| goto_racing | 05-23-2005 04:29 PM |
Broken front control arm. Seen it?
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There will be a full press release by tommorow, but I wanted to see if any of you have ever seen something like this, especially you rally guys. It was disapointing for us, especially after battling back from tons heat related problems, like melted intercooler hoses, and combing the phoenix area for the seemingly non-existent imprezza fuel pump...
but anyway, after getting the car back to running great and taking the 3rd spot on the grid, Brian goes to take the standing start and POP!, the control arm breaks at just near the ball joint. Even with this damage he goes under the OPak/Spoon car and makes it stick for the lead. Kid is crazy...
This is the stock WRX control arm BTW, snapped about half way through, then slowly tore off over the next few laps. Anyone ever experienced this?
Chris Lock
but anyway, after getting the car back to running great and taking the 3rd spot on the grid, Brian goes to take the standing start and POP!, the control arm breaks at just near the ball joint. Even with this damage he goes under the OPak/Spoon car and makes it stick for the lead. Kid is crazy...
This is the stock WRX control arm BTW, snapped about half way through, then slowly tore off over the next few laps. Anyone ever experienced this?
Chris Lock
| Storm | 05-23-2005 06:38 PM |
Could be from the lower balljoint binding up if you have a really low ride height. I've seen where the balljoint will only allow so much upward movement of the control arm before binding up and stressing the arm and hub assembly. Do you have marks on the inside of the rotors from something hitting it? That would be another indicator of serious flex at the end of the control arms.
Jay Storm
Jay Storm
| [email�protected] | 05-23-2005 07:04 PM |
[QUOTE=Storm]Could be from the lower balljoint binding up if you have a really low ride height.
What he said!!!
What he said!!!
| dentsport | 05-23-2005 07:09 PM |
You need some Titanium ones like these :)
Can you run the cast aluminum STi arms? What series is this in?
[IMG]http://www.werksrally.com/a_arms.jpg[/IMG]
Can you run the cast aluminum STi arms? What series is this in?
[IMG]http://www.werksrally.com/a_arms.jpg[/IMG]
| goto_racing | 05-23-2005 07:32 PM |
The series is USTCC. Thanks for the input. We run a pretty conservative ride height because we are aware of geometry issues, but this seems like a good lead to investigate with.
The rules stick us with stock control arms, so changing them out is not an option. We also can't change geometry, which is why we have a pretty high ride height for a road-racer.
How much articulation would you have to see to bind the balljoints? Extreme? This is very troubling because we have been running the car this way for months without a problem, and it was inspected prior to this race. I would like to track down the cause of this damage before we send our driver back out there. We were lucky it failed on the infield and not the oval(it was the right front, of course).
These wheels are 17s. Notice the fender gap. We are not that low... or are we?
[IMG]http://www.gotoracing.com/images/wrxrim.jpg[/IMG]
The rules stick us with stock control arms, so changing them out is not an option. We also can't change geometry, which is why we have a pretty high ride height for a road-racer.
How much articulation would you have to see to bind the balljoints? Extreme? This is very troubling because we have been running the car this way for months without a problem, and it was inspected prior to this race. I would like to track down the cause of this damage before we send our driver back out there. We were lucky it failed on the infield and not the oval(it was the right front, of course).
These wheels are 17s. Notice the fender gap. We are not that low... or are we?
[IMG]http://www.gotoracing.com/images/wrxrim.jpg[/IMG]
| AlexP | 05-23-2005 07:32 PM |
YIKES! Those fabbed control arm are terrible! That gusset at the knuckle joint is in the middle of the tube, that is very bad! Somebody needs to rethink that design... that inner box gusset isn't too hot, either.
| AlexP | 05-23-2005 07:35 PM |
[QUOTE=goto_racing]The series is USTCC. Thanks for the input. We run a pretty conservative ride height because we are aware of geometry issues, but this seems like a good lead to investigate with. [/QUOTE]
Can you take VERY detailed pictures of the control arm, the inboard mounting locations, and the knuckle? You need to look for binding evidence at the source. Also, detailed pictures of the crack can help determine what happened (if it was an impact rupture, manufacturing defect, just plain old fatigue, etc.)
I'm very interested in seeing pics, if you can.
Can you take VERY detailed pictures of the control arm, the inboard mounting locations, and the knuckle? You need to look for binding evidence at the source. Also, detailed pictures of the crack can help determine what happened (if it was an impact rupture, manufacturing defect, just plain old fatigue, etc.)
I'm very interested in seeing pics, if you can.
| goto_racing | 05-23-2005 08:05 PM |
[QUOTE=AlexP]Can you take VERY detailed pictures of the control arm, the inboard mounting locations, and the knuckle? You need to look for binding evidence at the source. Also, detailed pictures of the crack can help determine what happened (if it was an impact rupture, manufacturing defect, just plain old fatigue, etc.)
I'm very interested in seeing pics, if you can.[/QUOTE]
Upon first inspection, it looked like it tore from the rear forward. Which also went along with what we observed which was a wheel that was increasingly pointed left on right turns. It slowly got worse over two laps before it let go. The intial event was on the standing start, when the driver could here and feel a loud noise, but it hadn't fully failed yet. It seemed like wear. To get the car back in the trailer, we welded the control arm back together, destroying, in retrospect, some very useful forensic evidence. But by this point, most of the crew had heat exhaustion from trying to lug the damn thing into the trailer in 115 heat. So we said "fug it! weld the SOB!" :)
I haven't seen it myself since then, but apparently there are some witness marks still left on the leading edge. When I go to the shop tonight I will try and take some pictures of what is left, and also the uprights and rotor.
