Still uneven wear on tire
12-10-2009, 11:28 AM
#31
Rennlist Member
Now we are speaking the same language. i never said bump steer issues were not bad with the 928, and especially with a "slammed" 928. I said that it probably wouldnt provide uneven tire wear on the street, unless most of your driving was around cloverleafs on hyways. 
Yes, the bump steer in racing is a big issue, and for a refined car like a cup car, squeezing 10/10ths out of the platform, you certain wouldnt want both front tires pointed different directions, and you would want to reduce that occurrence as much as possible. hence, bump steer kits for race cars and porsches efforts to make a platform that handles better than anything out there in the racing world out of the box. For me, I already concieded that it desensitizes steering on the race track, as well as scubs the inside tire on constant radius turns, and thank you, yes, i have driven around many of the little things that some hold to a greater value .
EDIT: you point about the "tire changing directions" during a turn is a good one as well. Certainly that point would be more of a factor on a bumpy track like Sebring. in a turn, at the limit, and you hit an undulation, which changes the direction of the tire, can be bad. However, we are talking about wear patterns on street cars, where that wouldnt be much of a factor.
my 928 is slammed, and so its the 928 "Members Only" lemons car. 110mm front ride hight. We ran near 14 hours of all out racing on a pretty fast track and the tires looked brand new after we were done. I wonder why?
Yes, the bump steer in racing is a big issue, and for a refined car like a cup car, squeezing 10/10ths out of the platform, you certain wouldnt want both front tires pointed different directions, and you would want to reduce that occurrence as much as possible. hence, bump steer kits for race cars and porsches efforts to make a platform that handles better than anything out there in the racing world out of the box. For me, I already concieded that it desensitizes steering on the race track, as well as scubs the inside tire on constant radius turns, and thank you, yes, i have driven around many of the little things that some hold to a greater value .
EDIT: you point about the "tire changing directions" during a turn is a good one as well. Certainly that point would be more of a factor on a bumpy track like Sebring. in a turn, at the limit, and you hit an undulation, which changes the direction of the tire, can be bad. However, we are talking about wear patterns on street cars, where that wouldnt be much of a factor.
my 928 is slammed, and so its the 928 "Members Only" lemons car. 110mm front ride hight. We ran near 14 hours of all out racing on a pretty fast track and the tires looked brand new after we were done. I wonder why?
I agree his problem is probably from a bad alignment....unless the car is "slammed".
I do not agree that bump steer is an insignificant issue....nor would Porsche. Have you seem to the extent that they have worked to reduce the amount of "directional changes" on the Cup Cars? They aren't doing this for the sheer fun of designing and building stuff. The less the tire chages direction, as it sweeps through the camber curve, the more stable and predictable the car will be. You simply can't have a tire "at the limit" and suddenly change its direction without upsetting the slip angle.
While this apparently doesn't seem to impair your driving style....and I respect that....I assure you that it does impair most other drivers.
I do not agree that bump steer is an insignificant issue....nor would Porsche. Have you seem to the extent that they have worked to reduce the amount of "directional changes" on the Cup Cars? They aren't doing this for the sheer fun of designing and building stuff. The less the tire chages direction, as it sweeps through the camber curve, the more stable and predictable the car will be. You simply can't have a tire "at the limit" and suddenly change its direction without upsetting the slip angle.
While this apparently doesn't seem to impair your driving style....and I respect that....I assure you that it does impair most other drivers.
12-10-2009, 11:33 AM
#32
Chronic Tool Dropper
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Kibort's a great guy. He always seems very civil when he debates things....which is something you have to respect. You just have to realize, going in, that he will never conceed that he might be mistaken on an issue. He stands his ground....which is also something you have to respect. I suspect that he is gathering data from some obscure place to support his point, right now.
Anyway, back to work...
12-10-2009, 11:41 AM
#33
Race Car
My only problem with Mark's alignment method is IMO 19" of measurement difference is too little. The longer the measuring piece, the more accurate you can be. The markers on measuring tape are only so far apart. In reference to the wood not being completely straight, if you use the same piece of wood in the same orientation on both sides, whatever error there was will cancel itself out. You only need to make sure that the wood is well flat against the wheel. The piece I use is about 4 feet long.
Dan
'91 928GT S/C
475hp/460lb.ft
Dan
'91 928GT S/C
475hp/460lb.ft
12-10-2009, 11:49 AM
#34
Rennlist Member
anyway what is the ride height Brett??
I had another customer come in with the worn inner tires.
I replaced the shocks and returned the front end to original ride height spec..... he had it set at 130 now its 170 ,
guess what no more worn inner tires after the 4 wheel alignment and new front tires
all other front end pieces are tight .
Best to check the front end RH with a stock size tire diameter
I had another customer come in with the worn inner tires.
I replaced the shocks and returned the front end to original ride height spec..... he had it set at 130 now its 170 ,
guess what no more worn inner tires after the 4 wheel alignment and new front tires
all other front end pieces are tight .
