72 gmc 3500 car hauler hopping badly

[pinturbo75]

at about 35-45 miles per hour the rear end hops really bad.... shakes the whole truck...has the big leaf pack with overload pack and no shocks.... any ideas???? thanks. steve....

[LockDoc]

Originally Posted by pinturbo75

at about 35-45 miles per hour the rear end hops really bad.... shakes the whole truck...has the big leaf pack with overload pack and no shocks.... any ideas???? thanks. steve....

I would check for a loose wheel, inner or outer. There are two types of wheels that fit the big trucks "hub centric" and "lug centric" if the wrong type of wheels were installed for your setup it could cause that. Also check the tread on your tires and make sure one isn't coming loose from the tire casing..

This is just a couple of things to check. Hopefully others will chime in with other thoughts....

LockDoc

[pinturbo75]

ok.... just checked the rims and they are the wrong ones for the axle....the lug openings are bigger than the studs and the rims are off center .... so looks like I need rims....

[71swb4x4]

I put newer dually rims on my 67 C30, but I also went from the 1/2" studs to the 9/16" studs. The front was easy because I was going to discs anyway. The back was not very easy.

[pinturbo75]

ok.... now have the correct rims on the truck with good tires and the shake is still there although not as bad.....35-40 miles per hour..... im stumped.....

[LockDoc]

Originally Posted by pinturbo75

ok.... now have the correct rims on the truck with good tires and the shake is still there although not as bad.....35-40 miles per hour..... im stumped.....

OK, do you have the hub centric wheels or the lug centric wheels? Are your lug nuts flat on both sides or flat on one side and tapered on the other side? Hub centric wheels use the flat lug nuts and lug centric wheels use the tapered lug nuts....

LockDoc

[Lee H]

Why no shocks, by design? Can't imagine anything not hopping without some sort of damping. Even over the road semis have shocks.

[pinturbo75]

these are lug centric with the correct lugs and everything is seated correctly.... I verified the rims ere centered on the lugs and axles.... it all is as it should be so far....

could the lack of shocks really be the problem???? im not very knowledgeable on the bigger trucks.... I don't see any mounts here they would have been on the frame or axle

[pinturbo75]

nobody??? I know some had no shocks from the factory but can the lack of them be my issue???

[davepl]

Well clearly it should still drive properly without shocks if the factory offered them without shocks.

Are you certain you do not have 9/16" wheels on 1/2" studs?

After its shaken at least once, bring it home, lift it up, put a dial indicator on something like the bead, and rotate the wheel. I don't know what minimum vertical runout should be, but it shouldn't be a lot. That'll tell you if it's the wheel.

There are other possibilities like driveshaft critical speed but unless you have serious gearing I don't think you'd get there at that road speed. I'd still give the U-joints and carrier bearing a once-over just to be sure. I've never had a carrier bearing fail but if the driveshaft starts dancing it'd feel pretty wild.

What about the fronts? Sounds like you're confident it's the rear, but it's not always the easiest to tell.

I hated the Turbine wheels on my '75 Monte so bought a set of Superior wheels that were some horrible universal design. Not only was the hub center bigger than my hub, but the lug holes were elongated to fit anything as well. Stock rocks.

[Old Money]

they did not put shocks on a 1-ton back then. 40 years ago they beleived shocks would not last six monthson a truck. shocks were not a state of the art item back then. i drove a 68 1-t0n dual wheels and a 69 1-ton single rear wheel for work and neither truck had shocks. i was sixteen years old in 69. old money.

[pinturbo75]

I hadn't thought about the drive shaft..... that has me wondering.... it did have the rear swapped out to a dana 70 and I believe it had a spicer or corporate to start with...I just am not sure that with the violence of the shake that it wouldnt tear it out...if you drive through the shake it smooths out to a fairly decent ride above 40 mph..

[GASoline71]

Does it shake with the ramp loaded with a car? Or only when empty.

Spring eye bushings could be shot as well.

Gary

[pinturbo75]

I haven't had it loaded yet.... wanted to get it right before I put it to work and found out I shouldn't have..

[alsriv2]

Glad you found the problem with your rims, but what about the tires themselves? Possibility exists that even though they are still looking round and in good shape, they could be severly out of balance and causing your issues.

[pinturbo75]

talked to a drive shaft shop today and he thinks it drive shaft/ carrier bearing related.... so got home and crawled under it to check stuff out....rear drive shaft looked fine so reached up to the carrier bearing and its new.... looked up towards the trannie and holy crap!!!!! its got another section of drive shaft and a second carrier bearing..... yeah,,, 3 sections of drive shaft@!!!! and the front carrier bearing is toast...

[davepl]

Originally Posted by davepl

I'd still give the U-joints and carrier bearing a once-over just to be sure. I've never had a carrier bearing fail but if the driveshaft starts dancing it'd feel pretty wild.

If that's the case this davepl guy is some kind of wizard!

JK. Glad you found the problem and hope you get it back on the road such that you're confident driving it!

[pinturbo75]

ok, new carrier bearing and it still shakes like hell.... im stumped...

[ho70]

Had a 91 GMC it shook at about 45mph, couldn't figure it out then my mechanic buddy took a drive and the rear diff was cooked