Chris Lock
I'm very interested in seeing pics, if you can.[/QUOTE]
Upon first inspection, it looked like it tore from the rear forward. Which also went along with what we observed which was a wheel that was increasingly pointed left on right turns. It slowly got worse over two laps before it let go. The intial event was on the standing start, when the driver could here and feel a loud noise, but it hadn't fully failed yet. It seemed like wear. To get the car back in the trailer, we welded the control arm back together, destroying, in retrospect, some very useful forensic evidence. But by this point, most of the crew had heat exhaustion from trying to lug the damn thing into the trailer in 115 heat. So we said "fug it! weld the SOB!" :)
I haven't seen it myself since then, but apparently there are some witness marks still left on the leading edge. When I go to the shop tonight I will try and take some pictures of what is left, and also the uprights and rotor.
Chris Lock
| Storm | 05-23-2005 09:15 PM |
PM Scott from IMR ([email�protected]). He can get into more detail regarding what limits you have before binding up. Your second description sounds like it may have gotten cracked from a possible bottom out and then repeated braking forces pulled the hub assy forward seemingly "tearing" the control arm from the rear forward..... Sound feasible?
Jay Storm
Jay Storm
| Subie Gal | 05-23-2005 09:26 PM |
let's just put it like this...
i have 5 spare control arms... there's a reason ;)
sorry to hear of your woes
check them often for cracks/stress
change them if there is any doubt.
Jamie
[url]www.subiegal.com[/url]
i have 5 spare control arms... there's a reason ;)
sorry to hear of your woes
check them often for cracks/stress
change them if there is any doubt.
Jamie
[url]www.subiegal.com[/url]
| Draken | 05-23-2005 09:28 PM |
No offense, but simply judging the correct ride height off the wheel gap isn't quite accurate. Have you guys measured your ride hieght? Have you calculated your roll center, camber curve etc.? Is this Gary's old car? If so, get any setup notes from him?
Just curious, but even just for my limited autocross schedule, I ran through all these steps when I setup my STi. And I would say you guys are a touch more "pro" than me :)
Chris H.
Just curious, but even just for my limited autocross schedule, I ran through all these steps when I setup my STi. And I would say you guys are a touch more "pro" than me :)
Chris H.
| dentsport | 05-23-2005 10:07 PM |
"YIKES! Those fabbed control arm are terrible! That gusset at the knuckle joint is in the middle of the tube, that is very bad! Somebody needs to rethink that design... that inner box gusset isn't too hot, either."
Those are acropolis 2001 Spec Impreza WRC. Design a better arm and Prodrive will probably buy it from you.
Those are acropolis 2001 Spec Impreza WRC. Design a better arm and Prodrive will probably buy it from you.
| thechickencow | 05-23-2005 10:30 PM |
From the sound of it, this kind of breaking control arm is different from a rally-broken control arm that usually comes from hitting something. That said, the stock ones are pretty much known to be pretty week as Jamie said.
Jay
Jay
| Subie Gal | 05-24-2005 12:54 AM |
i've seen stress cracks on the welds of the OE arms
not from hitting anything... just from simple stress
weld over them or replace the control arms.
they are not super beefy
but if you have no choice, like me :)
(rules... rules... silly class rules....)
it's not a bad idea to keep lots of spares on hand.
when in doubt, swap em.
Jamie
[url]www.subiegal.com[/url]
not from hitting anything... just from simple stress
weld over them or replace the control arms.
they are not super beefy
but if you have no choice, like me :)
(rules... rules... silly class rules....)
it's not a bad idea to keep lots of spares on hand.
when in doubt, swap em.
Jamie
[url]www.subiegal.com[/url]
| goto_racing | 05-24-2005 01:18 AM |
[QUOTE=Draken]No offense, but simply judging the correct ride height off the wheel gap isn't quite accurate. Have you guys measured your ride hieght?
[/QUOTE]
I won't take offense, because you are right, this is not how we measure our ride height. I was mearly asking people for a "gut" feeling about how the car looked, and I gave them a reference with the rim size. I figured I wasn't going to bother giving people dimesions, because not everyone has an imprezza in their office to check measurements on :).
[QUOTE=Draken]
Have you calculated your roll center, camber curve etc.? Is this Gary's old car? If so, get any setup notes from him?
[/QUOTE]
It is Gary's old car, we did get alot of set up notes from him. They don't mean much anymore because with help from DMS, we pretty much rewrote the book on the ustcc wrx. There is virtually nothing the same on this car as when Gary owned it, except the shell itself. Even the cage was partially rebuilt. It really doesn't drive the same either because the rates, shocks, ride heights, track, and bars were all changed.
But back on topic. We went and inspected the ball joint on the side that failed, and there was a huge witness mark showing that the joint had severly bottomed out at least once, if not multiple times. Judging by the way the arm tore, this impact put a split in the top of the control arm where there is a stress riser near the ball joint taper. The force from the standing start lengthened the crack, making it wrap around underneath the trailing edge of the arm. The final failure left a twisted mark on the leading edge(like taffy) showing how the end of the arm separated as the final failiure occured. Thanks to the nasioc members for the tip!
