FP BMW suspension
 

Does anyone know if the really fast FP BMW uses the double balljoint lower suspension?

I have been playing with this a little in susprog and found it has some interesting ackerman characteristics. It gains ackerman as you steer it. The next question would be is this legal to do to a prepared car? I'm not really sure reading the rules.

I had intended to try something like this on my car but I can't get it all to fit in my little 13 inch wheels.

Cary

I'm not sure if Chris Cox's M3 has double balljoints, but it would be legal. Here's the rule:

1. Original suspension control arms may be reinforced, modified,
or replaced with components of unrestricted origin.

I can try and take a look in September when I see him again...

Tom

Thanks Tom, I wasn't sure you could get away with changing the connection at the bottom.

I watched a fast stock BMW the some time ago and climbed underneath and saw the double ball joints. I figured there had to be a reason why BMW did that and played a bit with it on susprog. It seems like it might be an interesting upgrade to our older cars.

Unfortunately I can't use it because I don't seem to have enough room. Personal problem using little 13 inch wheels full of brake. But for you big wheel guys this might be something to think about. It gains ackerman the more you turn it with a front steer rack.

Cary

Sounds interesting, but if all you're gaining is ackerman, and you don't have the room, why not move the steering rack closer to centerline? Would you acheive more ackerman the other way?

Hi Phil,

Some of this was thinking what I could do if I intended to keep racing my current car. In order to get to 100 percent ackerman (or more), which I can't get by moving the rack and/or the steering arms. To make matters worse with the 13s the steering arm and wheel want to share the same space.

For my EMOD car I fixed the problem by moving the rack up towards the top of the wheel. I can move the steering arm out more and it gets more advantage with the steering axis inclination.

The double ballljoint looks like you can get 100 percent without having to get too creative with moving the rack and it also has other interesting features. By playing with where the balljoints fall it can increase caster as you turn and change pin lead. I'm convinced it is part if why those BMWs are so fast. Perhaps I'm wrong.

Have you measured your car at all? I find that with the comp quicksteer knuckles I have almost parallel steering. I run a lot of toe out to compensate but the tires take a beating. Even moving the rack would probably help but it seemed like 60 percent was the max figure that I could get from the rack and knuckles being moved -- until I discovered the wheel problem. I need to find my notes to make sure that is correct. I'm not sure what just moving the rack would give for an ackerman percentage.

Cary

No I haven't measured anything yet. Hehe, I mentioned this in a way on the "other" board to so we get get to leap frog. I plan on diving into this stuff this winter, and just driving and having fun with the car now. I was thinking about getting REAL aggressive with just how far I could modify the crossmember and move the rack back, but I suppose the limit is when you hit the control arm pivot. Well, keep us posted on any further developments, I'm real curious. Oh, I still have the stock steering knuckles on too.

What's a double balljoint lower suspension?

The lower mount on some of the new BMWs uses two balljoints connected to the bottom of the strut. The actual balljoint pivot now becomes an instant center that changes as linkage moves. Real trick but it seems you need a computer to work most of it out.

The only reason I started looking at it was the performance of the BMWs and my realization that in autocross steering performance is a major performance factor. So for us things like scrub, SIA, pin lead, caster, etc. are all very important. Or said more simply -- how to go faster :-)

Cary

Any photos of the BMW setup?

So this took a long time and I couldn't find any of the pics I took. But I found a good thread on another forum that has pics. http://corner-carvers.com/forums/sho...t=24043&page=2

Cary

Oh. I get it now. It's a type of 4 bar linkage. I was thinking the ball joints were stacked or something. Interesting.