Paging GM truck experts, please!

So, a certain 2015 GMC Sierra that I am acquainted with has a fairly pronounced steering wheel shimmy at around 45-50 mph.  Suspecting a tire balance issue, had the wheels balanced and rotated.  Shimmy is the same.  Also passed state inspection, but if I jab at the brakes suddenly, the truck gives the impression that it wants to pull to the left, but standing on the brakes results in normal stopping without any noticeable pull.

Mileage on this truck is crazy low, like 31k miles.

What do I need to look at?

Ooops.  Forgot 2500 HD 4 x 4 base truck, 6.0 FlexFuel.

I wouldn't expect shimmy from balance.  Thats usually a bushing letting out, in my experience. 

Yep. I'd be looking at suspension joints and possibly wheel bearings.

That being said, presumably these are 10 year old tires?  Did you get a roadforce style balance, or did you just get your regular spin balance?

Mr_Asa said:

I wouldn't expect shimmy from balance.  Thats usually a bushing letting out, in my experience. 

We must be attaching a different meaning to the word "shimmy."  In approximately 50 years of driving (and maintaining vehicles) EVERY out of balance condition ALWAYS manifested itself as a shimmy in the steering wheel.

Given that the wheels were balanced today with no change in the "shimmy", I'm willing to entertain other possible causes. 

Mr_Asa said:

That being said, presumably these are 10 year old tires?  Did you get a roadforce style balance, or did you just get your regular spin balance?

Not original tires, but not new either.  Not inclined to place the blame on tires at this point, since in addition to being balanced, they were rotated also.

Modern tire shop.  No idea what kind of balancer they run. 

I would go thru the brakes very closely.  A pad worn funny would be a clue.  Loose caliper pin to housing clearance, sticking slide or pad  could put an uneven pressure at times but a hard stop over rides the minor rub.  

In reply to porschenut :

Yeah, I keep coming back to that little twitch to the left it takes when you first hit the brakes.  If it is worn suspension bits, I'm a little surprised this shop didn't find it with the inspection.  They've caught suspension issues on other cars they've looked at that weren't even this bad.

Bent wheels will balance and still shake the wheel. 

If you see it/feel it in the steering wheel, thats the front end.

If you feel it in your ass or your back, thats the rear end.  

If you move the wheels front-to-back, does the shimmy move?  If so, check the wheels and tires.  if no, its in the componentry.  

How is the alignment?  What is the tire wear looking like?

Caliper pins are seized in the mounting bracket. Just went through with this in my Avalanche.

93gsxturbo said:

Bent wheels will balance and still shake the wheel. 

If you see it/feel it in the steering wheel, thats the front end.

If you feel it in your ass or your back, thats the rear end.  

If you move the wheels front-to-back, does the shimmy move?  If so, check the wheels and tires.  if no, its in the componentry.  

How is the alignment?  What is the tire wear looking like?

 

Shop allegedly rotated the tires also, no change in steering wheel shimmy, so presumably not tires.  I specifically asked about bent wheels, I'd hope a skilled operator would point that out.  This shop is good about "yes, it passed inspection, but it could use ....  yadda, yadda." 
 

I say "shimmy".  The steering wheel will start to oscillate noticeably around 45 mph.  It smooths out at higher speeds, but is still perceptible at 70.  I will go over the caliper slide pins.

Tire wear seems fine.  I would put different tires on it, but these have tons of tread left.

Before you take the wheels off to check caliper pins spin them to see if you notice any out of round situations.  Then just for fun put them on the back and put the backs up front. 

I left a cotter pin off a center link joint once. The nut backed off, the taper popped loose, and I had a similar effect to what you're describing. I think those trucks have a rack and pinion (so no center link). Have the tie rod ends been thoroughly inspected for play?

Ranger50 said:

Caliper pins are seized in the mounting bracket. Just went through with this in my Avalanche.

So, thought I would bring this back up since the issue is still present.  I pulled the caliper pins.  All removed normally and were corrosion-free.  One was a little resistant once it broke free, but not what I would call difficult to remove.  It might have been a little "dry", but not remarkably so.  I cleaned and lubed them all up and re-installed.  No change.