We think the original damage happened at our test day this month before Phoenix. We were experimenting with ride heights and had set them 10mm down from where our normal setup is. We ran 1 session at Sears Point, and brian decided that it made the car too unpredictable with that geometry, so we put it back. Unfortunately, Brian takes a "hard" line through turn 3a at Sears, which always take him over the sharp overrun and gives the RF wheel a heavy jolt airborne. None of the other corners suffered any damage, so we think that a couple of good whacks minus those 10mm bottomed out the joint and made a small crack that went unnoticed in prerace. Probably was getting worse all weekend at phoenix till the big event. A lesson learned. It's only money...
Chris Lock
[/QUOTE]
I won't take offense, because you are right, this is not how we measure our ride height. I was mearly asking people for a "gut" feeling about how the car looked, and I gave them a reference with the rim size. I figured I wasn't going to bother giving people dimesions, because not everyone has an imprezza in their office to check measurements on :).
[QUOTE=Draken]
Have you calculated your roll center, camber curve etc.? Is this Gary's old car? If so, get any setup notes from him?
[/QUOTE]
It is Gary's old car, we did get alot of set up notes from him. They don't mean much anymore because with help from DMS, we pretty much rewrote the book on the ustcc wrx. There is virtually nothing the same on this car as when Gary owned it, except the shell itself. Even the cage was partially rebuilt. It really doesn't drive the same either because the rates, shocks, ride heights, track, and bars were all changed.
But back on topic. We went and inspected the ball joint on the side that failed, and there was a huge witness mark showing that the joint had severly bottomed out at least once, if not multiple times. Judging by the way the arm tore, this impact put a split in the top of the control arm where there is a stress riser near the ball joint taper. The force from the standing start lengthened the crack, making it wrap around underneath the trailing edge of the arm. The final failure left a twisted mark on the leading edge(like taffy) showing how the end of the arm separated as the final failiure occured. Thanks to the nasioc members for the tip!
We think the original damage happened at our test day this month before Phoenix. We were experimenting with ride heights and had set them 10mm down from where our normal setup is. We ran 1 session at Sears Point, and brian decided that it made the car too unpredictable with that geometry, so we put it back. Unfortunately, Brian takes a "hard" line through turn 3a at Sears, which always take him over the sharp overrun and gives the RF wheel a heavy jolt airborne. None of the other corners suffered any damage, so we think that a couple of good whacks minus those 10mm bottomed out the joint and made a small crack that went unnoticed in prerace. Probably was getting worse all weekend at phoenix till the big event. A lesson learned. It's only money...
Chris Lock
| goto_racing | 05-24-2005 01:26 AM |
[QUOTE=Subie Gal]i've seen stress cracks on the welds of the OE arms
not from hitting anything... just from simple stress
weld over them or replace the control arms.
they are not super beefy
but if you have no choice, like me :)
(rules... rules... silly class rules....)
it's not a bad idea to keep lots of spares on hand.
when in doubt, swap em.
Jamie
[url]www.subiegal.com[/url][/QUOTE]
Thanks for the advice. We had spares on hand, but that point it didn't matter :). We watched the rest of the race from a gravel trap.
We also have no choice, we have to run these(but we also reinforce with extra welds). But it is nice to get feedback and know what to expect now. We haven't been running a car this size for very long. Our experience was with much lighter cars before USTCC. We have expensed for some more spares and are going to add this to our list of inspection items. 2000lb cars never needed their control arms replaced, unless you hit someone, of course :).
Chris Lock
not from hitting anything... just from simple stress
weld over them or replace the control arms.
they are not super beefy
but if you have no choice, like me :)
(rules... rules... silly class rules....)
it's not a bad idea to keep lots of spares on hand.
when in doubt, swap em.
Jamie
[url]www.subiegal.com[/url][/QUOTE]
Thanks for the advice. We had spares on hand, but that point it didn't matter :). We watched the rest of the race from a gravel trap.
We also have no choice, we have to run these(but we also reinforce with extra welds). But it is nice to get feedback and know what to expect now. We haven't been running a car this size for very long. Our experience was with much lighter cars before USTCC. We have expensed for some more spares and are going to add this to our list of inspection items. 2000lb cars never needed their control arms replaced, unless you hit someone, of course :).
Chris Lock
| AlexP | 05-24-2005 12:52 PM |
[QUOTE=dentsport]
Those are acropolis 2001 Spec Impreza WRC. Design a better arm and Prodrive will probably buy it from you.[/QUOTE]
Well, since Prodrive does it, it must be correct, right? The biggest problem with those control arms is that the gusset on the outboard side is welded to the quadrant of the tubing, rather than the tangent. This is incredibly bad for stress distribution, as it "pinpoints" a line load right down the center of the tube. It's like taking a paper towel tube, laying it on a table, and then laying a ruler on top of it so that the ruler is "balanced" on the quadrant. It would look like this from the side:
O- (only, rotate it 90� counter clockwise)
Well, what happens when you push down on that ruler? Even the slightest amount of force will oval the tubing and it will flatten. That's what happens with those fabricated arms shown above. The correct way to do it is to gusset to the tangency of the tube. This way, the force flows tangent to the surface of the tube (no "right angles" for stress flows to negotiate).
Just because something is made by Prodrive doesn't mean it's a good design. It might be a PROVEN design, but if thats the case, then a control arm built with better gussets could be made, and it would be just as strong and less weight.
Other than "Prodrive uses it", which is a pretty weak argument, anybody have any suggestions as to why they might have done it that way (other than my characteristically negative conclusion of them just being poor designers)?
