Rear Double A-Arm Suspension ?

[GS guy]

PS: Jason,

The correct location for the tie rod/toe link is on a line that extends from the tie rod connection point at the spindle - that is the exact center of the "ball joint" of the tie rod end - to the virtual swing arm vertex point. The VSA vertex is determined by using a front view (or maybe rear view in your case) of the as-built suspension geometry.

The short version is: remembering you're working from the actual pivot point centers (which may or may not be directly in-line with the suspension members!) - you draw a straight line from the lower spindle (main) ball joint through the lower chassis a-arm pivot point, extending out the other side of the car. Now draw another line that passes through the upper strut pivot point and is 90 degrees to the strut axis (or more accurately 90 degrees to the line that passes through the upper and lower strut pivot points, the exact centers of those joints). Where those lines cross is the VSA vertex and will determine your roll center among other things. Going back to the tie rod - it should fall on another line that crosses that (imaginary) vertex point, but coming from the tie rod joint "center" at the spindle. You have to plot everything out on paper to determine these imaginary points, but the tough thing is doing the measurements from the chassis itself - to figure out where these points actually are in space! I just spent the better part of this past weekend doing just that on my aftermarket Mustang II suspension parts, to be able to reverse-engineer the design and create the proper pick-up points on my tube chassis buggy.

Cheers,
Jeff

[PJL54Oval]

Thanks for the feedback. GS, do you have a picture of the Chassis Shop upright?

[volkdent]

Thanks guys. I'm going to have to do it with the suspension travel method. Lasers are cheap these days, so I'm going to attach on on either wheel then have a white board behind to trace the travel path. The trouble is that this car has anti-lift(?) built into it. The rear inner lower control arm pivots make a V that points to the front of the car, when the V should point to the rear of the car. I'll just have to see how well I can make it work.

Jason

[beetle1303]

If i got it right this setup is used on the lotus elise/exige...(lucky enough to be able to see such a rolling chassis every day) the rear a arms have the V that Jason mentioned and viewed from the top the V goes well to the front of the car, but ALSO to the centerline (centreline). Actually the pivots are located at the front of the engine bay,similar to where the normal IRS trailing arms pivot points are on a beetle.

Chris

[volkdent]

Originally Posted by beetle1303

...but ALSO to the centerline (centreline). Actually the pivots are located at the front of the engine bay,similar to where the normal IRS trailing arms pivot points are on a beetle.

Chris

Yep, thats the trouble. My BMW is the same, there is a main pivot point almost in front of the rear tires. I might be able to do something like that later, we'll see. I believe that's the kind of wheel travel I need.

Jason