Best to check the front end RH with a stock size tire diameter
bump steer is not going to effect tire wear, but it will if you lower your car and dont realign it. ^^^^ Thank you, Greg. I got tired of beating that dead horse to death with MK.
To the OP--
Getting an alignment with new tires almost always means new tires installed THEN an alignment. They picked the car up to install the new tires, and stole the tire life from you in the process.
A car with a sagged front suspension will make the effects even worse, with results including the tramlining and bumpsteer issues you are experiencing. Since you are not racing the car and replacing tires every week or two, I recommend that you give MK's advice the respect it deserves as it applies to what you have and what you are doing. Spend a little time restoring the original ride height. Go drive the car 50+ miles, and take it to be aligned then WITHOUT RAISING THE CAR AT ALL DURING THE ALIGNMENT PROCESS. Alert the alignment guys that raising the car will cause you to go drive another 50+ miles before they get another shot at it, and you won't pay them if they raise it up. Simple as that. A reference to their revenue usually gets the right attention. Watch them and make sure they follow the instruction.
There is a great DIY alignmnet thread available with some searching here. There are some posts from MK that include his race experience, again his advice does not apply to your situation. There's also a decent video instruction from Andrew Olson that may help you. Even if you choose not to make adjustments yourself, you will be able to instantly identify a bad alignment before you trash another set of tires.
To the OP--
Getting an alignment with new tires almost always means new tires installed THEN an alignment. They picked the car up to install the new tires, and stole the tire life from you in the process.
A car with a sagged front suspension will make the effects even worse, with results including the tramlining and bumpsteer issues you are experiencing. Since you are not racing the car and replacing tires every week or two, I recommend that you give MK's advice the respect it deserves as it applies to what you have and what you are doing. Spend a little time restoring the original ride height. Go drive the car 50+ miles, and take it to be aligned then WITHOUT RAISING THE CAR AT ALL DURING THE ALIGNMENT PROCESS. Alert the alignment guys that raising the car will cause you to go drive another 50+ miles before they get another shot at it, and you won't pay them if they raise it up. Simple as that. A reference to their revenue usually gets the right attention. Watch them and make sure they follow the instruction.
There is a great DIY alignmnet thread available with some searching here. There are some posts from MK that include his race experience, again his advice does not apply to your situation. There's also a decent video instruction from Andrew Olson that may help you. Even if you choose not to make adjustments yourself, you will be able to instantly identify a bad alignment before you trash another set of tires.
Hey, you talk about beating a dead horse. yes, it is a dead horse! dont raise the car and align it! yet, thats what folks do or have done without their knowing it. just trying to help. My experience is with multiple 928 platforms with very low ride height, and no uneven tire wear. i would hope that some would benefit from those experiences and results.
12-10-2009, 11:55 AM
#35
Rennlist Member
My only problem with Mark's alignment method is IMO 19" of measurement difference is too little. The longer the measuring piece, the more accurate you can be. The markers on measuring tape are only so far apart. In reference to the wood not being completely straight, if you use the same piece of wood in the same orientation on both sides, whatever error there was will cancel itself out. You only need to make sure that the wood is well flat against the wheel. The piece I use is about 4 feet long.
Dan
'91 928GT S/C 475hp/460lb.ft
Dan
'91 928GT S/C
475hp/460lb.ftNo arguments there. But making a blanket statement for street cars based on race experience is a stretch. Backyard racers cut a lot of corners on things that a pro team wouldn't consider. Mark's 'alignment' methods and advice certainly fall into this category. I'm happy that it works for him, but it still doesn't mean that it's universal. I'll speculate that you seldom eyeball a setting and go race with it, except under extreme circumstances. Given half a chance to do it correctly, do you still go with a crayon and ruler measurement?
Anyway, back to work...
Anyway, back to work...
12-10-2009, 03:04 PM
#36
Former Sponsor
BTW...I agree with Kibort that there is a 90% chance that your alignment is screwed up. However, I have seen cars with badly worn inner tie rod sockets (very common) that will allow the toe to radically change when driving. Makes sure that all the suspension pieces are good and everything is tight.
12-10-2009, 07:18 PM
#38
Former Sponsor
No arguments there. But making a blanket statement for street cars based on race experience is a stretch. Backyard racers cut a lot of corners on things that a pro team wouldn't consider. Mark's 'alignment' methods and advice certainly fall into this category. I'm happy that it works for him, but it still doesn't mean that it's universal. I'll speculate that you seldom eyeball a setting and go race with it, except under extreme circumstances. Given half a chance to do it correctly, do you still go with a crayon and ruler measurement?
Anyway, back to work...
Anyway, back to work...
Really good drivers are fun....they will bitch about set-up and run your *** around trying to chase the "perfect" set-up, but once the race starts, they just shut-up and drive it. You never hear "Well, it would be better if".