Those are acropolis 2001 Spec Impreza WRC. Design a better arm and Prodrive will probably buy it from you.[/QUOTE]
Well, since Prodrive does it, it must be correct, right? The biggest problem with those control arms is that the gusset on the outboard side is welded to the quadrant of the tubing, rather than the tangent. This is incredibly bad for stress distribution, as it "pinpoints" a line load right down the center of the tube. It's like taking a paper towel tube, laying it on a table, and then laying a ruler on top of it so that the ruler is "balanced" on the quadrant. It would look like this from the side:
O- (only, rotate it 90� counter clockwise)
Well, what happens when you push down on that ruler? Even the slightest amount of force will oval the tubing and it will flatten. That's what happens with those fabricated arms shown above. The correct way to do it is to gusset to the tangency of the tube. This way, the force flows tangent to the surface of the tube (no "right angles" for stress flows to negotiate).
Just because something is made by Prodrive doesn't mean it's a good design. It might be a PROVEN design, but if thats the case, then a control arm built with better gussets could be made, and it would be just as strong and less weight.
Other than "Prodrive uses it", which is a pretty weak argument, anybody have any suggestions as to why they might have done it that way (other than my characteristically negative conclusion of them just being poor designers)?
| AlexP | 05-24-2005 12:57 PM |
oops, one more thing....
Where those fabricated arms spares? Or were they removed from a competing vehicle and replaced with new ones. I would surely hope it's the latter, as it is extremely bad practice to keep suspension components that are dented. In indycar, f3000, F1, etc. (where they still use alloy suspenion components), even the smallest SCRATCH warrants a replacement. At one point (not sure if they still do it), the Penske open wheel team did not paint their a arms or pushrods, but instead polished them (not chrome!) in order to find even the slightest scratchs and cracks.
Where those fabricated arms spares? Or were they removed from a competing vehicle and replaced with new ones. I would surely hope it's the latter, as it is extremely bad practice to keep suspension components that are dented. In indycar, f3000, F1, etc. (where they still use alloy suspenion components), even the smallest SCRATCH warrants a replacement. At one point (not sure if they still do it), the Penske open wheel team did not paint their a arms or pushrods, but instead polished them (not chrome!) in order to find even the slightest scratchs and cracks.
| Bimmubishi | 05-24-2005 06:14 PM |
The self imposed rules you're regurgitating regarding indy and F1 are most likely, if true, a result of the arms being composite and not resilient enough to withstand surface defects.
| AlexP | 05-24-2005 08:29 PM |
[QUOTE=Bimmubishi]The self imposed rules you're regurgitating regarding indy and F1 are most likely, if true, a result of the arms being composite and not resilient enough to withstand surface defects.[/QUOTE]
Absolutely not. I have no idea how composite suspension members are tested, and what the criteria is to retire them.
I made a point to indicate that this was in referance to alloy steel control arms... I know (from schooling and experience) that even the slightest surface imperfection on a properly sized cromoly tube can amplify stress quite significantly. I'm not talking 100's or 1000's of time here, I'm only talking maybe 20 to 50 percent. Doesn't sound like much, but when a control arm is built with only a factor of safety of 1.5 to 2, then that 50% could mean failure.
Penske polished their control arms so that with a simple wipe down of the arms, you can quickly tell if there is a reason to swap the arm (in all of a 30 seconds), which would later be magnafluxed or die pentrated or whatever.
What I've just written isn't speculation, hearsay, or conjecture... It's fact.
Absolutely not. I have no idea how composite suspension members are tested, and what the criteria is to retire them.
I made a point to indicate that this was in referance to alloy steel control arms... I know (from schooling and experience) that even the slightest surface imperfection on a properly sized cromoly tube can amplify stress quite significantly. I'm not talking 100's or 1000's of time here, I'm only talking maybe 20 to 50 percent. Doesn't sound like much, but when a control arm is built with only a factor of safety of 1.5 to 2, then that 50% could mean failure.
Penske polished their control arms so that with a simple wipe down of the arms, you can quickly tell if there is a reason to swap the arm (in all of a 30 seconds), which would later be magnafluxed or die pentrated or whatever.
What I've just written isn't speculation, hearsay, or conjecture... It's fact.
| GarySheehan | 05-24-2005 08:36 PM |
We had cracks show up on the control arm where the front arm connects to the main arm. Never had a complete failure, though.
Gary
Sheehan Motor Racing
[url]www.teamSMR.com[/url]
Gary
Sheehan Motor Racing
[url]www.teamSMR.com[/url]
| dentsport | 05-24-2005 11:00 PM |
"Well, since Prodrive does it, it must be correct, right? "
In regards to Subaru, yes. The R&D into that arm spans 13 years, hundreds of gravel events and thousands of miles of testing. In top level rallying teams use parts that work. They use the best motorsport engineers in the world to design parts, then use test drivers and teams that put thousands of miles on parts, check the parts using the latest technology and redesign accordingly. They then compete with the parts. They do this every year. This A-Arm design has evolved from the original prodrive engineered Legacy RS up until the current WRC05.
