Rear suspension redesign, Optimizing a semi-trailing arm suspension Options

[groot]

Finally, I'm actually constructing my rear suspension. This one's been in the works for a long time. I've had the parts, but lacked the mill/TIG time to make it happen. So, I'm starting with the 914 trailing arm instead of the 944 trailing arm (need the TIG for that one since it's alum).

Some background, the SCCA production rules state this under "Unrestricted Suspensions: Suspension Control":

2. Suspension pickup points on the chassis or structure may be relocated and/or reinforced. If such points are relocated to the driver/passenger/trunk compartments, such points and attendant suspension components shall be covered with metal panels.
3. The manufacturer’s original system of suspension, e.g., live axle, swing axle, MacPherson strut, A-arm, etc., shall be retained. The wheelbase of the vehicle shall not be changed or relocated in a fore/aft direction.
4. Suspension bushings are unrestricted. Adjustable spherical bearings or rod ends are permitted on all suspension components.
5. Any anti-roll bar, camber compensating device, panhard rod, watts linkage, and/or other suspension stabilizer is permitted. Attachment points of such components are unrestricted.

So, I can move pickup points, I have to run a semi-trailing arm suspension, but I can add a toe link.

The basic idea is that I can get the camber gain I want by playing with the location of the pickup points of the trailing arm to the chassis, but replacing the bulky semi-trailing arm with rod ends on tubes allowing easy control of static alignment settings. Simple geometry.... But, you're stuck with a crappy toe curve.

What suspension gives an excellent toe curve???? A real trailing arm (no toe change). So, allow the rear suspension knuckle to pivot in toe, but control that toe with an additional link (attached to the chassis in such a way that it mimics a trailing arm). This was the brainstorm that hit me one day.

I gathered all the parts last winter, but spent so much time in AZ for work I didn't get to do the work before the racing season crept up on me.

Here's the work I did this morning. That rectangle stock sticking out will be where I attach my toe link. The semi-trailing arm links will mount on the top and bottom of the knuckle (bearing carrier).

[Brett W]

Something else of note: learn to drive.

I disagree about the "compliant" rear suspension stuff on kinematic toe. I havent ran a GT3Cup car or GT3RSR with "rubber" in the suspenion (ever) Pretty easy to see what the suspension does on the alignment rack by loading and unloading the chassis. Physically watching it, the toe doesnt change front or rear. I *beleive* the toe does change in the rubber cars, but no where near that of the semi rear control arm of the 914.

B

I have to agree, seat time is the most important thing to a successful driver.

The GT3 uses a completely different suspension than the boxsters and Cayman. You should not see this compliance. It is not necessary. The GT3 is designed as a race car from the beginning. Hans didn't have to worry about grandma trail braking her GT3 Cup car turning into the church parking lot. But when she lift throttles mid corner he can't have the tail of her shiney Boxster convertible cruising around on her when she is headed to the grocery store. All production cars are designed with toe in under braking for stability. The 928 relied on elastokinematic deflection to create its toe in under braking. The front dog leg moved back and the rear dog bone actually bent under braking. Race cars do not typically have compliance designed in. It makes the cars harder to drive.

Kevin, I agree Sheridan's setup is for static settings only. Why he didn't design in a toe link, I don't know. Maybe Bertold could tell us if we knew where to find him.

Production is more for drivers where GT is more for builders. Kinda like the Mod class in autocross.