TJ Vibration: rear axle bent?

OK - I may have finally found the culprit.

I pulled the rear passenger axle (the vibration always felt like it was coming from the passenger side). First off, the bearing wasn't pushed up against the hub like it was on the driver's side -- it has a good

3/4" to slide back and forth on -- that would probably explain the weird side-to-side shake I feel every now and then.

More importantly, when I held a straight edge up to it, there is about a 1mm bend in the center of the axle. When I hold it on the opposite side, I have a 1mm gap at the top and bottom of the axle.

Could a 1mm bend in the rear axle cause violent shaking? Is that big enough to be the problem I've been hunting down for the past six months?

Reply to
Justin M
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It isn't 1 mm at the edge of the tire.... remember the calculations for leverage from physics class and since the wheel is rotating we have to factor in constantly changing vectors of acceleration.....

Reply to
billy ray

Hell yes. Same effect as a bent wheel, depending on where the bend is along the shaft. The closer to the hub, the worse it will be. If you hadn't gotten it apart yet, having a buddy drive alongside looking for the wobble would probably have given it away. If it's that much of a gap, you should see it if you slide the axle back in, mount a wheel and spin.

Reply to
Outatime

Wow.... That's a lot of work to find a bent axle...

That should show up in the wheel if you jack it up and spin it, more than enough to vibrate.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail >
Reply to
Mike Romain

The thing that's interesting about this is it doesn't show up when I do that. It's technically straight -- but there's a bend in the middle. The tire doesn't wobble, but you definitely can see it in the axle itself once it's out of the tube and slowly rolled. (I put it on my workbench and rolled it, and it bobs up and down).

Now I just have to find someone local with a hydraulic press. :)

Mike Roma> Wow.... That's a lot of work to find a bent axle...

Reply to
Justin M

The axle forging could be bent, but if the ends have been machined straight, you wouldn't have a problem. I don't think I've ever seen one like you describe though.

Earle

Reply to
Earle Horton

Hi Justin, You need to have the axle checked to see if the two ends run true to each other. It is possible that the ends run true but the middle runs out. We see this all the time in our machine shop. We take a slightly bent piece of shafting, put it up on centers in a lathe, and turn the ends true to each other but the middle will still run out. If it turns slowly in it's application, it's fine. If it spins fast, well, that's another story. If you have a shop locally, they should be able to put it in a lathe and indicate the axle to see what is really running out. If the wheel doesn't run out and wobble, I would look for something else like tire balance (it has been rebalanced?) or a worn suspension part. Just my two cents. Les '01 TJ

Reply to
Les

I have. Steve Seppala from this group did it to his YJ. There was runout on Steve's. I'll bet there is a slight runout on his too which could be the vibration. Even just the shaft bend could set up a harmonic at a certain rpm.

Mike

Earle Hort>

Reply to
Mike Romain

Using some creativity (and an extra set of bearings), I was able to hook the axle up to a drill and secure it down to my work bench. At low RPM's, it is fine... it spins nicely, the hub doesn't wobble.... hit about middle speed, and the axle starts to wobble the work bench -- but the hub is still spinning but not moving up/down. A little faster, and it started to resonate and finally broke free from the mount I built, almost leaping off of the bench.

After getting my (much more math inclined) roommate to crunch some numbers (it's nice having an engineer around sometimes), we determined it starts moving around 50mph, and at 63mph, it starts jumping around.

I imagine the 35" tires are keeping the wheel on the ground due to the weight ... but eventually the vibration gets bad enough where the wheel starts to bounce.

Man. This was a long trip to get here.

The first time I looked at the axle, I didn't notice the warp. I was using a piece of wood that I thought was straight. I finally went out and bought a true straight edge, and it was obvious.

At least I got a lot of parts replaced that probably needed to be replaced anyway (front bearings, u-joints, disc rotors and drums).

I've ordered new axles and bearings -- hopefully they'll get in this week -- and then I'm off to find someone with a hydraulic press to push the bearings on. And hopefully this will be the end of this adventure.

I'm also going to write a nasty letter to Chrysler about their "five star" dealer that charged me $320 to find the vibration, only to tell me it was the lift's fault. I fail to see how a 3" lift/35" tires could warp an axle. I would expect it to be bent -- not "sag" in the middle by a millimeter.

Finally, on the positive side... I now know tons more about the suspension, drive-train, and steering system of the Jeep. Before I started looking into this, I knew next to nothing. :)

j

Mike Roma> I have. Steve Seppala from this group did it to his YJ. There was

Reply to
Justin M

No pain, no gain??? :)

Reply to
RoyJ

It's a Jeep thing. ;)

Reply to
DougW

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