In regards to Subaru, yes. The R&D into that arm spans 13 years, hundreds of gravel events and thousands of miles of testing. In top level rallying teams use parts that work. They use the best motorsport engineers in the world to design parts, then use test drivers and teams that put thousands of miles on parts, check the parts using the latest technology and redesign accordingly. They then compete with the parts. They do this every year. This A-Arm design has evolved from the original prodrive engineered Legacy RS up until the current WRC05.
| Storm | 05-25-2005 01:41 AM |
With no source of the picture of parts being critiqued....isn't it kinda moot to be jumping on the soapbox? :( Those control arms may very well be retired pieces that are for sale to adoring fans worldwide..... :rolleyes: Certainly look like display parts to me....being ziptied to the chickenwire and all.... ;)
Jay Storm
Jay Storm
| dentsport | 05-25-2005 08:40 AM |
The control arms are spares and will be used again. The photo is from a service truck at rally GB 2001.
| Templar | 05-25-2005 09:38 AM |
There is also the fact that, despite being titanium, that control arm is designed to fail before the chassis does so that if the suspension takes a hit, the car can continue by simply swapping control arms. If the control arm were built stronger, the chassis would bend instead and the car wouldn't be much good.
| AlexP | 05-25-2005 12:17 PM |
[QUOTE=Templar]There is also the fact that, despite being titanium, that control arm is designed to fail before the chassis does so that if the suspension takes a hit, the car can continue by simply swapping control arms. If the control arm were built stronger, the chassis would bend instead and the car wouldn't be much good.[/QUOTE]
That's certainly good thinking. However usually the fasteners are designed to act in this capacity, as they are cheaper and easier to replace.
That's certainly good thinking. However usually the fasteners are designed to act in this capacity, as they are cheaper and easier to replace.
| AlexP | 05-25-2005 12:29 PM |
[QUOTE=dentsport]In regards to Subaru, yes. The R&D into that arm spans 13 years, hundreds of gravel events and thousands of miles of testing. In top level rallying teams use parts that work. They use the best motorsport engineers in the world to design parts, then use test drivers and teams that put thousands of miles on parts, check the parts using the latest technology and redesign accordingly. They then compete with the parts. They do this every year. This A-Arm design has evolved from the original prodrive engineered Legacy RS up until the current WRC05.[/QUOTE]
This, again, is not evidence of good design. This is pure opinion. I've laid out an engineering critique of why the design is bad, yet you insist that "prodrive does it, so its the best". Thats not a valid argument, it's your opinion. Some here are sharing their reasoning, and they have good ideas. Your argument that these are the best possible design are quite ludicrous. For example:
[QUOTE]This A-Arm design has evolved from the original prodrive engineered Legacy RS up until the current WRC05[/QUOTE]
According to this statement, this control arm has evolved (and I assume you also agree that it will continue to evolve). Why? If this is the best possible design, why would it evolve? Was it the best possible design back on the legacy RS? If it was, why did it change? You're very argument proves my point perfectly! Prodrive engineers, fabricators, and drivers are humans. They aren't perfect. They have poor judgement. They make mistakes. They get fired. They lie on their resumes. They put their pants on one leg at a time, just like the guys at Ralliart, Vermont Sportscar, Rocket, RalliSpec, Union 7, etc. etc.
I'm not trying to say that Prodrive isn't the best at what they do, what I'm trying to say is that every design can be improved upon. There may be a VERY good reason why those arms are like that, but quite frankly neither I nor anybody else can offer a solid argument for it.
This, again, is not evidence of good design. This is pure opinion. I've laid out an engineering critique of why the design is bad, yet you insist that "prodrive does it, so its the best". Thats not a valid argument, it's your opinion. Some here are sharing their reasoning, and they have good ideas. Your argument that these are the best possible design are quite ludicrous. For example:
[QUOTE]This A-Arm design has evolved from the original prodrive engineered Legacy RS up until the current WRC05[/QUOTE]
According to this statement, this control arm has evolved (and I assume you also agree that it will continue to evolve). Why? If this is the best possible design, why would it evolve? Was it the best possible design back on the legacy RS? If it was, why did it change? You're very argument proves my point perfectly! Prodrive engineers, fabricators, and drivers are humans. They aren't perfect. They have poor judgement. They make mistakes. They get fired. They lie on their resumes. They put their pants on one leg at a time, just like the guys at Ralliart, Vermont Sportscar, Rocket, RalliSpec, Union 7, etc. etc.
I'm not trying to say that Prodrive isn't the best at what they do, what I'm trying to say is that every design can be improved upon. There may be a VERY good reason why those arms are like that, but quite frankly neither I nor anybody else can offer a solid argument for it.
| Bigspin | 05-25-2005 12:45 PM |
I have a set of S9 front lateral links, they do not look like that at all !! I think that might be a knock off, or mine are from a TARMAC car or a GRAVEL car and those are the other type... Jeffery Shu at Chaste Automotive would know.
GOTO: Let me know when you get those pics, I have seen these arms fail like SubieGal mentioned, but not from what you describe.
mark
GOTO: Let me know when you get those pics, I have seen these arms fail like SubieGal mentioned, but not from what you describe.
mark
| M3GTR | 05-25-2005 01:06 PM |
[QUOTE=AlexP]This, again, is not evidence of good design. This is pure opinion. I've laid out an engineering critique of why the design is bad, yet you insist that "prodrive does it, so its the best". Thats not a valid argument, it's your opinion. Some here are sharing their reasoning, and they have good ideas. Your argument that these are the best possible design are quite ludicrous. For example:
According to this statement, this control arm has evolved (and I assume you also agree that it will continue to evolve). Why? If this is the best possible design, why would it evolve? Was it the best possible design back on the legacy RS? If it was, why did it change? You're very argument proves my point perfectly! Prodrive engineers, fabricators, and drivers are humans. They aren't perfect. They have poor judgement. They make mistakes. They get fired. They lie on their resumes. They put their pants on one leg at a time, just like the guys at Ralliart, Vermont Sportscar, Rocket, RalliSpec, Union 7, etc. etc.
