Thứ Bảy, 4 tháng 3, 2017

Auto-X with pad knockback? part 1

Capt Crunch 04-02-2004 10:47 PM

Auto-X with pad knockback?
I have a stoptech BBK and I'm getting pad knockback like crazy. I don't have enough money to replace the bearings, is there anything I can do to prevent the pedal going to the floor after the slolam?

Thanks
angryfist 04-03-2004 06:22 AM

lightly tap teh brakes real quick with your left foot before heavy braking.
Storm 04-03-2004 09:52 AM

I'm having a hard time figuring out what the help you're talking about.....WTH is brake knockback. First I've heard of this term....

You have a BBK, ok, got that. No money for what bearings? brake bearings?....seriously.....what bearings have changed or gone bad because of the BBK?

Pedal to the floor after a slalom....Why? Heat? Capacity probs due to stock M/C and BBK? Are you driving thru the slalom in a "stomp, jerk, stomp" method?

I don't have any problems with stock stuff, so I'm just trying to learn something myself if I can't help you.

Jay Storm
[url]www.sourcemotorsports.com[/url]
D_REX 04-03-2004 10:23 AM

He's talking about his wheel bearings. When they go bad they allow the rotor to move relative to the caliper and push the pads in so that they have a lot more distance to travel before real braking.

The short and simple solution is to stop auocrossing your car. There is a reason that wheel bearings are one of the things they are supposed to check in tech. Fork out the cash and fix your problem or stop competing until you do.
Strepto 04-03-2004 10:48 AM

I know it's not what you want to hear, but I have to agree with D_REX. You shouldn't be autoXing with bad wheel bearings. That's a serious safety issue.
Capt Crunch 04-03-2004 11:59 AM

[QUOTE][i]Originally posted by D_REX[/i]
[B] He's talking about his wheel bearings. When they go bad they allow the rotor to move relative to the caliper and push the pads in so that they have a lot more distance to travel before real braking.

The short and simple solution is to stop auocrossing your car. There is a reason that wheel bearings are one of the things they are supposed to check in tech. Fork out the cash and fix your problem or stop competing until you do. [/B][/QUOTE]

I've autocrossed once. Apparently this is a widespread problem with WRXs. Stoptech described the bearings as "inferior" and they are not on their way out, they are just not built with enough precision to handle fixed caliper systems. They swapped the brake kit from an old WRX that was having problems to a new one that wasn't and the problem stayed with the old WRX.

I've gotten in the habit of left foot braking, hopefully I'm dexterous enough to do well.
Cosworth 04-03-2004 12:16 PM

Would a floating caliper remedy this then since the ST BBK is a fixed caliper whereas oem is a slider?
D_REX 04-03-2004 03:02 PM

[QUOTE][i]Originally posted by Capt Crunch [/i]
[B]I've autocrossed once. Apparently this is a widespread problem with WRXs. Stoptech described the bearings as "inferior" and they are not on their way out, they are just not built with enough precision to handle fixed caliper systems. They swapped the brake kit from an old WRX that was having problems to a new one that wasn't and the problem stayed with the old WRX.

I've gotten in the habit of left foot braking, hopefully I'm dexterous enough to do well. [/B][/QUOTE]
The WRX might not have the best wheel bearings ever but they are far better than you describe. I run a fixed 6 pot caliper on the front of my car with rotors the same size as the stoptech kit. I've never experienced any pad knockback in auoX use and I've autocrossed a good bit.

What you are describing is very bad. It's not just regular operation. The time between brake applications is very short in autox, if you have to actually pump the pedal to get braking you have a problem. Something is not operating cprrectly. Please fix it before you hurt yourself or somebody else.

I don't mean to be harsh, I just want to make sure that you know this isn't normal. Some pad knockback "might" be normal with the stoptech kits, personally I think it's at least partially due to the floating rotor design, but you should never have to actually pump the brakes to get them back.
Capt Crunch 04-03-2004 05:18 PM

I withdrew from the auto-x. I don't have the money to replace the bearings, Subaru resuses to cover it under warentee, and I don't have the money to even replace the god damn brakes with the stock units.

