Strange shimmy

[altiain]

So, for the last week or so, I've had this odd shimmy in the front end of the car. It only turns up under braking from 50mph+, and if I hold the brakes steady, say from 70mph to 30mph, it goes away under about 45mph. Doesn't matter if I brake lightly or at the threshold, it still shows up, but only in that speed range (50mph+).

My first thought was that it was a warped rotor, but it only happens above 50mph, and I can roll the car along at 5mph or so and lightly apply the brakes, and I don't get the "pulsing" feeling that usually accompanies a warped rotor.

I've got an old set of well worn stock rotors that I can remount to see if it is a warped rotor, but before I did that I wanted to see if any of you might be able to think of something else that could be causing the problem.

Any ideas?

[tailchaser]

Sounds like the rotor problem we have in the Civic...

Good Japanese rotors don't suffer much from warping, but the surfaces will develop high and low spots. If the high spot is small and "not very high," the pulsing effect (in the case of the Civic, severe enough to make you want to get out and walk when the brakes are applied between 70 & 80 mph) will only be felt where it creates a vibration at the right frequency to set up resonance.

I'd try the spare set of rotors.

[POS Racing]

Did you run a dial indicator on the rotors? I'd be interested to see how much run-out you may have to give you the shimmy!

[altiain]

Well, TC - you were right. It was the rotors. I replaced them today with my old OEM ones, and no more shimmy. I learned a couple of things from this:

First, I learned that you can make a remarkable improvement in how the car tracks over uneven pavement merely by tightening all four of the front swaybar mounting bracket bolts... instead of three of them, which is what I obviously did when I put the big front bar back on before the last autocross.

Second, when it comes time to replace your rotors, go OEM. The "Spec Miata" replacement rotors I bought from Adrenaline Racing lasted 8000 miles before they warped. In addition, they seized to the hubs so badly that I needed a 3-foot demolition bar and a 5-pound mallet to remove one of them, damaging it beyond repair in the process.

By constrast, my OEM rotors have 80,000 miles on them and they're still true (although they are scored pretty badly from track pads). Plus, I've only ever needed a couple of light taps with a rubber mallet to remove them from the hubs.

[channelmaniac]

Shameless plug time...

I've gotten over 25k on my Powerstop cross-drilled rotors with no warping or any issues on my 92 B&T.

Loved 'em after putting a set on my 97 Grand Cherokee which would warp rotors like clockwork... Now with 70k on the rotors... no warping. In the 120k before that we had the rotors resurfaced 3 times, replaced, and resurfaced again.

Raymond

[POS Racing]

Shameless plug time...

I've gotten over 25k on my Powerstop cross-drilled rotors with no warping or any issues on my 92 B&T.

Raymond

That would be nice but they would not be stock legal for autocross. I've had great luck with the Brembo OEM replacement rotors! Plus they are cheap!