From neilmaller_at_postoffice.worldnet.att.net
Subject: Re: [M3] E36 Suspension Failure...on going (LONG!!)
Date: Thu, 25 Jun 1998 16:52:54 -0500
From: Neil Maller <neilmaller_at_worldnet.att.net>
cc: <M3_at_Mailing-List.net>

On 6/25/98 3:09 PM xxx quotes his friend's message:

>I have the infamous rear end failure (rear toe bushings or upper trailing arm
>failures or shock mount failures). The car sways to the right on
>acceleration
>and then back left when you let up on the gas pedal. Plus, you hear a clunk
>from the rear end when the body flexes, at low speeds, like when entering a
>gas station driveway, inclined, at right angles.

Whoa! Let's stop a moment before this goes off in too many wrong directions.

There are 3 suspension members in the rear:

• The upper control arm, which goes from the rear subframe at the final drive to the wheel upright and carries the rear spring;
• The lower control arm, which also goes from the rear subframe to the wheel upright;
• The trailing arm, which is part of the rear wheel upright and extends to just forward of the wheel well, where it mounts to the body. (The shock mount has nothing to do with any of these, although a low speed clunk is characteristic of the upper shock mount rubber mounts failing, a simple repair to do.)

The major failure in question concerns only the trailing arm mount.

I took this topic to the Performance List a week or two ago, and have also had some discussion with David Finch privately. Here's the situation as I understand it so far.

The trailing arm has a rubber bushing - sometimes referred to as the toe bushing - at its forward end. This bushing mounts to a U-shaped bracket, which has a broader mounting plate welded to the horizontal part of the U. The mounting plate has 3 holes, through which bolts are threaded into 3 cylindrical steel tubes spot welded to the body sheet metal. The failure apparently occurs when one of these tubes tears its welds. The remaining tube attachments then become over-stressed, and tear out of the surrounding sheet metal. You can see the ugly results at <www.njbmwcca.org>, click on Scrapbook.

Repair is difficult and very expensive once the sheet metal is torn.

One possible mode of this failure is as follows: the toe bushing wears out and disintegrates. The trailing arm bolt then hammers directly on the mounting bracket, and the resulting shock loads cause the weld failure and sheet metal tearing. 96 and later cars have a heavier duty toe bushing (P/N 33 32 2 228 153) which can be used to replace the 95 part.

Toe bushing failure ought to be apparent from inspection: there should be no free play in that area. You may also hear a knocking noise. However the toe bushing is recessed into a cupped area of the body, and not readily visible. Replacing the toe bushing itself is relatively simple, 3 bolts each side, use a puller to remove old and install new bushings.

However cracking of the mounting tube welds isn't that easy to see, since they're covered by the bracket plate, and also have underbody coating on them. I think it would be hard to find beginning cracks without at least unbolting the trailing arm and mounting plate, and putting the car up on a lift. It may be possible to reweld (TIG) them if no other damage has yet occurred.

All but one of the failures involves cars with modified suspensions, i.e. higher stress. Only one failure is know so far on a post-95 model year car, but that may simply be a function of age and miles, not the improved rubber bushing.

The best conclusion I can draw at present is that if you have a modified suspension and/or track your car, inspect the trailing arm toe bushings for play every time you change your track wheels. The older your M3 is, the closer attention you should pay to this area. If your car has more risk factors, consider having the toe bushings changed to the later version. If you have local access to an experienced performance shop like Dinan, have them inspect the weld areas for you.

I know I'll be keeping a close eye on this area of my car.

Neil
96 M3