I'm stuck. I have a billion frustrations right now and this is just another pleasantry.
Capt Crunch 04-04-2004 11:55 AM

Am I right in assuming that the BBK could have no impact on the condition of the bearings? I tracked the car once, and I did quite well (not stupid with the brakes). If I can prove the the brakes would not affect the bearings then it should be covered under warentee. Do I have a shot at this?
HoRo1 04-04-2004 12:18 PM

If anything StopTechs will slow bearing wear. The rotor assembly has more mass than stock, it has better cooling than stock. These factors should limit heat flow into the hubs and let the wheel bearings run cooler and last longer.
Alleggerita 04-04-2004 01:20 PM

Having had pad knock pad with bearings on their way out I've certainly never had my pedal go to the floor as well as with rotors that didn't run true. In neither case did I ever have apedal go anywhere near the floor. As someone suggested gently tapping the brake with you foor prior to breaking ceratinly works on the long straights on a raoad course ... I have my doubts about slalom or auto X - there is just too much going on.

Anyway, if you pedal goes to the floor it's more likely than not that you have air in your system - maybe introduced when you installed the new brakes. Try to bleed. If there is still no improvment try a vacuum bleeder that often helps to dislodge reluctant air bubbles. And do put in new wheel bearings ...
angryfist 04-04-2004 02:26 PM

i have the stoptech bbk and also get some knockback. not to the point i have to pump the brakes but it engages really low after mulple tirns without braking. if you have to pump the braeks thats defilitly bad. but low and random engagement of brakes unfortunatly is common and there isnt really much you can do about it.

as for the wheel bearings, they should not be worse after installing the bbk. the stoptech will help the wheel bearing life becasue of less heat in the system hwile braking. gary sheehan said that his wrx's bearings would go bad after each race but after he put the stoptechs on they would be good for multiple races.
Capt Crunch 04-04-2004 06:29 PM

So I have a case for the bearings. Also when I say the pedal goes to the floor, I was exagerating, it just goes down really far as if I had massive brake fade. The things still brake, up to the point of ABS going off, but it makes it impossible to heel-toe and I have almost no feel. I'll call the dealer again.
jmott 04-04-2004 06:40 PM

the pedal going to the floor could just be due to your fluid being cooked or your pads being tapered from the track day.



[QUOTE][i]Originally posted by Capt Crunch [/i]
[B]So I have a case for the bearings. Also when I say the pedal goes to the floor, I was exagerating, it just goes down really far as if I had massive brake fade. The things still brake, up to the point of ABS going off, but it makes it impossible to heel-toe and I have almost no feel. I'll call the dealer again. [/B][/QUOTE]
bullseye17 04-06-2004 01:53 PM

bearing is cooked, I have / had the same problem.
ITWRX4ME 04-07-2004 10:26 AM

It's funny what you get used to. My bearings aren't bad (that I know of) but I get pad knock back too. I have the stock calipers and OEM slotted rotors. I just got in to the habit of tapping the brake once or twice before I start my braking. No big deal.
D_REX 04-07-2004 10:38 AM

[QUOTE][i]Originally posted by ITWRX4ME [/i]
[B]It's funny what you get used to. My bearings aren't bad (that I know of) but I get pad knock back too. I have the stock calipers and OEM slotted rotors. I just got in to the habit of tapping the brake once or twice before I start my braking. No big deal. [/B][/QUOTE]

This really sounds like you are just driving your car, which you know has a real problem, and telling yourself that all is fine.

Have you checked your wheel bearings? I'll say it again, this is not normal, something is wrong. It could be something other than wheel bearings, but something is not right. Stoptech claims that the stock wheel bearings are crummy, this causes some knockback in their systems partly because the rotor diameter is larger and the issue is magnified. You are on stock sized rotors so your condition is worse than those with a BBK and the same issue.

