Vibration Diagnosis

Diagnosing the vibrations in my Jeep

For a while now I have had severe vibrations in my street Jeep. I have corrected several problems like a bad front pinion bearing and a drive shaft U joint. However the vibrations have persisted.

Every time I drive, I get a shimmy at 50 to 55. On the interstate I can’t go over 70 as it feels like the whole rear of the Jeep is coming off the ground.

I got a new set of BFG AT’s thinking my old worn ones may have contributed to the vibrations. However if anything it made the vibrations worse.

I started with the vibration diagnosis chart in the Jeep Shop manual. Everything there pointed to the tires. But since I had new tires I discounted that. I put the tires back on my bubble balance machine just to make sure but they read fine.

I then set to looking at all the U joints, wheel bearings, ball joints, drive shafts and any thing else that moves with the suspension. I even changed rear springs thinking maybe my sagging springs could have disturbed the pinion angle.

I swapped drive shafts and installed new shocks. The only part not changed was the steering stabilizer.

Last night I decided to look for a bent wheel. I set up my dial indicator on a jack stand and read the rim. It had about .020 maximum run out on any wheel. I figured that was not too bad.

I also checked for out of round on the rims. They all read pretty good.

I also put the indicator against the sidewall of the tire. It was difficult to read due to the moldings on the tore but they seemed to be fine as well. I never figured out a way to read the out of round of the tire due to the tread pattern of the AT. There is no clear path to rotate the tire and not have the indicator fall into the tread.

I also checked the Vibration Analysis chart on the Tire Rack web site. Their chart also indicated a tire problem.

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I happened to have another Jeep in the shop that had a fresh set of Goodyear tires that had been dynamically balanced. I decided to put them on and just see.

At 55 it was smooth as could be. Not even a hint of shimmy. I made a quick dash to 75 and it was smooth there as well. No pounding like I have experienced with the new BFG’s.

While I am glad I have found the problem, I am pretty disappointed with my new tires. This is the second set of BFG tires I have dealt with that had a dynamic balance problem. The ones I put on my daughter in law’s Jeep were BFG Long trails and they will not run smooth at high speeds. I have not had that trouble with Kumhos or Michelins.

I guess my next step will be to try a liquid balance on the tires. I really hate to spend $50 or more to get them dynamically balanced knowing that I will have to keep doing it as the tires wear.