I have a horrible vibration at high speeds on my 72 Sport Custom Longbed with 360/C6 combo. Anything over about 60 mph shakes the bed, the hood, and the doors so much that it's embarrassing to drive at highway speeds. Below that speed it feels and drives awesome. It seems to randomly get weaker and stronger but it is always there. I can't really get it to change consistently based on whether I put the power on, cruise, or decelerate. I can barely feel it in the steering wheel but I can really feel it in the seat of my pants. I just finished having the wheels and tires balanced. I just re-installed the rebuilt and rebalanced two-piece driveshaft complete with a new carrier bearing and all new U-joints. The U-joints are all in phase and greased and the slip-joint is tight and greased too. It honestly feels just like unbalanced tires until I get up to about 70 mph and then it feels the truck is going to shake itself apart.

There is a rythmic thrum, thrum, thrum sound there but the big, violent vibration feels like it's from a different source. I'm 99% sure the thrum, thrum vibration is a worn rear axle bearing. Those will be replaced as soon as I can find a decently priced 9" with 3.25s and a posi. What else could it be?

Next week, I'm planning on replacing the front axle pivot bushings and radius arm bushings because they are completely worn out. Could those be setting up some sort of vibration? Is there a chance the guys balanced the wheels incorrectly? What else should I check? Is there a chance that one of the rear axles is bent? I would think I would feel that at lower speeds. I'm looking for some ideas here, because I'm pretty much out.

 

You know the shake is in the seat , Not the steering wheel from The trans back is where to look ..Can you put it on jacks and let it run ? My F-100 some how had a bent axle between the third member and the wheel bearing ??? My trk would shake at 70 MPH and over then coming through town a guy driving beside me shouted that my right rear tire looked like it was coming off the trk ..
Thats when I found the bent axle ....

 

Just a thought but I found that a broken belt in a rear tire can set up a shake that seems to be coming from the front . Be sure to check all of the tires to eliminate a freak tire problem

 

I'd chock the front tires on a level surface and put the rear axle on jack stands. You could examine the tires and wheels while they spin. It could be a bent wheel, but I'd look at the tires very closely.

 

Process of elimination that will not cost money but some time, rotate tires front to rear and test again. If it changes from a cold tire to hot like highway normal temps it sounds like a tire may be delaminating. Careful crap tires can get you or others killed. At the speed you stated it sound likely to be tires unless the rim is bent with a flat spot. You rotate tires to check this? Even with shot radius arm bushings if tires are balanced you should have a smooth ride.

 

I've seen an out of round tire(brand new)balance up with only a little extra weight. Many times you get some kid running the balancer and as long as the weight readings magically go to zero they don't care if the balancer is shaking. I agree with others on the broken belt, but don't rule out an out of round tire. There has been the odd case of a tire being so out of balance that the cycle of vibration no longer seams to come and go. Do you trust your tire guy? Take it back down to the tire shop and let them find the problem. They have the tools, let'um figer ot haw ta use um. Let us know what you find.

 

I had a "Goober" place wheels (8 lug) with 9.50 and 12.00 x 16.5 tires on a adjustable sloppy linkage spiders then tell me the steel rim was bad as the tread jumped 5/8". Well so did the bolt circle as the spider was a worn out piece of crap. Them said "Goober" went to a cone that hit on the flashing in the wheel center which didn't allow for proper centering, again jumping wheel assembly but not as bad. I got the wheels and left. Mounted wheels on front hub of truck spun they spun like a top. With the help of a friends work lathe, spare hub assembly with bearings and machined axle to fit he removed the punched flashing on all 4 rims until they became concentric, then remounted tires and had them balanced.

New balanced tires will wear round and become slightly out of balance by app 5,000 miles (my guess).

With the static balancer I built for motorcycle wheels, balanced disc brake hub and axle slight imbalances can be corrected plus when mounted on a truck front hub spun up with one of those old "shoes" to verify perfect balance.
Sorry long story and maybe off track.

What got me started in rebalancing was years ago with Uniroyal's Ameri-Steel 9.50 x 16.5 tires that wouldn't hold a balance, problem they delamimated (3 out of 4) the company had a problem they wouldn't correct or even talk to me about as I complained many times until I took videos of the jumping treads and one huge blister plus the rebalance report a different tire shop made on the defective tires.
Enought in court for the judge to award me 100% money back plus all rebalance charges. Got smart and run Michelin's which take very little to balance and run many miles longer that lower cost tires.

Back to your problem, any tire that has a thumping, wop wop sound beyond tread singing is a defective tire that will only get worse and go out of balance until failure strikes. Have you tried to have them what they call "load balanced", I bet the operator will hear that noise under load and verify delaminating and pending failure?
All these tests aren't free so keep your bills should it end up in small claims court as tires are a big safety issue and investment.

 

My assumption was that a bad/delaminated tire would not balance correctly. I'm not really getting a thump-thump sound as much as the violent shaking. I'll jack it up and run it that way and see what I find. My plan is to put new rims and tires on her anyway, so maybe I'll put it in the garage until after Christmas. That'll give me a chance to fix the front end and replace the rear end and bearings.

 

It's normal to have some vibration when your near or approaching Mach 1, when you break thru it's smooth sailing again.

On a delaminating tire you'll chase your tail as it will change balance between the tire getting hot from cold besides the growing delamination until a total failure. Not something you want to play around with.

If there is a lot of tread left you should get a replacement almost free minus your percentage of tread useage.

Question; does this brand and style of tire have a track record of failures of delaminations? If so I would switch brands.

 

All good, sound advice. Sometimes it does'nt pay to dink around trying to fix something that is not repairable. I used to run recaps in high school to save a buck. I would'nt touch them now, unless the rig never hit the highway. Can't believe I never peeled one off at 135mph. Silly kids!

 

have our tires rotated and balanced, front to back, and have the tire store check for seperation and bent wheels. i work at a local tire shop, and it usually costs about 20 bucks, but well worth it.

 

78Ford; they wanted $15 a wheel to just balance in my area 5 years ago for 9.50 front and 12.00 rear x 16.5's even when I roll all 4 to their wheel balancer. They call them "big heavy truck wheels" hence their higher price.
Do my own thank you even though it may take longer.

On another vehicle:
The tough one is just breaking the beads alone on Rover's Disco alloy rims, they act like a 16" tire stretched on a 16 3/8" rim deep safety groove. I use my friends home made hydraulic powered "C" clamp bead breaker he build for his 18 wheeler.

 

I already spent $70 for a "lifetime" balance at Goodyear. Tonight I'm going to change the third leaking power steering pump in three weeks, then I'll jack her up and run it on jack stands.

 

Those remanufactured pumps or used pumps?
Best to have a quiet pump that isn't scored up inside and rebuild it with a kit that includes the bushing.

 

Remans. I wanted to go new, but they've been free since the warranty was still active on the one that my dad put on it when he had the truck. I knew that buying the crap Cardone ones was going to be a crapshoot with really bad odds, so I have gotten what I paid for...nothing.

The one that was on it started leaking at the low pressure return fitting where it goes into the reservoir. The first replacement had chatter on the sealing surface of the pressure fitting so it drained the whole system before I got two blocks away. That was a rookie mistake. I should have checked the fitting before I put it all together. It didn't leak until I started turning the steering wheel and building pressure. Now the second replacement is leaking from the low pressure return fitting again. Maybe I wasn't careful when I was installing the pulley on it...or maybe it's that outstanding Cardone quality rebuild. Oh well, I'm getting to be a pro at changing the PS pump now. If this one doesn't work, I'll actually go spend some money on it I guess.