03-20-2010, 07:54 PM
#39
Drifting
Thread Starter
I finally got the time and nice weather to start into my list of maintenance and upgrades. The first on the list was checking and setting the ride height, since I'm curious to see if that's contributing to my tire wear issue. The results:
LF 165mm
RF 163mm
LR 171mm
RR 170mm
The fronts are definitely way out of spec. I adjusted them both to 190mm and will recheck and set the rears if it's changed.
Now, I assume I'll probably need a new alignment, correct?
LF 165mm
RF 163mm
LR 171mm
RR 170mm
The fronts are definitely way out of spec. I adjusted them both to 190mm and will recheck and set the rears if it's changed.
Now, I assume I'll probably need a new alignment, correct?
03-20-2010, 08:48 PM
#40
Chronic Tool Dropper
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Brett--
Adjusting ride height is an iterative procedure, at least around here. The same stiction/friction in suspension bushes affects the way the car responds to adjustments as it does to settling after raisng the car for a tire swap. Go drive it a bit and remeasure. When the car is at the height you want and it's not changing with 20 miles of driving, you are OK to go get that alignment. For grins, drive straight home after the alignment and measure the ride height again. If it's higher, they raised it and you'll need to go at it again.
FWIW, the initial front numbers you post at 165 are close enough that I might not have adjusted if you have decent shocks. The factory-acceptable range for warranty purposes is 190 to 170, about a 3/4" range.
The goal, for those playing at home, is to functionally have a level lower control arm. "Level" in this case is the center of the lower ball joint pivot height same as the center of the inner bushing pivot height. Deviation from level means more bump steer, and worn shocks means bigger vertical deflection on impacts and therefore more steering.
Adjusting ride height is an iterative procedure, at least around here. The same stiction/friction in suspension bushes affects the way the car responds to adjustments as it does to settling after raisng the car for a tire swap. Go drive it a bit and remeasure. When the car is at the height you want and it's not changing with 20 miles of driving, you are OK to go get that alignment. For grins, drive straight home after the alignment and measure the ride height again. If it's higher, they raised it and you'll need to go at it again.
FWIW, the initial front numbers you post at 165 are close enough that I might not have adjusted if you have decent shocks. The factory-acceptable range for warranty purposes is 190 to 170, about a 3/4" range.
The goal, for those playing at home, is to functionally have a level lower control arm. "Level" in this case is the center of the lower ball joint pivot height same as the center of the inner bushing pivot height. Deviation from level means more bump steer, and worn shocks means bigger vertical deflection on impacts and therefore more steering.
03-20-2010, 08:56 PM
#41
Under the Lift
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The 165-163 you found was well within spec. I have always run that height. The factory spec is: 180 +/-20 (i.e. 160-200) when "new" and 180 +20/-30 (i.e. 150-200) for us. Regardless, this probably has nothing to do with your problem.
In the original message you show uneven wear right versus left, with inside wear only on the right front tire. That's puzzling. I've seen it a few times but never understood how it could happen unless the camber were way off on that side.
In the original message you show uneven wear right versus left, with inside wear only on the right front tire. That's puzzling. I've seen it a few times but never understood how it could happen unless the camber were way off on that side.
03-20-2010, 10:07 PM
#42
Drifting
Thread Starter
LF Camber -.6 RF Camber -.4 LR Camber -.7 RR Camber -.6
From the alignment settings done right before the new tires. I made sure they didn't raise the car, so the alignment should have remained good. I wondered if the less than desirable offset of the 18" wheels might be making the car ride a little on the inside of the tires and contribute to the wear and to the car's tendency to tramline, but it's puzzling that it has only worn the RF tire.
I'll have the alignment checked/set and replace the front tires and see how it feels and wears.
From the alignment settings done right before the new tires. I made sure they didn't raise the car, so the alignment should have remained good. I wondered if the less than desirable offset of the 18" wheels might be making the car ride a little on the inside of the tires and contribute to the wear and to the car's tendency to tramline, but it's puzzling that it has only worn the RF tire.
I'll have the alignment checked/set and replace the front tires and see how it feels and wears.
03-22-2010, 06:16 PM
#43
Race Car
Keep in mind that every time you change the ride height, you are changing the toe. So if you raised your front height by ~20mm, your toe will be off of what it was before by a LOT.
Dan
'91 928GT S/C 475hp/460lb.ft
Dan
'91 928GT S/C
475hp/460lb.ft
03-27-2010, 04:24 PM
#44
Drifting
Thread Starter
I just noticed that, even though the ride height was only 2mm difference FL to FL and I raised it the same # of turns + a little more for FR, the # of threads exposed on the adjuster is quite different, 7 threads on the FR and 3 threads on the FL. I would think that would put the ride height difference quite a bit off between FL and FR, correct? I will drive the car a bit to settle the suspension and check the new ride height, but if it's equal FL to FR now, what could account for such a difference in the adjustment?
03-27-2010, 05:03 PM
#45
Under the Lift
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I think a discrepancy between ride height and adjuster position is common. Some have suggested that you get better corner balance when the adjusters match despite a ride height uneveness. Porsche allows a 10mm difference side to side.