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  #1  
Old 08-10-2008, 12:20 PM
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Non-MB tech issue: Front wheel shimmy on high speed braking

On my '87 BMW 325i, now pushing 280K and running quite well, engine just a purrin' along.

I've renewed a few parts in hopes of solving this -- new front bearings, rotors and pads, as well as new rear pads, and new front control arms.

I put new tires on the front a few days ago. All of these moves seem to offer some improvement but it's still there. I'm wondering if there's some age in the struts.
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  #2  
Old 08-10-2008, 12:26 PM
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I'd suspect rotors as the first culprit...........and, yes........I realize they are new. With the general lack of quality control these days, it would not be all that surprising to find one with a runout of .003" on the face. That would cause all your problems.

Have an indicator that you can put up against them?
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Old 08-10-2008, 01:49 PM
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I second that, just because theyre new, don't rule them out, where'd you buy them?
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  #4  
Old 08-10-2008, 02:17 PM
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On E30 BMWs, this always seems to be a toxic combination of control arm bushings and brake rotors- the rotors are the real culprit, and the bushings amplify it once the shimmy starts. But if you've done the bushings then i think the rotors are to blame. Either setup a dial gauge to check runout or get another set of new rotors. I use only Zimmerman, Balo, or ATE parts on these brake systems.
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  #5  
Old 08-10-2008, 08:23 PM
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bushings

Quite some time back I had an E32, 1988 735. It wasn't the rotors. It was some combination of bushings in the front end.
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  #6  
Old 08-10-2008, 08:25 PM
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Sturdy little cars though...I hear shimmy complaints on BMW forums on all sorts of 80s and 90's era BMW's, the only one I had a problem with was a 1988 735iL that had so many other issues I never got around to chasing down the shimmy.

My 325ic with 220xxx on a broken odometer doesn't shimmy at all, but needs front shocks desperately, it floats around like an old Buick...any thought on a good brand for shocks on the little beast?
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Old 08-11-2008, 03:36 AM
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Quote:
Originally Posted by Brian Carlton View Post
I'd suspect rotors as the first culprit...........and, yes........I realize they are new. With the general lack of quality control these days, it would not be all that surprising to find one with a runout of .003" on the face. That would cause all your problems.

Have an indicator that you can put up against them?
Oh man, I hate to say it, among all the auto tools I've bought and rarely use, my dial indicator with the flex/locking chain attached to vise grip somehow got away from me.

I've been debating buying another, found one locally for $200. I'm not sure how much the eye can see -- when I had the tires off, I spun the rotors and damn they looked straight. Not too scientific I know.

This shimmy feels like maybe the tie rod is not holding the wheel securely enough, like the wheel is whipping back and forth a small amount, but more violent than merely a light bump-bump from an out of round rotor.

I also put in new inner tie rod ends a year ago because of my parts guy's recommendation to cure a creaking sound in the steering. It worked.

I hope my rack is not bad, as they come with inner tie rod ends, I'm pretty sure.
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Old 08-11-2008, 03:36 AM
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Originally Posted by Johnhef View Post
I second that, just because theyre new, don't rule them out, where'd you buy them?
I have a discount OEM parts guy in Berkeley. I forget the brand, I can check my records later when I'm back in Cal.
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  #9  
Old 08-11-2008, 08:51 AM
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Quote:
Originally Posted by cmac2012 View Post
This shimmy feels like maybe the tie rod is not holding the wheel securely enough, like the wheel is whipping back and forth a small amount, but more violent than merely a light bump-bump from an out of round rotor.

.
Sure, if the tie rod is the problem, it will cause the front end to shake at discrete speeds, but this is going to occur without brakes.

Once the issue occurs during braking actions, your possible culprits are usually limited.

BTW, you cannot eyeball .004"............

Also, Harbor Freight has some inexpensive indicators. The mounting arm is probably as much as the indicator.
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  #10  
Old 08-11-2008, 09:29 AM
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Brake shimmy is like the bane of my existance. Seems like everything I own has had it at one time. One thing that gets over looked is the caliper mount. Many calipers are of the "floating" type and have some mechanism to allow them to move on their mount - either mounting on long pins or sometimes they slide onto the steering knuckle.

Anyway, when these mounts get a little rust on them, the caliper can't effetively slide and every movement is translated into the piston and the fixed assemblies. If you compress the piston completely with the caliper mounted, you should be able to grab the whole thing and slide it side to side by hand. You might have to take the pads back out to give some room. It should be kind of tight but most of the time you can move it.

Try taking the caliper apart from it's mount, go after it with a wire brush and put a dab of wheel bearing grease on the sliding surfaces/mounting pins.

Sorry for being vague but most all calipers are different.

Last edited by KarTek; 08-11-2008 at 09:57 AM. Reason: Updated, more accurate info.
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  #11  
Old 08-11-2008, 09:38 AM
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Originally Posted by KarTek View Post
One thing that gets over looked is the caliper mount. All calipers have some mechanism to allow them to "float" on their mount - either mounting on long pins or sometimes they slide onto the steering knuckle.
This is true only for floating calipers. All M/B calipers, to my knowledge, are fixed calipers. They are rigidly attached to the steering knuckle. Any runout in the rotor is immediately transferred to the steering knuckle during braking.

I'm not sure about BMW's..........but, I'd suspect that were fixed.
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  #12  
Old 08-11-2008, 09:53 AM
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Interesting, I've never run across anything but floating... I've only had my car about a year so I haven't had occasion to take the calipers apart. I'm due to rotate my tires so I'll have a closer look at it at that time. And, I changed my previous post for more accurate information.
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  #13  
Old 08-11-2008, 10:01 AM
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Originally Posted by KarTek View Post
Interesting, I've never run across anything but floating... I've only had my car about a year so I haven't had occasion to take the calipers apart. I'm due to rotate my tires so I'll have a closer look at it at that time. And, I changed my previous post for more accurate information.
Floating calipers are cheaper...........they use a single piston on one side of the caliper..........so, the caliper has to "float" over toward the piston to allow the opposite side pad to contact the rotor.

The more expensive vehicles use a dual or quad pistions.........one or two on either side........for better braking action.

The fixed calipers with the dual or quad pistons are far more sensitive to runout of the disc.
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  #14  
Old 08-11-2008, 10:07 AM
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OK, now I remember years ago I worked on a car that had a setup like that and I accidentally removed the wrong bolts and split the caliper. There were O-rings and passages to allow the brake fluid to pass to the outside pistons.
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  #15  
Old 08-11-2008, 01:00 PM
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Did you tighten your lug nuts in the correct pattern with a torque wrench? If not, see if it helps.
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