Fixing the rear geometry once and for all...

[matt]

I'm thinking of building an unequal length a-arm rear suspension for my e30.

Anyone ever tried this before or have any pictures? I know how I'll do it, but I'd like to see how it's been done before.

I've heard rumors that e30 M3 rally cars used such a setup, but I've never been under one so I don't know.

[xLibelle]

what wheel hub would you try and use?

[matt]

Not sure. Some sort of BMW fitment. Possibly e30 M3, e36, or e28.

The uprights will be custom fabricated anyway, so I can put in whatever size bearing I need to. I am still looking into CV axles also. Stock ones are OK, but e28 ones might be better.

[xLibelle]

ive been mulling over redesigning the trailing arms for some time with the addition of a cross bar tied into the top of the hub upright, i suppose you call it.
My idea started with using the stock knuckle (m3, e28, what ever) and cut the cylinder that comes immediately out of the bottom of the hub knuckle right before the weld bead, then weld on a steel cylinder (same diameter) that is more accommodable for mounting the attaching arms.
With any setup regarding modifying the suspension, it still seems logical to mount the coilover where the shock is and cutting out the framework to make room as needed.
At this rate, using the e28 suspension sounds that much easier, but thats not what this thread is about.

[matt]

Mount the pushrod where the stock shock is maybe.

But I don't intend to use any of the stock trailing arms. I will just use the subframe as a base to bolt the diff to, and I'll fabricate additional mounting arms to the rest of the subframe. It'll end up looking more like the e36 one.

[Jean]

Would love to see this happen :)

[Mike B.]

Subscribed!

[xLibelle]

Mount the pushrod where the stock shock is maybe.

But I don't intend to use any of the stock trailing arms. I will just use the subframe as a base to bolt the diff to, and I'll fabricate additional mounting arms to the rest of the subframe. It'll end up looking more like the e36 one.

i didnt suggest that you would use any of the stock trailing arms - my post was, in part, suggesting what i have had in mind in the past.

I dont suggest you use that rear subframe for anything if you will not be using it for trailing arms. There is too much mass there for just holding the differential up.
A diff mount frame would be more appropriate. The diff bolts to this frame and then to the body's frame to the left and right. a diff cover with left and right frame mounts would be ideal as well. (simple diff bolt-on and frame tabs welded on)
the shorter, upper a-arms could mount to the other side of the frame where the other side would already be inforced by the diff-frame.

the more i think about your project, the more of a picture i have. still, this is going to be a PITA to build!

[xLibelle]

btw, what car is this for? i dont suggest you hack up your good one.
...grm?? if this is true, i just might have to show you guys up again ;)
dude, kevin is suggesting he pay my entry fee. lol, i dont have time this year with a wedding and all. i'm still trying to put the new oilpan and bringing the car home on the calendar

[matt]

I have enough spares... I'll reply with more of my plans later.

This is not for the WR car. The budget would be blown on decent rod ends and sphericals alone.

[matt]

i didnt suggest that you would use any of the stock trailing arms - my post was, in part, suggesting what i have had in mind in the past.

I dont suggest you use that rear subframe for anything if you will not be using it for trailing arms. There is too much mass there for just holding the differential up.
A diff mount frame would be more appropriate. The diff bolts to this frame and then to the body's frame to the left and right. a diff cover with left and right frame mounts would be ideal as well. (simple diff bolt-on and frame tabs welded on)
the shorter, upper a-arms could mount to the other side of the frame where the other side would already be inforced by the diff-frame.

the more i think about your project, the more of a picture i have. still, this is going to be a PITA to build!

I am on the fence about using the stock subframe. I will defintely use the beefy ass piece that surrounds the driveshaft and mounts the diff. I am trying to figure out a way to make it a bolt in kit that leaves the diff in the same location WRT the driveshaft. I figured keeping the entire stock subframe would be the best way to do that, but I'm seeing other options now.

The other thing is deciding how fancy to get with it. I am thinking about building a 3d model out of balsa or something to get the geometry right. The other option is to model the whole thing in Solidworks and use CAD to figure out how it will act.

Another issue is shocks. I'm thinking of using a rocker arm/pushrod system. The other option is a standard coilover mounted to the same place the stock upper shock mounts go, which is definitely the easy way. Either way, the shocks are going to be kinda spendy. Option two would let me use used circle track coilovers, but option one would let me use motorcycle shocks (resprung and revalved).

[xLibelle]

while in the market for suspension for the GRM, i had that idea of rocker geometry open game as motorcycle components are easy to come by and rebuilt relatively cheaply/easily. i already have some sketches for component design in that regard, in case you want to discuss that topic further.
But thats all second hand to what has to happen first, the suspension arms and frame.
What about a trailing arm from stock subframe (Groupe-A style) for lower-front a-arm, a cross bar from equal distance perpendicularly for lower-rear, and the upper arm mounts perpendicularly to the trailing arm from under the car's frame. ..the geometric complexities from the resulting setup makes my head spin :)
lets work with a different car that already has this suspension, eh?

[matt]

My suspension guy goes "if you aren't going to actually take the time to model the car and develop the suspension, just copy lotus."

And the problem with adding a motorbike shock based rocker arm system is that it would be sprung very softly, so the damping and spring rate would be way off for a pushrod deal on a heavier car.

[xLibelle]

no, you just adjust your rocker geometry... your loss is suspension travel though

thats the main reason i gave up with considering it for the GRM, too much shit to learn and understand to design the thing with such little time

[e304me]

Ive thought about this too. I was going to wait till i designed FSAE suspension first to really start thinking about though. I think with brain storming we can make this happen. The question is how much of a benifit would it be and would the car be as fun to drive. If you intend on designing the suspension, i really hope you know what your doing, not just from a stay together standpoint, but also from a roll center camber gain standpoint