Later,
Dustin
ITWRX4ME 04-07-2004 10:52 AM

[QUOTE][i]Originally posted by D_REX [/i]
[B]This really sounds like you are just driving your car, which you know has a real problem, and telling yourself that all is fine.

Have you checked your wheel bearings? [/B][/QUOTE]
Yes, the only way I know how. I grab the top and bottom of the tire and try to twist it laterally as hard as I can. I grab the sides tire and try to twist it laterally as hard as I can. No movement whatsoever. Supposedly, this is the way to test it. It is more intense that the way tech inspectors at events test it and it has always passed.
So, I'm not just driving a car that has a problem. Pad knock back isn't unusual. In fact, it's relatively common.

[QUOTE][i]Originally posted by D_REX [/i]
[B] Stoptech claims that the stock wheel bearings are crummy, this causes some knockback in their systems partly because the rotor diameter is larger and the issue is magnified. You are on stock sized rotors so your condition is worse than those with a BBK and the same issue.

Later,
Dustin [/B][/QUOTE]
Don't know if their claims are true. But, the bigger rotor, has a longer radius and therefore more deflection, given the same bearings. Therefore more knock back.
D_REX 04-07-2004 11:33 AM

[QUOTE][i]Originally posted by ITWRX4ME [/i]
[B]Yes, the only way I know how. I grab the top and bottom of the tire and try to twist it laterally as hard as I can. I grab the sides tire and try to twist it laterally as hard as I can. No movement whatsoever. Supposedly, this is the way to test it. It is more intense that the way tech inspectors at events test it and it has always passed.
So, I'm not just driving a car that has a problem. Pad knock back isn't unusual. In fact, it's relatively common.[/quote]
If you mean that it's relatively common in hard track driving on super sticky race rubber, then I agree. If you get this driving to work or even autocrossing on street tires then I disagree. It's all about context and I neglected to ask yours, sorry.
elgorey 04-07-2004 01:13 PM

I have never bought the "wiggle wheel" method of testing wheel bearings.
Great way to test tie-rod ends and balljoints, but Im not so sure its effective at diagnosing a wheel bearing problem, especially with the car on the ground. JMO

FWIW, my car has seen many many days on tack and autoX and has never had a knockback or wheel bearing problem. YMMV.


Evan
NotFast 04-07-2004 01:28 PM

Geez, why don't you guys go right to the source to find out what pad knockbad is:

[URL=http://www.stoptech.com/whitepapers/knockback.htm]Stoptech article on knockback[/URL]

Unless you've got a race car, there's a chance you will experience knockback on the track at some point.
D_REX 04-07-2004 01:38 PM

[QUOTE][i]Originally posted by NotFast [/i]
[B]Geez, why don't you guys go right to the source to find out what pad knockbad is:

[URL=http://www.stoptech.com/whitepapers/knockback.htm]Stoptech article on knockback[/URL]

Unless you've got a race car, there's a chance you will experience knockback on the track at some point. [/B][/QUOTE]

How is this ANY different than what I've been saying here?
GarySheehan 04-07-2004 02:08 PM

Knockback is very common in the WRX. It is not necessarily indicitive of a bearing failure. Some WRXs have quite a bit of knockback, even after having all wheel bearings replaced, and it can be felt in normal street driving. Other WRXs are less suseptible and really need to be leaned on hard to feel knockback.

The only reason I can come up with regarding differences between chassis is some hub carriers must be more flexible than others.

Gary
Sheehan Motor Racing
[url]www.teamSMR.com[/url]
Capt Crunch 04-07-2004 06:55 PM

[QUOTE][i]Originally posted by GarySheehan[/i]
[B] Knockback is very common in the WRX. It is not necessarily indicitive of a bearing failure. Some WRXs have quite a bit of knockback, even after having all wheel bearings replaced, and it can be felt in normal street driving. Other WRXs are less suseptible and really need to be leaned on hard to feel knockback.

