Why did the car stop shimmying?

Since Click and Clack have gone to reruns only, I have no recourse but to ask the Dope.

The other day, my husband wanted to trade cars with me because his car had developed a serious shimmy (which he assumed as a bad CV joint, because the last time he had it in to the mechanic, said mechanic said the CV joint was on its way out).

Anyhoo, this is a 5-speed 1999 Subaru Legacy with about 120K miles on it. When I was driving it, I noted that it did indeed have a serious shimmy that could be felt strongly in the steering wheel. The shimmy seemed related to the drive train, in that it only shimmied when the engine was engaged.

I drove it about 15 miles and got it up to highway speeds of about 65. When I drove home a couple of hours later, no shimmy. Drove absolutely normally.

The next day, my husband e-mailed me at work to ask “What did you do? The shimmy is gone!”

Any idea what caused the shimmy and what made it go away?

The drive train is always engaged. Taking it out of gear, which is what I assume you meant, only disengages the transmission from the engine.

Yes, that’s what I meant.

It’s funny you phrased your question like that. The other day they played a show where the “Stump The Chump” section was about a car that shimmied, I thought maybe you were calling about that. They guessed that the steering box was loose. When they talked to her the seconded time, she told them that the mechanic said three of the four bolts that were holding it in were loose (missing?). He tightened them up and added some shims to account for the wear from it sliding around.

Perhaps there’s an off chance that the same thing happened with yours and maybe it got stuck in one place for now. If that’s the case, it’ll probably return sooner of later.

Shimmies that disappear can be caused by mud or other gunk stuck on the inside of a wheel. It unbalances the wheel until it falls off. (see “My Cousin Vinnie”)

See: Me.

On the way to Florida I spun out, through the (grass) median that divided the highway and decided that it might be a good time to pull over and sleep for a while (lack of sleep had nothing to do with it, but it was late and I was kinda shook up).
The next day I hit the road again and the car was kinda shaking. I stopped at a mechanic and asked them to balance the tires or align them or something. A few minutes later I wondered why they were rolling one of the tires out the back door. Turns out I had scooped up a bunch of mud and it was making the tire bounce a bit.
The steering wheel was shaking earlier in the trip so I really hadn’t connected the shaking now to the spin out earlier.

I’ve had this happen both with snow inside the rim and with a chunk of tar stuck to the tire.

They explain this in the movie My Cousin Vinnie. “You got mud in your tires”.

Did it do it when slow? Mud would only make it shimmy at speed.

I’d generally agree with the gunk in the tires theory, but that wouldn’t explain why it seemed to go away with the car out of gear.

Loose or broken engine mounts then?

Keep in mind that when you take the engine out of gear, you also take your foot off the gas so the engine isn’t at such a high RPM and the car starts slowing down.

My old Honda Civic started to develop a shimmy. It started at extreme highway speeds, and by the time I had it looked at, it had become annoying at 45mph.

Turns out to have been the steering arm on driver’s side. they are connected by a ball joint at the wheel end. This ball joint was wearing loose; the looser it got, the lower the speed at which the wheel would shimmy.