Topic: shimmy

[arkypyro]

My 97TJ just recently developed a sporadic shimmy.  I have 32's with a 2 inch terralift.  Since the shimmy started I replaced the steering shaft(all the u-joints were shot), had it on a lift and all tie rod ends, knuckle, trac bar, and steering damper appear good according to the guy at the tire shop who would have sold me a repair job if he could have.  The steering damper is only about 6 months old and when pulled loose at one end is still good.  My tires still have about half the tread left, and show no signs of cupping or uneven wear.My shock bushings were shot but I replaced all my shock this evening.  It drives straight with no pull and runs straight while braking.  On the way from my shop to the house the shimmy/wobble showed up as bad as ever.  As I said, it is sporadic.   It seems to manifest itself at speeds above 45 or so, usually needs a small bump to set it off.  Does often go off in a long sweeping left turn but never in a right turn.  Sometimes starts while braking and braking never helps just makes it worse.  the shimmy is very violent, jerking the steering wheel left and right.  with the front end jumping around.   I, all my buddies, the manager at Auto Zone(my cousin), and the guys at the tire shop are all stumped.  If anyone can offer any advice it will be appreciated.

[getstucksome]

it's called bumpsteer.  it has to do with your steering and track bar angles.  it they're off (un-arallel), your axle fights the steering over a bump and you get the resulting "shimmy"

you can fix this with a drop pitman arm usually.  

if i'm wrong in my description of bump steer, someone correct me

[Jeffy]

Well, since he's got a TJ and I'm guessing the shimmy occures long after the initial bump in the road.  I'd do some searches on TJ 'Death Wobble'.  I forget the particulars but it's common on coil sprung Jeeps that are lifted.

Oh and bumpsteer occures because the distance between the tie-rod and drag link aren't parallel.  As the suspension compresses, the distance between the point on the tie-rod and the pitman arm reduces.  Since the drag-link is unable to collapse, it pushes the tie-rod right.

If you happen to be holding the wheel tight, it will steer the Jeep right because it tries to turn the steering wheel to the left.

As for the drag-link and the trackbar, they also need to be parallel.  Otherwise they will bind since they are both connected to similar locations are  so they need to swing in the same arch.  The trackbar helps reduce bumpsteer buy preventing the suspension from moving too much.  This keeps the distance between the tie-rod end and the pitman arm close to what it should be.

[black99tj]

This happened to me only on my old grand cherokee, same exact axle set up though with the coils front/Rear. It was like you said, only on a left turn. At high speeds it would never do it. only like 30-45, after that it would not happen. The cherokee had no lift, stock tire size, and no steering modifications. It did have 165,000 Mi. on it, and i'm sure things were worn out. I sold it and bought a TJ, and have not had any apparent problems so far with the steering. What everyone has said makes sense. I would jack the front end up and look for play in anything you can find.

[Bounty Hunter]

It's not Bumpsteer, sounds like early signs of death wobble.  Bumpsteer is the cycling of the steering wheel to the side as the front suspension compresses and extends and is causes by what Jeffy said.

Here's the answer I gave you on another forum:

95% of the time I've found it's usually a combination of bad front track bar and bad control arm bushings. Have somebody turn the steering wheel back and forth off of full-lock while you watch both ends of the track bar for movement.

Inspect the control arm bushings for tears with a flashlight and a screwdriver.

Both are very likely on a 9 year old jeep.

[arkypyro]

I have not had a chance to re-visit my issues in a few days.  I will get to it this weekend and report my findings.  Thanks for the input.