Can someone help me out please? My IROC seems to have a killer appetite for front tires. The front ones have nearly no tread left on them while the rears are still in pretty good shape. I would say the rears have alteast another 5000 miles left on them. And I don't take it easy on the rear tires - they get lit up at least 3 times a week. But I don't drive my car hard into the corners either. I know that's hard on the front tires and bearings so I take it easy when going around curves. The unibody was slightly twisted when I put the new tires on it so the wear on the right front tire is slightly more than the left one, but the unibody was straightened at a frame shop a few thousand miles later. My alignment has also always been spot on, and all steering components are in good shape (no slop in the tie rods, drag link, idler arm, etc). Yet the car wears out the edges of the front tires really fast, the outside edge a lot more than the inside edge. I've seen people with other cars drive the snot out of them yet not see this kind of wear on the front tires. Is this a problem with all thirdgens or just mine?

What could be causing this is also the fact the left front tire rides about 3/8" inboard. It doesn't line up with the lip of the fender like the other tires, and after everytime I have had my alignment done the camber on it seems positive (top of tire leans out). I know the car was in a front end collsion before I got it and thats why the unibody needed to be pulled straight. Yet there is no visible suspension damage, except for a bent knuckle on the passenger side spindle. The guy at the frame shop looked the car up and down and couldn't figure out why the tire doesn't stick out as much as the others. The spindle in question has a bent knuckle that would affect steering geometry only in lock-to-lock turns not in a straight line. The car also likes to wander on rutted roads, trying to the cross over the crests of the ruts. I can usually tell the road's rutted even if I can't see it because the car behaves so awefully on the surface. The front subframe has been looked over countless times. Still nobody can tell what's wrong with it. I'm assuming all thirdgens had the same size lower A-arms, or that would be the culprit right there. A shorter A-arm would screw up front end geometry a whole lot.

Please people, help me out here! I'm at my wits end :lala: trying to figure out how to save my next set of tires from a certainly premature demise. Thanks in advance

 

The stock alignment specs really are terrible. It calls for too much positive camber and it kills the tires if you corner very hard at all. Try this:
-.5 camber
+4.5 caster left +5 caster right
toe +1/16

 

88,

Well written. Thanks for including the history and details. That always makes it easier to analyze. You may have several issues involved causing the variety of symptoms.

First, you have presumably already checked the obvious items, like inflation pressure, wheel bearing adjustment, tread width versus wheel width, and correct wheel offsets. If you are using all factory wheels and no spacers, and the tires are the same as the original equipment in width and aspect ratio, there should be no problems. Wheels with different offsets will place the tread centerline in a different position with respect to the steering angles, and handling can be affected (especially in cornering). Tires that are too narrow for the wheels will tend to experience outer edge wear, but more likely on both edges. Combinations of problems can cause pretty erratic symptoms, however. Throw in some loosely adjusted front wheel bearings, and the front end can become very sloppy. Pay close attention to the rubber bushings in the lower control arms, and be very suspicious of the bent knuckle.

Maintaining adequate tire pressure is important to prevent shoulder wear on both edges, and assures that the sidewalls are as rigid as they were intended to be to prevent rolling on turns.

The wandering on some roads is something that I experience regularly. The relatively large treads on a rigid tire carcass are always going to provide a lot more feedback, and try to be more compliant with the paved surface. Some of this is almost always present, and isn't a problem unless it has become more noticeable or severe to the point of not being able to control the car without excessive correction. This can be amplified by worn suspension parts. Don't overlook the total lash on the steering gearbox when checking for worn components. Excessive lash can allow a wide "dead spot" in the steering and increase the wandering tendency.

