Rear suspension and compression

#1

Camber. Years ago I found a graph that showed our rear camber changes under compression. After poking around several times in the years since to find that info, I kinda gave up. I vaguely recalled deciding that the graph indicated a rule of thumb for us of -1deg of camber for every inch of rear compression. Recall that our old springs would compress only about 1.5". Obviously our new springs can compress more. The way mine are set up they could prob compress 3".

This assumes horiz trailing arms, and ours are very close to horiz. If the trailing arms are compressed or unloaded a fair amount, the camber change increases. So camber gain from 2-3" of compression is more then it is from 0-1" of compression.

Tonight I found an old post by someone that seems to know what he’s talking about. He says -0.9deg of camber for each 1" of compression, and that this is pretty accurate for 3" of vertical movement, call it 1.5" of compression and 1.5" of unloading. So if you have -3 rear camber, under an inch of compression you can expect it to become -3.9. Contrast that to the front that gains no camber under compression.

Toe. The same post talked about toe. Folks told me for years that our rear end toes in under compression. But I tried to measure it once and the way I measure toe is super accurate because I shoot a laser beam forward towards a reference that multiplies change by 6x. When a toe problem is multiple by 6, you can see the magnified problem really easily. I moved the car up and down a bit but could not spot toe changing. This guy says that our car changes total toe 1/16" (1.6mm) under 3" of compression. That’s 0.8mm of toe change on each wheel with 3" of suspension travel. I suppose he could be right. That’s so little toe change that I might not have spotted it. I don’t recall for sure if I compressed the rear suspension 3" to observe toe change.

http://zeeasylum.com/albums/bimmerforums/Gustave/E30%20Semi-Trailing%20Arm%20Geometry%20-%20Page%203.pdf

#2

I don’t know if it’s my software, but there are some hyperlinks in that PDF that aren’t working.

Edit: Found a bypass: Index of /albums/bimmerforums/Gustave

Here are my thoughts and finding on the rear suspension over the last season. (new springs)

Adjusters are not worth the hassle. They just aren’t. So many fellow drivers had failures related to those adjusters, that’s why I have always put off “upgrading” to them. Luckily my car has been straight so far.

And lower rear = better. Being limited to ride height and sway bar adjustments in the rear made it easy to test, but everytime I would raise the car, both the loss of static camber and higher ride height made it for a worst driving experience.

Changes not mentioned when lowering the car:

  • Lower anti squat. As a track car, I can’t care less. When my car used to be on the street, it would drive me nuts, any little throttle input and the car would squat.
#3

Imo adjusters at the inboard rtabs are fine because they are easy to get to. If I had it to do over again, I would not have put toe adjusters at the outboard rtabs. Too hard to get to. If I had a toe problem, I’d just have a shop with a frame machine bend the trailing arm to my liking.

Re. lower rear = better. Afaik, no one is advocating raising the rear. We get can get more roll via more rear compression not because we can raise the rear, but because the new rear springs do not coilbind after 1.5" of compression.

#4

Re. lower rear = better. Afaik, no one is advocating raising the rear. We get can get more roll via more rear compression not because we can raise the rear, but because the new rear springs do not coilbind after 1.5" of compression.

I can’t find enough time to do the roll center calculation, but what I meant is it does not have the same weakness as the mcpherson setup up front as too low is not an issue. I would imagine once the arm passes the subframe horizontal you might have that issue, but I don’t think that is possible. But you are correct, it is certainly more linear spring rate.