Alignment after lowering rear

GRN69CHV · March 2, 2015

2015 LT DC 4x4, Last week removed the 1.25" factory blocks, no other mods. Truck steering wheel had been notably off center since I got it with a slight pull to the right. Going to be logging some miles this week so figured best to get it on an alignment rack. Bottom line, everything was within factory spec except right side toe-in and caster on both sides. Caster specs are: LF (2.2*-4.2*), as it sits with the rear lowered 1.25", mine is 4.3*; RF (1.2*-3.2*), mine is 3.3*, just the upper limit of factory stock specs for an unloaded vehicle. Tech asked me about the ride height, I explained that I wanted to try running it a while more at the lower rear ride height, so he left the caster as set from the factory just in case I add some incremental height back to the rear axle.

Guys that have done the 2" lowering shackles, would assume caster on yours is moved yet another 2*m maybe in the 5-6* range on the driver side and 4-5* on the passenger side, same goes for trucks that have 2" front lift kits?

Jon A · March 2, 2015

What was your thrust angle of the rear axle? That's likely what changed when you did the shackles.

GRN69CHV · March 3, 2015

I didn't use shackles, just removed the factory 1.25" blocks. Thrust angle stock spec is -0.40* to 0.40*. Mine was .030* before, .040* after. Right in the middle, well within spec. Only thing I can see that changed from lowering it was added positive caster. As it sits with bed unloaded it's right at the upper edge of factory specs, loaded, it's going to have more positive caster, not sure how much but would assume it could easily add 2-3* depending on how much weight, even more if it got to the bump stop. That may the shortcoming of lowering the rear via removing the lift block, suspension still has full travel.

POS VETT · March 3, 2015

Whoa. I never thought about that. My truck came with wrong alignment from the factory; steerin wheel was crooked when going straight. I wanted more positive caster and it is now set "in the red", close to 5 degrees IIRC. Lowering the rear will effectively increase the caster in positive direction.

Interesting.

GRN69CHV · March 3, 2015

I've only logged couple hundred miles (rural driving) since lowering it. Seems adding positive caster should make the truck more neutral on the highway. Heading out of town for a couple days, have a 150 mile one way highway run to make so will be interesting to see how it drives. I can tell you this, for the little bit of highway driving I've done, this truck is far more straight line stable than the 2012 Caddy SRX that I traded for it. Caddy was FWD, quick steering response for around town, but almost too responsive on the highway to the point where every little movement of your body in the seat could initiate the car to move out of the lane. Add to it, the 5.3L running in this Silverado is only about 2 mpg less than what I was seeing in real life with the 3.6L in the SRX. Going forward, I'd think a set of Timbrens would be beneficial if truck gets used to haul/tow anything significant just to keep it from getting rear end sag.

Jon A · March 3, 2015

Yeah, you didn't change the thrust angle enough to do much, but something still doesn't sound right. Was anything else done to the truck at the same time?

Lowering the rear will add a little positive caster, equally to both sides. It should still track straight. These are pickups, when you put a load in the back you sink the rear 1-2 inches and they still drive fine. If that caused some sort of problem they couldn't do their jobs. I think something else is at play.

GRN69CHV · March 3, 2015

Yeah, you didn't change the thrust angle enough to do much, but something still doesn't sound right. Was anything else done to the truck at the same time?

Lowering the rear will add a little positive caster, equally to both sides. It should still track straight. These are pickups, when you put a load in the back you sink the rear 1-2 inches and they still drive fine. If that caused some sort of problem they couldn't do their jobs. I think something else is at play.

Only thing I did was remove the rear blocks to lower it. GM thrust angle spec is +/- .40*, so a little less than 1/2 degree to either side is within spec. Mine is set at .040*, that's darn near perfect square for unbolting the rear and re-attaching it in thegarage on jackstands given that the only thing locating the rear are the leaf spring bolts and recess in the housing. Road trip got cancelled, but I did run it about 20 miles up I95 today. I think the only thing at play here was the toe-in was off. Centering the wheel and adjusting the toe has it tracking dead straight now.

EXSlider400 · March 3, 2015

Check the locator hole and leafs are in correct position. Just removing rear blocks should not have changed any tracking.

GRN69CHV · March 3, 2015

Check the locator hole and leafs are in correct position. Just removing rear blocks should not have changed any tracking.

Checked all that before I snugged it all up tight. Took measurements from the front spring eyes as well as the body. I got it squared to 4/100th of a degree. Point of this thread was just to share the detailed results of setting the truck on an alignment rack after playing with the suspension and to provide a baseline for others to use if/when they raise/lower theirs. Needless to say, I am very happy with the way it runs down the road now. 5 weeks and 2400 miles in, I've even gotten over the 3.08 gears and actually figured out how to use the gearing to advantage.