I'm not trying to say that Prodrive isn't the best at what they do, what I'm trying to say is that every design can be improved upon. There may be a VERY good reason why those arms are like that, but quite frankly neither I nor anybody else can offer a solid argument for it.[/QUOTE]
Alex,
I am the first to admit I am not an engineer. I'm not claiming to have any knowledge about control arms either. Your "engineering critique" is coming from circuit/formula car motorsport engineering knowledge. I never claimed that this was "the best control arm design". You claim these are a bad design, even after knowing that they are from a gravel car. There aren't books or classes offered on gravel rallycar control arm design. You need to put your ego behind you and realize that prodrive has thought of every design you have and improved upon it.
According to this statement, this control arm has evolved (and I assume you also agree that it will continue to evolve). Why? If this is the best possible design, why would it evolve? Was it the best possible design back on the legacy RS? If it was, why did it change? You're very argument proves my point perfectly! Prodrive engineers, fabricators, and drivers are humans. They aren't perfect. They have poor judgement. They make mistakes. They get fired. They lie on their resumes. They put their pants on one leg at a time, just like the guys at Ralliart, Vermont Sportscar, Rocket, RalliSpec, Union 7, etc. etc.
I'm not trying to say that Prodrive isn't the best at what they do, what I'm trying to say is that every design can be improved upon. There may be a VERY good reason why those arms are like that, but quite frankly neither I nor anybody else can offer a solid argument for it.[/QUOTE]
Alex,
I am the first to admit I am not an engineer. I'm not claiming to have any knowledge about control arms either. Your "engineering critique" is coming from circuit/formula car motorsport engineering knowledge. I never claimed that this was "the best control arm design". You claim these are a bad design, even after knowing that they are from a gravel car. There aren't books or classes offered on gravel rallycar control arm design. You need to put your ego behind you and realize that prodrive has thought of every design you have and improved upon it.
| GarySheehan | 05-25-2005 01:59 PM |
You guys need to step off the ProDrive soap box and listen to what AlexP is saying. AlexP stated that as far as he can tell from the photo and without knowing exactly what the engineers were trying to accomplish, there are some very basic engineering issues he has with the design of the piece. There COULD be something that the engineers were thinking of in order to design the part contrary to what is learned in basic physics, but it is not apparent to anyone on this forum.
All anyone has said to contradict AlexP's claim is that "since it's from ProDrive, it must be the best it can be", yet no one has come up with any reasoning based in physics or from ProDrive to back it up. The arrogance of some of these replies with no inkling of the design criteria defines fanboyism.
ProDrive has a lot of credibility with me. AlexP raised some valid points regarding the design. Now I'm curious why it was built that way. Putting a ProDrive sticker on it does not satisfy my curiosity...
Gary
Sheehan Motor Racing
[url]www.teamSMR.com[/url]
All anyone has said to contradict AlexP's claim is that "since it's from ProDrive, it must be the best it can be", yet no one has come up with any reasoning based in physics or from ProDrive to back it up. The arrogance of some of these replies with no inkling of the design criteria defines fanboyism.
ProDrive has a lot of credibility with me. AlexP raised some valid points regarding the design. Now I'm curious why it was built that way. Putting a ProDrive sticker on it does not satisfy my curiosity...
Gary
Sheehan Motor Racing
[url]www.teamSMR.com[/url]
| goto_racing | 05-25-2005 02:08 PM |
[QUOTE=Bigspin]
GOTO: Let me know when you get those pics, I have seen these arms fail like SubieGal mentioned, but not from what you describe.
mark[/QUOTE]
Here are the pics!
this is the front right arm.
The top pic has too many welds to be good evidence, but the bottom shows the fissure, including showing us which way it came off. The outer case of the ball joint shows a huge whack happened somewhere, maybe repeatedly. So I think the ride height was right on. Thanks again guys.
[URL=http://www.gotoracing.com/cgi-bin/gotogallery/index.cgi?mode=album&album=/Phoenix2005/Damage]Control arm pics at GOTO:Gallery[/URL]
GOTO: Let me know when you get those pics, I have seen these arms fail like SubieGal mentioned, but not from what you describe.
mark[/QUOTE]
Here are the pics!
this is the front right arm.
The top pic has too many welds to be good evidence, but the bottom shows the fissure, including showing us which way it came off. The outer case of the ball joint shows a huge whack happened somewhere, maybe repeatedly. So I think the ride height was right on. Thanks again guys.
[URL=http://www.gotoracing.com/cgi-bin/gotogallery/index.cgi?mode=album&album=/Phoenix2005/Damage]Control arm pics at GOTO:Gallery[/URL]
| Joel Gat, 1.8L | 05-25-2005 03:05 PM |
Hello,
Huh. I stared at the pics for a while and tried to think things through. Have you looked at the axles? If this was caused by excessive force pushing outwards at the ball joint, that force has to come from somewhere and I would assume that to be through the axles. The axle joints might be excessively worn, now.
That would also indicate that you're riding too low or you don't have enough roll stiffness. If the control arms are far past parallel and you're pulling the hub in against the axles, that could generate the right forces to cause that tear in the control arm... The axles can only "shrink" in length a little bit... they're quite long in order to reach the hub in the stock max droop configuration without pulling on the joints themselves.
