Determining Bent Subframe/Trailing Arm

[cunninglinguist]

Any tips on determining whether a trailing arm is bent while the arm is still installed?

Car was never invoved in accident or pothole incident, yet the rear passenger corner is sloppy and has excessive negative camber compared to the other side.

When a subframe gets bent, is it that only the trailing arm tabs get bent at an angle? Or can the subframe actually bend along its axis, like a bow?
For trailing arms, what axis usually shows damage from normal (non-collision) wear&tear.

I'm hoping just the TA bushing are blown out on the one side, but when i go to replace them, I'd like to also measure the subframe for straightness and the arms, too. It would be nice to have the cash to replace the subframe and arms and be done with it, but I can't really swing that at this time.

Thanks guys.

[1991 318is]

Finding bends.

Suspension parts get a road film and scale on them over time. When a part gets bent, that film or scale layer cracks open. Rear control arms are likely to bend at the arrows in the pix. M3 arms had extra bracing there. It's most likely RCARM bushings if it's new and you haven't "curbed" a wheel.

[mspiegle]

Just an FYI - I slammed my driver's rear wheel into a curb when I first got my car. I ended up with excessive positive camber on that side and it was the trailing arm tabs on the subframe which were bent. It was very easy to visually see that they were bent.

Here is a picture of my rear subframe. You can clearly see that the tabs are not straight:


I ended up replacing the entire rear end with parts I got from a low mileage '90 IS using urethane bushings all around. Just as a heads up, all urethane in the rear will make your suspension uneven (if you don't do anything to the front). My car is very unpredictable now and it is pretty loud. If I had to do the whole thing all over again, I would have used OEM BMW stuff. This was also for my daily driver... if I had a street/track car, it would be a different story.

[ye12]

Michael Spiegle,
I'm in the process of replacing my rear cross member and all bushings on it with IE urethane, because it got bent after an accident. After reading your reply (above), I would like to know more about your situation with the urethane bushings.
1. Could you give me more information on unpredictable
2. Is the increased road noise quite a bit more than stock rubber bushngs?
3. What front bushings can be changed to urethane to balance the rear urethane bushings? I've already changed my control arm bushings with M3 type and I don't think it was urethane.
Thanks!

[cunninglinguist]

Gents, thanks for the diagram and picture. This will give me a good starting point on what to check out.

Michael, I'd also be interested in hearing more about your experience, as i was contemplating the IE bushings, too. (I've now decided on OEM rubber)

[redhatpat]

I suspect mspiegle means having a urethane stiff rear suspension and a stock style soft rubber front (and stock springs?) will cause some funky suspension inbalance. Makes sense.

For my car, I'm going to use IE urethane rear subframe bushings, but keep the stock style rubber in the trailing arms for my daily driver. Since the subframe is the foundation for every other rear suspension part to bolt to, I think it makes sense to make that stiffer than stock for better handling purposes. But by retaining rubber trailing arm bushings, it will still have enough compliance to be a nice daily driver.

We'll just have to see how it turns out. This car will go to an occasional autocross, so I want to make it handle better than stock, but I don't want to give up too much of the nice, BMW ride.