While caster angle is not a wear item, it can contribute to the wandering. Caster that is too low (negative) will tend to eliminate any negative camber on turns, especially on a car with front struts. This can cause unnecessary outer wear on both front and rear tires because of the unequal weight transfer on hard cornering. If you are not pushing the car hard in corners, the tread wear may only appear on the front end. It also can make the car feel loose in cornering, since the rear tires are taking more of the side loading from excessive weight transfer. More negative caster does help a little on bump steer, but the gains there are wiped out by the lack of overall control in hard cornering on normal surfaces. I would suggest setting the caster as high as you can within the specified range, or just short of the point of losing high speed stability. Caster is the "black art" of alignment angles, and while it doesn't affect wear when the car is traveling straight down the road, it can certainly affect cornering wear and handling.

Camber should be 0° or less, even if you don't push the car hard through the corners. Again, the front struts tend to turn the camber positive on the outer wheels under cornering load (the opposite of "A" arms), so setting a little negative can have a beneficial effect. It will also tend to put more weight inboard

It should be mentioned that just like the compression from cornering, sagging springs will greatly affect the camber angle on strut equipped cars, so ride height is important to consider when determining the camber you want. It may be advisable to weight the car as it would normally be during driving before adjusting any steering angles. If that means loading the fuel tank full and placing a 200 pound weight in the left front seat and a 100 or so in the right front, then try to convince your frame shop to do that. It is also very important to have true tires when aligning the steering. Worn tires can cause unpredictable loading of the suspension and make correct adjustments almost impossible.

Toe is obvious. The tighter your steering and suspension components, the closer to zero toe-in you can run. Excessive toe-in can cause outer tire wear, but should also be evident in feathering wear across the entire tread surface.

As for the apparent unequal positions of the wheels in relation to the fenders, you did include the point that the car was in a front end accident. The subframes needed to be pulled back in place, and may have been put exactly back into their correct positions. This doesn't necessarily mean that the fenders were bolted on with the correct shims and alignment, so the body sheet metal may not be symmetrical, and could the cause of the apparent unequal tire positions. Basically, the steering and suspension can be dead-on straight, and the body is still racked a bit. That shouldn't affect handling at all unless you are driving in a more aerodynamic mode at 130 MPH or higher. Even then, the slight misalignment would likely cause very little push to one side or the other. Just a possible explanation for the symptom if everything else is correct.

 

Very helpful information. I'll try to have everything looked at by a competent shop. I still have a few questions though.

I didn't know caster was adjustable on these cars . Can you tell me how it's done? I did notice that the camber on my car remains neutral or just slightly positive when the wheels are turned to lock. On the other hand, the camber on my friend's 86 IROC became even more negative with the wheels turned to lock.

I'm not running factory wheels either. They're 225/60R15 tires on 15" x 8" or 9" rims with slightly less than factory backspacing. Hence, all my wheels except for the left front ride flush with the lip of the fender, i.e. stick out just a little more than factory wheels. I have checked the rim in question too. It's exactly the same as the others. Tire pressure is 35 psi all around. This seems to have worked beautifully for the rear tires, because they are equally worn across the entire tread.

The wandering really gets to me! I have to turn the wheel quite bit to get the car to stay in line, about 15-20 degrees either way depending on where the car is pulling, especially during braking. That makes it even worse. I did adjust the lash in the steering box. There was a dead spot and getting rid of it made the steering a whole lot better. I still have a worn out rag joint in intermediate shaft that I have been trying to replace with something more rigid, like a universal joint but I haven't found a shop yet that's willing to undertake the job. Why anybody would use something that looks like the Flintstones made it in the steering is beyond me .

I suppose toe-in is within specs because the tires are worn smoothly. There is no feathering across the tread. BTW, did I mention the car also has a tendency to 'walk' in tight turns? It's evident only in really tight turns in parking lots etc, especially on rough surfaces.

Well, the front end bodwork has always been suspect, but if the fenders were that badly misaligned then the doors wouldn't have lined up properly with the fenders either. Not that they are perfectly aligned right now, but a wouldn't a difference like that be quite apparent? I guess the only way to find out is to measure the relationship of the rear tires to the front tires, or to pull the front end body work off.