Joel Gat
Crew Chief
Sheehan Motor Racing
Huh. I stared at the pics for a while and tried to think things through. Have you looked at the axles? If this was caused by excessive force pushing outwards at the ball joint, that force has to come from somewhere and I would assume that to be through the axles. The axle joints might be excessively worn, now.
That would also indicate that you're riding too low or you don't have enough roll stiffness. If the control arms are far past parallel and you're pulling the hub in against the axles, that could generate the right forces to cause that tear in the control arm... The axles can only "shrink" in length a little bit... they're quite long in order to reach the hub in the stock max droop configuration without pulling on the joints themselves.
Joel Gat
Crew Chief
Sheehan Motor Racing
| goto_racing | 05-25-2005 03:17 PM |
I think that it was the binding of the ball joint actually, that caused the first fissure on the top by bending the control arm down. Only then did the braking and accelaration forces finally make the metal weak. The launch cause the fissure to extened to the trailing edge, then it only took a few laps of accelarating and braking to finally tear loose the leading edge. You can see on the leading edge that it was definately the control arms "last stand" because of the twisted metal, as apposed to the clean break on the trailing edge.
But that is good to know you had problems with the axles too, Thanks. More reasons to continue to improve the suspension.
Chris Lock
But that is good to know you had problems with the axles too, Thanks. More reasons to continue to improve the suspension.
Chris Lock
| GarySheehan | 05-25-2005 03:43 PM |
[QUOTE=goto_racing]I think that it was the binding of the ball joint actually, that caused the first fissure on the top by bending the control arm down. Only then did the braking and accelaration forces finally make the metal weak. The launch cause the fissure to extened to the trailing edge, then it only took a few laps of accelarating and braking to finally tear loose the leading edge. You can see on the leading edge that it was definately the control arms "last stand" because of the twisted metal, as apposed to the clean break on the trailing edge.
But that is good to know you had problems with the axles too, Thanks. More reasons to continue to improve the suspension.
Chris Lock[/QUOTE]
The welds along the sides of the control arm are not stock, correct? I wonder if the welding created a stress riser or embrittlement in the seam, which cracked under normal loading and initiated the failure.
Gary
Sheehan Motor Racing
[url]www.teamSMR.com[/url]
But that is good to know you had problems with the axles too, Thanks. More reasons to continue to improve the suspension.
Chris Lock[/QUOTE]
The welds along the sides of the control arm are not stock, correct? I wonder if the welding created a stress riser or embrittlement in the seam, which cracked under normal loading and initiated the failure.
Gary
Sheehan Motor Racing
[url]www.teamSMR.com[/url]
| goto_racing | 05-25-2005 03:47 PM |
[QUOTE=GarySheehan]The welds along the sides of the control arm are not stock, correct? I wonder if the welding created a stress riser or embrittlement in the seam, which cracked under normal loading and initiated the failure.
Gary
Sheehan Motor Racing
[url]www.teamSMR.com[/url][/QUOTE]
Actually, the welds were AFTER the incident. We welded them to get the car back into the trailer :). It was all stock.
Chris Lock
Gary
Sheehan Motor Racing
[url]www.teamSMR.com[/url][/QUOTE]
Actually, the welds were AFTER the incident. We welded them to get the car back into the trailer :). It was all stock.
Chris Lock
| dentsport | 05-25-2005 04:49 PM |
[QUOTE=GarySheehan]You guys need to step off the ProDrive soap box and listen to what AlexP is saying. AlexP stated that as far as he can tell from the photo and without knowing exactly what the engineers were trying to accomplish, there are some very basic engineering issues he has with the design of the piece.
There was no questioning as to what the arms were intended for, just the statement that they were a bad design.
There was no questioning as to what the arms were intended for, just the statement that they were a bad design.
| Joel Gat, 1.8L | 05-25-2005 05:03 PM |
[QUOTE=dentsport]There was no questioning as to what the arms were intended for, just the statement that they were a bad design.[/QUOTE]
Hello,
Alex pointed out that the design violates the basic rules of how to distribute forces in a mechanical structure for maximum strength. Gary pointed out that no one has offered a reason for that violation. That set of facts remains.
There is probably a reason for the design, since any engineer can see the flawed design, and I doubt ProDrive would make such a noob mistake. Unfortunately, no one here seems to know that reason or even be able to guess at it.
Joel
Hello,
Alex pointed out that the design violates the basic rules of how to distribute forces in a mechanical structure for maximum strength. Gary pointed out that no one has offered a reason for that violation. That set of facts remains.
There is probably a reason for the design, since any engineer can see the flawed design, and I doubt ProDrive would make such a noob mistake. Unfortunately, no one here seems to know that reason or even be able to guess at it.
Joel
| GarySheehan | 05-25-2005 05:44 PM |
[QUOTE=dentsport]There was no questioning as to what the arms were intended for, just the statement that they were a bad design.[/QUOTE]
The fact that they are to be used for rally is just one component of all of the different things considered that go into the design criteria. Weight, loading, planned failure modes, life cycle, etc., etc. SHOULD go into designing that type of piece.
Who knows what they were thinking when they designed this piece. Or what they weren't...
Gary
Sheehan Motor Racing
[url]www.teamSMR.com[/url]
The fact that they are to be used for rally is just one component of all of the different things considered that go into the design criteria. Weight, loading, planned failure modes, life cycle, etc., etc. SHOULD go into designing that type of piece.
