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Old 06-23-2005, 12:10 PM   #1
IchibahnWRX
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Default Funny Tire wear and alignment question:

Ok, I have the following suspension bits currently on my car:
KG/MM Coilovers
Cusco front and rear strut bars
Cobb front and rear sway bars
Rear pillow ball endlinks (Cobb prototypes)
17x8 rims with 225/45zr17 kumho tires.

The problem I have been having is that I keep wearing my front tires, on the outside edges. I recently had the car aligned really well, I have 0 toe front and rear and 0 thrust angle (obviously). I set the camber in the front at -2.5 degrees and the rear at -1.7. My castor was set to about 4.5 degrees. The wear I had in the rear went away after it was alligned, but the front is still wearing like crazy. I rotate my tires every oil change, but this is getting really annoying! My only thought is that my ride hight is too low, so that when my suspension actuates down in the front, the control arm goes way below parallel with the chassis and causes toe in or out (not really sure which one). Or maybe the opposite is happening, when the front becomes unloaded, it lengthens and causes the same problem. I am lost here. As a reference, I there is about 1 inch from my tire to the fender and I have not noticed any rubbing. Any help would be appreciated, thanks!

Ryan
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Old 06-23-2005, 12:46 PM   #2
DrBiggly
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Ok, now that just sounds bizarre. Have you gotten under your car to measure the toe for yourself? i.e. get out the tape measure?

I've seen some racks be really off before. For example, this rack claimed I had -5 degrees of camber on one side and couldn't get more than -2.5 on the other. Yeah, it was off a bit. I will say that this typically isn't the case though.

1" from the tire sounds like a lot; are you sure it's not rubbing? The car's suspension, even on the stiffest of coilovers, has to travel more than 1" for any normal driving. The only thing I can think of that would cause you to have tire wear like that if your toe was truly 0 is a combination of a few things:
1) Car is too low and your camber goes positive during a turn, chewing the outside edge.
2) You have coilovers, so you drive it aggressively and this adds to the problem
3) You have positive camber to start with (possibly, but less likely) that compounds problems #1 and #2.

What is your exact alignment? Do you have a printout from the last time it was done?

-Biggly
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Old 06-23-2005, 12:48 PM   #3
drees
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Wearing the edges is usually a sign that tire pressures are low, what are they set to? I'm not sure how much bump steer there is on the car at those ride heights, but it can't be good. Will you measure ride height f/r from center of the hub to fender lip?
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Old 06-23-2005, 01:01 PM   #4
IchibahnWRX
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Quote:
Originally Posted by DrBiggly
Ok, now that just sounds bizarre. Have you gotten under your car to measure the toe for yourself? i.e. get out the tape measure?

I've seen some racks be really off before. For example, this rack claimed I had -5 degrees of camber on one side and couldn't get more than -2.5 on the other. Yeah, it was off a bit. I will say that this typically isn't the case though.

1" from the tire sounds like a lot; are you sure it's not rubbing? The car's suspension, even on the stiffest of coilovers, has to travel more than 1" for any normal driving. The only thing I can think of that would cause you to have tire wear like that if your toe was truly 0 is a combination of a few things:
1) Car is too low and your camber goes positive during a turn, chewing the outside edge.
2) You have coilovers, so you drive it aggressively and this adds to the problem
3) You have positive camber to start with (possibly, but less likely) that compounds problems #1 and #2.

What is your exact alignment? Do you have a printout from the last time it was done?

-Biggly


# 2 seems to ring slightly true; I need to be nicer to my tires! How would # 1 happen, from a mechanical point of view? Just curious, thanks for the input!

-Ryan
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Old 06-23-2005, 01:04 PM   #5
IchibahnWRX
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Quote:
Originally Posted by drees
Wearing the edges is usually a sign that tire pressures are low, what are they set to? I'm not sure how much bump steer there is on the car at those ride heights, but it can't be good. Will you measure ride height f/r from center of the hub to fender lip?
The last time I looked, I was at 38psi f/r. That was about 3 weeks ago, so I will check again. I am not sure about bump steer, the coilovers are REALLY stiff and are designed to be low; I am relatively high up on them in the front. I will go measure from the hub by lunch and get back to you.

-Ryan
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Old 06-23-2005, 01:27 PM   #6
drees
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If you've got even fender gap front/rear, you are most likely too low in the front. What spring rates are in your coilovers?

You can also try going up 2-3 PSI in the front, usually works well to have a bit more pressure in the front than the rear.
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Old 06-23-2005, 01:50 PM   #7
IchibahnWRX
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I will get back to you on the rear to front comparison for ride height, but I am pretty sure the front is higher. The front springs are 8 kg/mm and the rear are 6 kg/mm. Thanks for the help.

Ryan
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Old 06-23-2005, 01:57 PM   #8
IchibahnWRX
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OK, I measured the heights:
Front: 14 in
Rear: 13.5 in

That is from the hub center to the fender lip. Thoughts?

-Ryan
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Old 06-23-2005, 02:46 PM   #9
DrBiggly
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Ryan,

Those heights aren't too low although I feared they would be from the way you talked earlier. On a Subaru when the suspension compresses, it has a camber curve. When the corner compresses a little, it goes a little negative in camber initially (Good!) and then positive as the car compresses further. (Very bad)

So say you're turning right. The left front wheel starts off as | before turning, then goes / under initial load and then \ (not quite as drastic as the back and forward slashes, but it's just a simplistic illustration.)

The amount of positive camber at the maximum end of that camber curve is first determined by ride height, and then by springrate. So going from a 217# spring to a 250# spring really means very little as far as bodyroll is concerned if one is dropping the height of the car with the stiffer spring. The spring with the best handling is typically the tallest in those cases. Get up to the springrates that you have, and the car can be lowered just a bit. But really you're about as low as you'd want to go. To see how much travel the car has, you can do the ziptie test on the strut shaft and go take a few good turns. I have 10k/12k rates plus 22/24 bars and I still had about 2" of travel, which I never thought I could possibly have. It was an eye-opening experience and I highly recommend that anyone with coilovers do the same.

That said, your heights really look fine to me. I can't really figure out why you would have such drastic outer edge wear unless your alignment is whacked. Your coilovers are stiff enough I believe to support your ride height without a serious detriment to handling.

Which, now having gone full circle, I can only think that it's a bizarre alignment issue. I'd suggest getting out the tape measure and measuring the toe and see what you have there that way. If it's way off, it should be plainly obvious using that method, however imprecise it may seem.

-Biggly
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Old 06-23-2005, 03:44 PM   #10
IchibahnWRX
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Biggly,
Wow, thank you for the explanation, that was exactly what i needed to hear, and sort of what I was thinking. Well, I guess I will double check the allignment and I think I may switch to a stickier tire, and see if maybe I am just asking too much of these kumho's. Thank you again for the help!

-Ryan
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