The only reason I can come up with regarding differences between chassis is some hub carriers must be more flexible than others.

Gary
Sheehan Motor Racing
[url]www.teamSMR.com[/url] [/B][/QUOTE]

Exactly what Stoptech told me. Basically I'm SOL.
Bob Lee 04-08-2004 06:26 PM

Knockback is common to many different platforms and is a well known industry phenomena. Knockback has been observed on Stock WRX systems driven at the limit. We swapped the StopTech brakes from a low milage WRX with no knockback to a higher milage WRX with Knockback and the knockback followed the platform. We knew it would but we did the work to show the owner it was platform dependent.

Knockback is not a StopTech phenomena and certainly not a result of floating rotors. In fact some racers believe that high axle float prevents Knockback. We have not been able to prove this claim.

Knockback is less with floating calipers and this is why a WRX with stock calipers may not exhibit knockback, and then later exhibit the phenomena with a fixed caliper upgrade.

Knockback is directly proportional to the rotor radius. We know many showroom stock racers that see knock back with stock calipers and have learned to tap the brakes before the next braking zone.

I used to race a McLaren M-20 team CanAm car with AP brakes and rotors and had to install antiknock back springs to lessen the knockback. AP has had Knockback springs available for their calipers for years. This was a full on purpose built race car and not a production street car. This is a phenomena that is part of your challenge to race a production car.

Bob Lee
StopTech LLC
Capt Crunch 04-08-2004 07:31 PM

I remember talking with you Bob, you were very helpful. I don't mean to imply that the problem is inherent in your brakes.

There are so many small annoyances with my car that I am thinking of selling it, and building up a cheaper car and getting everything right the first time. If StopTech would be so kind as to make a rear BBK I would like that very much. :)
teamscr 04-09-2004 10:47 PM

WRX knockback comments
Hi all. Thought I would chime in after lurking for a bit. In the knockback article I wrote (and that was nicely linked in this thread) my example case was my old Cobra, but I have been tracking my WRX wagon for the past 2 years and have a few comments on its knockback issues...

I instruct at a few high-speed schools per year, and when I first bought my WRX I ran Watkins Glen with stock brakes (but uprated pads and fluid) and 17" Potenza S03's (not a race tire, but better than stock). Even though the car was relatively new, the knockback was so pronounced that I was having to tap the pedal both going into the bus stop and into the toe of the boot. In most other braking zones the pedal was OK, but in those two turns it really felt low.

The bearings checked out no worse than what the dealer expected, so they stayed on the car. However, even around town there was a noticeable difference in pedal height if I simply hit the brakes once for a stop or if I gave them a light tap and then applied them more firmly when coming to a stop. Note I am not talking about racing around town here - this is all normal around-town driving where the difference could be felt.

I have since been in a few other WRXs around town and they all pretty much do the same thing to varying degrees. In fact, I just wrote an article for Car&Driver's "Boost" magazine in which I was evaluating the Praxis System on a WRX and sure enough their car displayed it as well. All on stock brakes.

Now, after installing my StopTech BBK on the WRX last year I returned to the Glen, and sure enough the knockback was worse. Of course, I had 3 other track days and 30,000 miles more on the bearings at that point, but the larger diameter rotors do increase the amount of lateral deflection which causes the knockback issue. Again, tapping the brakes was the cure, but no fault of the brakes themselves.

I also consult for a few professional race teams, and it is surprising how many front-running cars have this same issue. Of course nobody talks about it for fear of exposing a weakness, but it's out there all around. Nobody is immune.

Now, the amount of knockback is due to a number of different factors, but everything else equal the faster you are through the corners, the more issues you will see. More lateral force = more lateral deflection. For this reason, some people will claim to never have a problem, but driving style plays a role.

That pretty much exhausts my comments. Thanks for reading.

james walker, jr.
[email][email�protected][/email]
[url]http://www.teamscR.com[/url]

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