I will add that the PO installed a "leveling kit" which makes it possible to raise the front end a couple inches by cranking on the torsion bars.  I feel this may be problematic, as the driveshaft angle and the position of the upper control arm looks extreme to me.  I backed the bolts off one turn while I had the truck in the air, and I may go another turn, or more, though I do not expect that to solve my shimmy problem.  I'm also assuming that will necessitate a change in alignment?

I took the time to shoot some grease in the zerks I could find on the front suspension.  That seemed to reduce the amplitude of the shimmy slightly, but it could also be my imagination. 

Truck is super smooth up to 45 mph, where the shimmy starts--the steering wheel oscillates several degrees back and forth.  It stays pretty noticeable up through about 50 mph, where it begins to diminish.  I had it up to 75 yesterday, and it's quite smooth there.

Kinda leaning toward tie rod ends, but I'm having trouble believing the shop didn't find an issue there, as they are usually good at finding worn suspension bits.

Any other ideas?

Until I lay eyes on it myself I don't believe anyone.

Jack it up, crawl underneath, inspect the whole front end.  

You can use pry bars to lift the wheels up and check for balljoints.

Rock the wheels back and forth and check tie rod endplay.  Should have none.

Have someone saw the steering back and forth with the wheels on the ground and look for play in everything.  Could even be something dumb like A-Arm bushings.

Or load up the parts cannon and rebuilt the front end nuts to soup.  Its not particularly hard or expensive.

I would still swap wheels front to back just to eliminate a potential bent wheel.  I had a bent wheel on my Mercedes (thanks Milwaukee) and it did exactly as you describe.  Smooth as glass, then vibratey, then smoothed out again.  

In reply to 1988RedT2 :

yes, you will need to adjust the alignment after adjusting ride height. However, the driveshaft angle should be unaffected by changes to the torsion bars as the differential is mounted to the frame independent of the suspension.

Do you have the ability to evaluate the tie rod ends yourself? If you have a jack/jackstands, it isn't exceptionally difficult, and I believe that is likely where your problem lies.

budget_bandit said:

In reply to 1988RedT2 :

However, the driveshaft angle should be unaffected by changes to the torsion bars as the differential is mounted to the frame independent of the suspension.

I misspoke.  I said driveshaft when I definitely meant drive axle.

Update:

After watching a few videos of owners having similar issues, I learned that GM installed steering stabilizers on these trucks.  They quit putting them on in 2011 or so, but the hole in the center link and the bracket on the frame are present on my 2015, so why not?  Seems a lot of people running larger than stock tires are getting this wobble. 

It might be a band-aid, but it was the easiest install ever.  Bilstein steering stabilizer from Rockauto for a hundred bucks.  Symptoms are about 90% improved.  Eventually I'll spring for more road-friendly tires, but for now, this seems to be the fix.

Definitely look to the brakes.  Low mileage means that the rotors spent a lot of time with surface rust.  Since the pads drag slightly on the rotors, a gunky rotor can certainly cause this.

My 06 Express does this all the time.  The brakes pulse when applied, but they "catch" the crappy part of the rotors even without brakes applied.

Sticky calipers can do this, and that's also a common side effect of minimal use.  I would also take a good look at the tires as well.  They might balance perfectly, but if there is a belt that is starting to come apart, it won't show up on a balancer, it will only act up with weight on it.

K-trucks are notorious for front bearing failures, but I've never had one cause a shimmy.  Usually they start growling long before you get play.

Every one of these with a leveling kit needs new upper control arms. Every one. Instantly. 

QuasiMofo (John Brown) said:

Every one of these with a leveling kit needs new upper control arms. Every one. Instantly. 

I can imagine reasons you'd feel this way, but even so, I'd appreciate it if you could expand on your statement with particulars.

In reply to 1988RedT2 :

There is a design "issue" with ball joint that causes the joint to be loaded too aggressively when the vehicle is lifted. As that happens they fail somewhat regularly.

They addressed the problem on factory "lifts" by changing from a stamped steel to either an alloy or cast steel arm but I have heard people running over 2" leveling still having issues.

The aftermarket has a design improvement that allows for better articulation of the joint with less damage resulting from normal operation.