Noise and Vibration at 50-55MPH?

[TwoFiftyShifter]

I bought my '87 last week and have driven her about 50 miles or so, and the truck makes a lot of extra vibration and a little noise at exactly 50-55MPH, and at no other speed. You can hear it a little and feel it on the pedals and seat, not on the wheel. U-joints are good..... What can it be? Normal possibly?


Also, my seat is horrible..... middle and passenger are fine (bench seat) but the driver portion feels like I'm sitting on a lunch box. Should I just find some foam and stick it under the seat cover?

[TwoFiftyShifter]

Also the vibration is a lot more prominent decelerating than accelerating.

[78 Big10 454]

My first thought is tires/wheels. Are they all the same size? Properly inflated? How old are the tires? If the truck sat for awhile, the tires could be prone to cupping or developing a flat spot. You can decipher the age by looking at the sidewall. The DOT identification number is usually 12 characters long and includes the date of manufacture on the end. So a 1501 would specify that the tire was made in the fifteenth week of 2001. It could be a issue of tire balancing and/or a bent wheel. Eliminating the U-Joints was a smart move. Next to check are the wheel bearings. It might be worth the $$$ to have a reputable alignment shop check out your front end components. You never know what the PO may have done to your ride before you bought it, especially off-roading. Even a build-up of mud and gunk can cause a front-end shake.

Unless you decide to reupholster or replace your existing bench seat, head to your local fabric shop, get a piece of foam rubber, and build up the padding. I had the same problem and suffered through years of being poked in the behind by the seat springs. One good thing is I never fell asleep at the wheel!

[TwoFiftyShifter]

Yeah thanks, I greased the front end and check the ball joints, not too bad, but I figured it could use an alignment. It wasn't used for much offroading, just as a work truck, and the tires seem to be actually fairly new.

Thanks. Anyone know ballpark what an alignment costs at a shop?

[Keith Seymore]

My first thought is driveline.

The reason I say that is because with tires & wheels usually once they start to vibrate they continue to get worse the faster you go; that is, if they start at 50 the vibration will keep getting worse on up to 55, 60, etc. That's because they have a lot of relative mass to drive the imbalance.

Driveline, on the other hand, doesn't have as much energy so it will typically just excite a resonance. So it will cause a vibration to build, and then subside, as you pass through that point.

Driveline is also torque sensitive - you could feel it more accelerating or decelerating (typcially due to bad u joints or improper u joint working angles). Tires are not torque sensitive - they would vibrate the same regardless of throttle position.

Driveline also drives the energy up through the suspension and into the floor and seat, where you would feel it. Tires (front tires, mostly) you would feel in the steering wheel. You can't "hear" a tire imbalance (because it's too low a frequency, outside of your hearing range) but you can hear a driveline disturbance. It will manifest itself as a low frequency "boom".

I'd play around with the driveshaft a little bit. Possibly rotate it 180 degrees in the pinion yoke, or add some hose clamps/weights randomly and see if that changes it; maybe take one more good hard look at those ujoints.

My $.04 -

K

Edit: One more thought - work on that seat and see if it makes any difference in that vibration. By it being loose, it could make you more sensitive to feeling whatever disturbance might be coming up through there.

Edit: One more "one more thought" - there are some occasions where this is normal. If a driveline disturbance happens to line up with another distubrance, say engine firing frequency, then you will get a "boom beat" (where you can hear the low frequency come in and out). We built that into our M vans, where the torque converter would lock up in a range where it excited the driveline (around 30 - 35 mph). There's not much you could do there other than (a) drive in a different gear or (b) change rear axle ratio. I doubt that's the case here.

[Tony_SS]

I owned a car that had been sitting for awhile and it has flat spots in the tires and they vibrated around 50-55 like you described. When they heated up the vibration went away.

[Keith Seymore]

One more thought: if it is driveline then you should be able to duplicate it without going down the road.

Put the rear axle up on jackstands and try running it in the driveway. Run it up to speed and see if the vibration is still there. You can do this without the rear wheels on, too (just put some lug nuts on so that the drums don't come off.

If it's still there then that's pretty conclusive. If it is not still there then you know to look elsewhere.

K

[TwoFiftyShifter]

Thanks for all of that Keith, you really went into detail on my problem. Yeah it happens at 50-55, and is gone at 56mph and doesnt come back, I've been up to 75mph and no viration. I'll jack the rear end and try it. Thanks so much.

[Oilbrnr]

Has the truck been lifted? Sounds like driveline to me. Can you feel the buzz in the 465 or TC sticks?

I just had my K5 rear DL fitted with a Spicer double Cardin (CV) joint and the 50-55 mph buzz is gone. Then again, my rear pinon points pretty much dead-on to the TC after the shackle flip.

[68 TT]

I had one that did that. It needed the wheel bearings adjusted.