Why E34 rearends are loose

[Robert K]

Just finished up reassembling the entire rear end of my 1991 535i. Thought I'd share some hard learned lessons about E34 rear suspensions. The whole purpose of all of the work I just did was to stiffen up the rear end of my car. It had always been a bit mushy feeling, like the rear end was about a half step behind the front. I decided to replace the trailing arm bushings (2 per arm), differential mounts (3), and rear subframe mount. Ordered all the bushings from BMA. So I put it up on jackstands and started tearing it apart. I took the trailing arms out and took them to various machine shops to try to find someone to press the old bushings out and the new ones in. I finally found one that said they'd give it a try. They couldn't budge them and since I told them I wasn't wanting to spend hundreds of dollars to get the job done, I took them back from them. Then I took them to an ag dealer. They had a small, heavy-duty C-clamp looking device that they'd used for similar jobs. However, they'd need a pusher block. So back to the machine shop to get that made. While I was waiting for it to get done, I decided to put a bolt into the old bushing, then put a pipe over it to use as a lever to see how much give they actually had in them. Once I had a feel for that, I put the new bushing in a vise and did the same test. Guess what? There was hardly any difference in deflection versus the old bushing. Since I figured it was still going to be a fight to get the new bushings, and the test seemed to indicate that I'd pick up very little stiffness, I decided to leave the old ones in and put the trailing arms back on.

Then I decided to look at the differential bushings. I used a lever between the diff housing and subframe to see how much deflection there was in the old bushings. I could hardly even move the thing. So I decided once again to leave the old ones in. Finally I moved to the subframe rear mount. I loosened all of the subframe mounting bolts/nuts and used a jack to lower the rear of the subframe so I could access the rear mount. When I got it out, I mounted it in a vise and did a similar test to what I did to the trailing arm bushings. I then repeated the test using the new mount. Guess what??? The old original was stiffer than the new one. So again, I put the old one back in. So after about 10 hours work, my car is back together with all of the original parts that were in it before I started this project and all the new bushings are laying on my bench.

While I still had the car on jackstands, I decided to put a lever between the subframe and the unibody near the large front subframe bushings. That's when I discovered where all of the movement in the rear end is. It's the stupid subframe bushings. I had just replaced mine a little over six months ago with new Mahles. But using a 18" lever, I could move the subframe at least a 1/2 inch. So, the lesson I learned is that although the subframe mounts probably contribute to a cushy ride, the absolutely suck in the handling department. I'm going to send the bushings I didn't use back to BMA and order some of Bruno's subframe inserts. I figure they can only help. I may even try to engineer something up myself to really stiffen them up. I've read about people pouring urethane in the voids in the OE subframe bushings and would probably recommend that to anyone intent on changing them out. I may try to figure out how to pour that stuff in mine while the bushings are in the car. After this whole ordeal, I suppose I'd recommend upgrading your subframe bushings if you're trying to stiffen up your E34 rearend. The OE trailing arm bushings, diff mounting bushings and subframe rear mount all appear to be well engineered and long lasting. In my opinion, the subframe bushings are just poorly designed and contribute the majority of the looseness of the E34 rearend.

One last thing. Even though I experienced quite a bit of frustration in this wrench session, I hope you'll notice that I did not find it necessary to use profanity in my post to explain what I'd learned. I have noticed more and more people on this board who can't seem to say anything unless they pepper their message with profanity. Guess what? It's not necessary and in the past, was rarely found on this forum. I suppose if someone just doesn't have the verbal capacity to say anything without profanity, they could always hop on over to Roadfly. I'm sure they wouldn't mind.

Grace and peace,

Robert K
1991 535i

[myles]

Interesting info Rebert. Thanks for sharing. I've often wondered about how all of those bushings are lasting in my car but have never had the time to look further into it.

[repenttokyo]

could it be that you just got a bad set of subframe bushings when you replaced them 6 months ago?

[Tiger]

Geez and no profanity at all? Not a peep? So you are saying that the front subframe bushing are bad... even though you put in new one... So that bruno's inserts are probably the easiest way to solve the problem?

[Robin-535im]

Well Robert... you are perhaps a wiser man than I...

I did the same as you, but in fact did replace all the bushings. It was a bit stiffer after all 10 (big diff support too) bushings were renewed, but only a bit. I had the benefit of help from Don here on the board, which outweighs a whole city of machine shops as we were able to finally get the trailing arms replaced. My only regret is that I used stock e34 and not m5 bushings as replacements - different part number and presumably stiffer.

In retrospect it was marginally worth the effort, mainly because I had Don's help. Without that it would have been d**n (darn) hard.

I do sleep better at night though, knowing I have another 100k miles before I have to worry about the rear end!

- Robin

[Boone.Msi]

What?! A forum full of Aussies and Yanks with no profanity? ...Blasphemy

[ThoreauHD]

So, M5 bushings good. Subframe bushings need urethane filler or Bruno's inserts. Works for me. I wonder if the M5 has the same subframe bushing looseness?

[pundit]

What?! A forum full of Aussies and Yanks with no profanity? ...Blasphemy

Working on cars without profanity is simply not normal!

[BigKriss]

Winfred and Bill R have both told me the trailing arm bushes almost never wear out. I have yet to fit my subframe bushes with liquid urethane. I don't know what rear swaybar your running Robert, maybe you can fit a larger one to get the rear of the car firmer. As for profane language, I'm young and dumb. I don't mind it.

[Robert K]

I don't think I got a bad set of subframe bushings, I just think they are that flexible by design. As I remember, the subframe moved around with the old bushings in too and they weren't degraded that badly. I think BMW just carried away with trying to provide a smooth ride. Also, on the E39, they used four bushings to mount the subframe instead of two and the center rear mount. I think they learned their lesson at our expense. I have a feeling that the E39 does not have the slop in its rearend setup.

Grace and peace,

Robert K
1991 535i