bradleyheathhays

Stock 96 XJ here and the old girl's had this problem ever since I bought her 11 years ago. There's a whump sound that matches exactly the rate of tire rotation and I have no idea what's causing it.

Here's the facts ma'am...

- It's been happening since before I got this recent set of tires so I know it's not an unbalanced tire.

- It seems to vary somewhat but is always present

- At speeds under 10-15 you can sometimes feel it in the seat with the truck body bobbing slightly up and down in rhythm with the sound

- Does not translate to the steering wheel

- I just got all the wheels off the ground w/ jack stands, first just the rear then all 4, had it in both 2 wheel and 4, gassed it to get the tires spinning and didn't hear or feel any whumping at all.

My buddy that I bought her from and a couple other yahoos loaded her down and went tearing through a field one fourth of July, you could imagine, and ended up bottoming it out on a pile of cut logs. Best I can recall, and by that I mean not being able to recall anything at all, I think those logs we had to yank out of there were jacked up under the drive shaft. I thought that might have something to do with it, but talking with him recently he says he didn't notice any problems with it the next day so now I really don't know.

Any ideas what this could be or maybe what I can do to start testing/examining parts for the problem?

TrailerTrash

My guess would be a bent wheel

IJM

That was my first reaction as well. Are you running steel or alloy wheels?

'90Cherokee

A bent wheel should have been spotted by the tire shop, but maybe not. A bent axle, which is what I'm guessing it has, they wouldn't have noticed. When you had it on stands & the tires spinning did you get out & look or just sit in the jeep? If you didn't get out & look that's what you need to do.

If it were the driveshaft the speed of the sound would be much faster than the tire speed, 3.55:1 being the most common.

bradleyheathhays

I believe they're alloy wheels but not entire sure. They're the 5 thick spoke wheels that come in either lighter brushed steel or a light gray color.

Little confused about something though...concerning it being an out of balance wheel. When a wheel balance is done, would it matter whether it was the wheel or tire that was causing the out of balance state? Once a weight is put on I would think the entire thing as a unit would be balanced.

Yeah '90 looking back I should have got out and had a close look at the wheels. Next step is to get it off the ground again and do that.

On the bent axle idea... now would that be a bent axle shaft where I could just put a new one in, or are we possibly talking about a bent tube in which case I'd probably have to replace the whole axle?

Tbone289

A bent wheel and an out of balance wheel are two different things.

Bugout4x4

Yep, this is what it sounds like, and it doesn't take much bend at the axle to make a huge difference at the tire/road contact patch.

Your point about the ratio difference in driveline sounds/vibrations and wheel sounds/vibrations is something very important to help judge this stuff!