gears on the way down a neccesity with the dogbox along with double clutching or is it easy just to toss it from 5th to 2nd? I ask because I've never driven one but have heard they can be hard to work with for someone like me who knows nothing. Thanks.
| waktasz | 08-24-2006 12:33 AM |
He said he does it to keep his rythym (omg 1 am cant spell) I do the same thing but sometimes I dont let the clutch out in every gear...its easier to get the revs right perfectly gear to gear then it is going 5 to 2.
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