Brake Chatter

[weirdjim]

Probably should have *****ed this problem with the dealer, but didn't.

New (to me) '99 OB Wagon has a chatter when braking between 30 and 40 mph. The chatter starts at 40, peaks at 35, and diminishes as you get below 30. It had the problem from the getgo, but seems to be getting worse.

The local high school auto shop mech miked the rotors a couple of months ago and found the front starboard rotor a few tenths of a thousandth out of limits, so we replaced both front rotors and both front padsets with no change in the vibration whatsoever.

(FYI, spec on runout is 2.8 thou, we had 3.2 thou on the old rotors and the new ones measured 1.2 thou after installation.)

The problem is with light to normal braking only; if you really clomp the binders, it doesn't seem to vibrate as much.

Any suggestions on where to look next?

Jim

[weirdjim]

Sorry, that's a '98, not a '99. The Miata is a '99.

Also, didn't know we had a naughty word filter on the machine or I wouldn't have used the perfectly legitimate word for a female dog in the post.

Jim: 9:

[rockhopjohn]

Well, It never hurts to have your new rotors turned, often times they can be a little uneven, but I doubt that is the problem. I had that same problem one time when the shop that did my brakes didn't torque the calipers properly and the caliper bolts loosened up.

[The_Lizard]

*G* I was tempted to edit your post to * out "female dog" just to mess with your head...

[weirdjim]

Originally posted by rockhopjohn
Well, It never hurts to have your new rotors turned, often times they can be a little uneven, but I doubt that is the problem.

As I said, we miked the rotors after installing them and they came out less than half of the allowable runout. I don't think I can get them much better with another turning.



I had that same problem one time when the shop that did my brakes didn't torque the calipers properly and the caliper bolts loosened up.

I personally torqued them with the spec book in one hand and the torque wrench in the other. The shop teacher was looking over my shoulder as I was doing it.

[weirdjim]

Well, here we are a year later and the problem persists. It hasn't gotten better, it hasn't gotten worse, but it is really annoying.

One clue that has surfaced (didn't notice it last year, but on the Alcan, you don't notice minor vibrations) is that there is a constant very light vibration in the steering wheel when you get between 60 and 70. Since that is almost exactly double the speed where the brakes start to give trouble, I thought it might be a resonance or a harmonic of some sort.

Thoughts appreciated.

Jim

[porcupine73]

Subaru's position on your issue (per TSB Brakes - Vibration Diagnostics NUMBER: 06-32-03 DATE: 11/01/03 APPLICABILITY: All Models; All Years) is to

"inspect and resurface rear brake rotors. If out of specification, replace."

This is based on:
Can vibration be felt when applying brakes above 50 mph? Answer: no
Is vehicle equipped with rear brake drums? Answer: (I'm pretty sure is no)
Can vib be felt ... around 37 mph (Answer: yes)
then......"inspect and resurface rear brake rotors. If out of specification, replace."

Also, there is something to lose when having rotors turned. That is rotor mass. The rotor's ability to absorb heat is reduced as its mass is reduced.

[weirdjim]

Porcupine, many thanks. It looks like that nail is square on the head.

I searched all over ****'s half acre for this information and you found it quite simply it seems. How did you do that? I looked at the maintenance boards and forums here, googled it, and came up dry.

Gimme the secret if you are willing to share.

Jim

[porcupine73]

I have an alldatadiy subscription, which has many of the Subaru TSB's. I read through all the TSB's before because they're great troubleshooting tools and a lot of times they address common issues. I recalled seeing one for brake vibration diagnostics.

The TSB is Brakes - Vibration Diagnostics NUMBER: 06-32-03 DATE: 11/01/03 APPLICABILITY: All Models; All Years.

The TSB had a troublshooting chart, which I have attached.

Another good research tool is endwrench.com. click archives, then enter a search term. They have articles on many common Subaru questions and issues.