Who knows what they were thinking when they designed this piece. Or what they weren't...
Gary
Sheehan Motor Racing
[url]www.teamSMR.com[/url]
| speedyHAM | 05-25-2005 08:48 PM |
AlexP- I think your analysis is spot on for a circut car type suspension, but a rally car suspension is completely different. The direction that the major loads act upon the suspension are 90 degrees from what they would be in circut cars. More importantly, as Gary mentioned, the failure modes are more important in rally than in circut cars. In a rally car you would never want the bolts to break as this would end your day (missing a wheel on one corner = very hard to drive), where a even severly bent control arm would just slow you down until the next service area. The A-Arm pictured would be far from optimal for weight and stiffness, but nearly perfect for a survivable impact. I looked at the picture and thought they were very well designed and built, then I read your posts and realized that coming from a different perspective that they would look pretty weak. But after changing out a few control arms at service areas in a cow pasture or on the side of a road in the middle of nowhere, those arms look nice to me.
| duncangrant | 05-26-2005 04:06 AM |
Its interesting to see the spherical joint in the outboard end of that WRC control arm. Think what that does for roll-centre, camber curve and bending moment compared to the standard offset (raised + inverted) ball-joint.
The binding issue highlights the need to check suspension movement without springs and bumpstops, and also with bumpstops. What struts were fitted?
The binding issue highlights the need to check suspension movement without springs and bumpstops, and also with bumpstops. What struts were fitted?
| MattNJ2.8 | 05-26-2005 09:38 AM |
I'd like to agree with the other posters who said Prodrive did their homework.
For one main reason: The arms are from a non-tarmac, non-circuit racing vehicle.
It looks to me that the arms are designed to take the punishment and save the more expensive (and some parts are bloody expensive) suspension systems from a "shunt" or a nasty encounter with a rock.
It seems to me, as a businessman and not a engineer, that the control arms are designed to become a 'wear' item, and save the more expensive bits from undue, and budget-breaking (as well as rally ending) damage or failure.
Don't forget, racing is a business, and budgets are everything. I'm sure with enough cash, some misguided knucklehead could make a WRX take on Formula One cars.
For one main reason: The arms are from a non-tarmac, non-circuit racing vehicle.
It looks to me that the arms are designed to take the punishment and save the more expensive (and some parts are bloody expensive) suspension systems from a "shunt" or a nasty encounter with a rock.
It seems to me, as a businessman and not a engineer, that the control arms are designed to become a 'wear' item, and save the more expensive bits from undue, and budget-breaking (as well as rally ending) damage or failure.
Don't forget, racing is a business, and budgets are everything. I'm sure with enough cash, some misguided knucklehead could make a WRX take on Formula One cars.
| Joel Gat, 1.8L | 05-26-2005 01:22 PM |
Hello,
The points about wanting the control arm to fail first are good ones. Any ideas on why, if all you want is the control arm to be certain to fail before anything else fails, not make it lighter weight than the pictured arm? Put "proper" gusseting but reduce the thickness or OD of the tubing?
Anyway, the point was, it's not "right because ProDrive made it." It's hopefully right because it accomplishes what it was designed to do, and one of the ways it does that is by inducing a specific failure location, possibly.
Joel
The points about wanting the control arm to fail first are good ones. Any ideas on why, if all you want is the control arm to be certain to fail before anything else fails, not make it lighter weight than the pictured arm? Put "proper" gusseting but reduce the thickness or OD of the tubing?
Anyway, the point was, it's not "right because ProDrive made it." It's hopefully right because it accomplishes what it was designed to do, and one of the ways it does that is by inducing a specific failure location, possibly.
Joel
| platypus | 05-26-2005 04:49 PM |
that would be my estimation, that they're attempting to induce a known failure mode which can be compensated for (or at least anticipated).
| xtorted-legacy | 05-28-2005 03:47 AM |
Much like the article in Sport compact car in 2003 highlighting how DMS designed the bracket on the bottom of the coilover which bolts to the hub to crack and fail first. Therefore a new one may be shipped out to the customer and the "broken" one could be returned, and repaired easily and live to see another day.
Hell, it could just be poor engineering. I am not an engineer.
Why doesn't someone email prodrive? :lol:
Hell, it could just be poor engineering. I am not an engineer.
Why doesn't someone email prodrive? :lol:
| XT6Wagon | 05-28-2005 05:18 AM |
Joel, one of the more important things is the ablity to inspect the arm quickly with a high degree of certainty that nothing is wrong. I wouldn't be suprised that by doing it "wrong" they have found ways to create visible damage early and ALSO prolong the time from visible damage to complete failure from stress cycles. Then there is the idea that it might be designed to FLEX as part of its normal operation. Making it the "right" way could be a disadvantage if they need the A-arm to flex as the tire hits rocks or whatever
And yes its definitely designed to fail before the car. Changing an A arm could be a 15 minute task or less. How long does it take to replace a engine subframe???
Which just made me think. For gravel you don't have quite the fore-aft stresses you would find on tarmac with a powerful car. But you do have impacts from rocks, and a requirement for withstanding lateral impacts as you slide across ruts or whatever.
And yes its definitely designed to fail before the car. Changing an A arm could be a 15 minute task or less. How long does it take to replace a engine subframe???
Which just made me think. For gravel you don't have quite the fore-aft stresses you would find on tarmac with a powerful car. But you do have impacts from rocks, and a requirement for withstanding lateral impacts as you slide across ruts or whatever.
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