Chasing Vibrations - ‘78 K10 Suburban

[RyanAK]

Update: New Cragar wagon wheels and 265/75R16 General Grabber A/TX rubber now on the truck.

The more I drive it, the more I'm pretty darn pleased with how this rig is riding up to about 65. I think the new wheels and tires made a bigger difference in how the truck rides that I first thought. That mid-range vibration at 45-55mph isn't hardly worth mentioning any more. 65mph and up it starts to vibrate again... which has me back to thinking about tire balance. It was hell to get the new wheels and tires to balance and two of the set never did balance perfect on the machine. They went on the rear as 'close enough'. This was on a pretty modern machine, but not a road force balancer.

Now these aren't the biggest tires (31.6"x10.5"), but they're a pretty beefy high profile tire compared to what most modern trucks are running. And I'm not even entirely sure the U.S.-made Cragar wheels are perfectly round and true. Like I said, we had a hell of a time getting them to balance and one or two took a bunch of weight. These are guys that started with bubble balancers in the '60s. Family friends. They spent more time on this than they would have for anyone else, and they still aren't 'prefect'. So...

Would a 'close but not perfect' balance job more-or-less eliminate the vibrations at 45-55mph but allow the vibration at 65+ to remain? Should I consider finding a shop with a road force balancer? How about balance beads? There's a thing called something like a 'maintenance level' dose of beads to help get the last few percent of balance on tires already balanced with weights.

Anyway... was just thinking about tire balance again. Still gonna chase the front driveshaft CV and consider driveshaft balance. Just couldn't help think about the new wheels and tires.

Then... I thought I noticed this before, but with the new wider wheels/tires it's more apparent...

The front axle isn't centered under the truck.

The passenger side tire projects from the wheel well by 1-1/4" to 1-1/2". Driver side is tucked in. Truck tracks down the road pretty well... but could this be contributing to the vibration at 65mph+? I'd assume that this would keep the front driveshaft from being in proper alignment between the transfer case and the front diff...

Ahhhh... Old Trucks!

[1976gmc20]

Hate to mention this, but did the shop that replaced your body mounts get everything put back together straight ?

I'm just trying to figure out how your front axle could possibly be 3/4" off center from the frame without a lot of other problems.

Also, is the backspace on all four new wheels the same?

[RyanAK]

Everything seems like it went back together right with the body mounts. We only did the ones that didn’t require lifting the body, so it was bolt out, bolt in sorta stuff. I had noticed the axle seemed off before that work was done, but sorta forgot about it while doing other things. The new wheels and tires make the issue more obvious.

Wheels were all ordered with the same backspace... and like I said, I noticed this with the stock wheels. It definitely seems to be drivetrain-to-frame rather than body-to frame.

I don’t know how this could even be... I’ll need to spend some time underneath and inspect/measure everything... but on first glance nothing seems bent and the passenger side inside u-bolt mounts around the axle AND front diff pumpkin... so how could it all slide in relation to the leaf springs?

Hmmm...