Alignment Issue after Lift

WhineyTTwins · July 20, 2016

Got to installing my lift over the weekend and went ahead and got the alignment done today. You can review the before and after printout sheet (ignore the spilled water). Unfortunately, I still have pull to the left and he told me it is because of the thrust angle of the rear. He does a lot of track cars and not a lot of trucks so he said it could be a couple things I don't agree with.

My questions are:

1) I have a blown passenger side rear shock, could this be causing it?

2) If not, could this have happened when i installed the lift blocks?

3) How can I fix this? I am guessing I am going to have to eyeball it and then go back and have the alignment checked but I have always had cars with Independent suspension so this is a first. Any way to measure this for self adjustment?

Vic15 · July 20, 2016

4 Wheel Parts told me that once the truck is lifted you can't use the stock factory alignment settings. I don't know what those settings are or what size lift you have.

The thrust angle is an imaginary line drawn perpendicular to the rear axle's centerline. It compares the direction that the rear axle is aimed with the centerline of the vehicle. It also confirms if the rear axle is parallel to its front axle and that the wheelbase on both sides of the vehicle is the same.

Tire Tech Information - Alignment

I don't see how a bad shock can change the thrust angle. I will say having my truck aligned at 4 Wheel Parts made a big difference on my truck. It didn't pull to either side but it was really wearing the tires on the inside from the day I bought it and I purchased it new with a Fabtech 6 inch lift. It also eliminated the tire rub on the drivers side at full lock to the left.

WhineyTTwins · July 20, 2016

Right rear just has a tremendous amount of negative toe. I am going to measure the distance to the lift blocks to see if I can get any measurements that can tell me what is going on. Maybe its just a matter of loosening the u bolts and trying to shift the axle a bit (as much as the alignment pins allow) but regardless, it will still be excessive for both sides.

If I would have gotten hit there, I think I would feel some vibration (more than the famous regular chevy shake). I am gonna be really upset if this isn't fixable and I have to purchase a) a new axle, b) parts for the axle or c) fight with the dealer to try and get it done under warranty as a defective axle. Never had my truck aligned before but it has always pulled a bit to the left, much more noticeable now with the big tires.

WhineyTTwins · July 24, 2016

Got it up on the lift this weekend and took some measurements. Looks like the axle is perfectly straight. Measured from the same spot on both sides to the same fixed points that are uniform on each side of the truck. Loosened the u bolts, took some axle straps and tried to twist the axle slightly. Still getting some pull but not as much. It will be getting back on the alignment machine this week to check. Really doesn't make any sense to me. If something was bent, I should be able to feel it or at least see it.

Silverado915 · July 24, 2016

Why is the right front caster still off? Camber and caster are pull forces. Cross camber and cross caster should be at 0.

WhineyTTwins · July 26, 2016

He did it to help compensate for the bent axle supposedly.

WhineyTTwins · August 26, 2016

Saw on other forum someone had lift blocks that had the alignment pins off. Called rough country and they said they highly doubt thats the cause. Going to see where my alignment is since we used the strap to twist the axle some and then take them out, measure them, reinstall stock blocks and then check alignment again. That should either confirm or eliminate the possibility that it's the lift blocks causing it. Shocks are also going in this weekend

WhineyTTwins · November 18, 2016

Just an update:

Last weekend I went and installed: ProComp ProRunner SS struts/shocks on the front and rear which removed my leveling kit. I also changed out the rear lift block to Rough Country's 5 3/4" Anti-Wrap Blocks.

Just got my truck back from getting the alignment and the rear axle is now dead-on. Thrust angle is 0.02 on both ends. So I know for a fact that it was either the lift blocks OR the shocks causing it. (I had 1 rear shock completely gone and the other right on the fence of going). Could the rear shock extension been contributing as well? Could the dramatic difference between the front and rear lift have caused it? Cannot be sure of either but I do know that my issue is now fixed. Figured I would update everyone in case someone experiences this.

Truck now drives like a dream. Very happy with it.

Towerhog · November 19, 2016

I guarantee that a rear shock or shocks will not affect the thrust angle. That thrust angle is dependent upon the positioning of the rear axle on the leaf springs. The hole in the axle leaf pad is at least an 1/8 of an inch larger than the block pin or leaf pack pin.

Thats why sometimes you see trucks ahead of you look as though they are driving at an angle. Thrust angle is out. Anything more than .04 is too much.

Heres an explanation I found on another site.

The front wheels aim or steer the vehicle. The rear wheels control tracking. This tracking action relates to the thrust angle (3). The thrust angle is the path that the rear wheels take. Ideally, the thrust angle is geometrically aligned with the body centerline (2).

In the illustration, toe-in is shown on the left rear wheel, moving the thrust line (1) off center. The resulting deviation from the centerline is the thrust angle.

If the thrust angle is not set properly the vehicle may "dog track", the steering wheel may not be centered or it could be perceived as a bent axle. Thrust angle can be checked during a wheel alignment.

Positive thrust angle means the thrust line is pointing to the right hand side (RHS) of the vehicle.

Negative thrust angle means the thrust line is pointing to the left hand side (LHS) of the vehicle.

If the thrust angle is out of specification, moving the axle to body relationship will change the thrust angle reading.

If the vehicle is out in the Positive (+) direction-moving the RHS forward and/or LHS rearward will move the thrust angle towards zero degrees.

If the vehicle is out in the Negative (-) direction-moving the RHS rearward and/or LHS forward will move the thrust angle towards zero degrees.