Tires: Uneven tire wear, alignment settings, thrust angle

Question
I was wondering about my 1999 Chevy Tahoe and my uneven tire wear I am having with it. It seems to be mainly on the front end and even more so on the passenger side more so than on the drivers side. It is cupping real bad and then wearing out the outside edgo of the tire to the point wear its tread is gone. I bought this truck and didnt have that problem and then I take it and get it aligned at the dealership and another place where I bought my tires and now its a disaster and cant seemed to be fixed. Do you have any idea what is going on here? Please let me know, I would appreciate any help you can provide.

Answer
Jim,

This is probably alignment related.

First is the normal 3 items that we think of when we think of alignment - camber caster, toe.  Each of these has tolerances - HOWEVER, my experience is that these tolerances are too wide.  Alignment settings in the outer half of these tolerances tend to develop irregular wear, but most technicians aren't savvy enough to realize that the tolerance is a matter of degree, rather than kind, so they think in terms of "In Spec / Out of Spec".

But there are also other static alignment settings - such as thrust angle - that can affect the way the front end actual addresses the forward motion of the vehicle.  Many tech's don't really understand this - or give it too low of an importance.  Again, they think in terms of "front end alignment" but the rear axle will affect the way the vehicle goes forward.

Also, there are dynamic forces involved and the alignment will change when the vehicle is moving.  Usually, the static alignment settings are adjusted to compensate, but worn components will be affected more in the dynamic situation than the static one.

And last, but not least is steering geometry.  There are several issues here, but the most prominent one is "Ackerman".  This is also called "toe out on turns".

When the steering wheel is turned and the vehicle negotiates a corner, the inside tire has to be angled more than the outside tire.  This is called Ackerman and it is defined by the links in the steering system.  It is not adjustable without replacing or bending parts - not something an inexperienced person should do.  You may see the effect of Ackerman when you turn the steering wheel to full lock and hear the tires squeal.

For a given radius turn, and a given speed, there is one and only one optimal Ackerman setting.  There will be a while series of radii / speed combinations that are optimal for the Ackerman setting of your vehicle.  This becomes problematic for sharp turns, even though they are low speed.

If you frequently make sharp turns, then the Ackerman may not be suitable for the speed you are going.  Slowing down will reduce the amount of tire scrub.

If I were you, I would go somewhere that is willing to work with you on your alignment.  You need to get the "Before" and "After" printouts.  Examine them closely and make sure they are within the inner half of the tolerance.  Also look at the thrust angle, and "toe out on turns".

Hope this helps.