The rear suspension uses pushrods and bellcranks (aka rockers) rather than the conventional configuration in which the shock absorbers are mounted directly to the lower control arms. This is an exotic architecture found on Indy Cars, formula cars, prototypes and other pure race cars. While a few exotics have pushrod suspensions most production-based racers such as the Porsche GT3 Cup, a Spec Miata, or a Spec E30 have a conventional configuration.
Pushrod suspensions have the following advantages:
Packaging: pretty much all open wheel race cars have pushrod suspensions because they allow aerodynamics to be optimized. Specifically, the shock can be placed inside the body (i.e., out of the air stream) and the width and height of the body can be drastically reduced because there is a high degree of freedom with respect to shock placement. If you look at the picture above, it’s clear why packaging is critical for open-wheeled cars.
Unsprung weight is reduced because the shocks, springs and reservoirs are supported by the chassis rather than the lower control arm.
Ride height can be adjusted by changing the length of the pushrod without changing the pre-load on the springs.
Corner balancing can be done via adjustment of the pushrods.
Body roll is reduced by relocating some weight towards the center of the car.
Wheel rate can be easily changed by modifying the bellcrank’s motion-spring ratio (i.e., changing the length of one of the bellcrank’s arms). Wheel Rate = Spring Rate * (Motion Ratio ^ 2) * Spring Angle Correction. Since the notion-spring ratio is squared, small changes have a big impact. That said, the SL-C’s bellcrank has a 1:1 ratio.
The SL-C is basically a race car, so it’s not surprising that it has a pushrod rear suspension. The bellcrank is mounted via a large shoulder bolt which is retained by a billet aluminum bushing welded into a 1/8” x 2” x 2” chassis tube. The bushing takes high loads and the bellcrank is in single shear. It would be a nightmare if the bushing deformed because the only way to fix it would be to cut the entire tube out and replace everything. The chassis was welded in a jig at the factory and doing it properly on a finished car would be a challenge. Like some of the other builders, I decided to add a support arm to put the bellcrank in double shear.
Agile Automotive went through several iterations on their endurance SL-Cs and they offer 4130 chromoly parts for their latest iteration:
2x barrel welded to a cone (aka misalignment) washer
2x tube nut
2x bent and notched tube
There is a fair amount of variation from chassis to chassis and even from left-to-right on the same chassis, so they shipped me the parts with only the cone washer and barrel welded. The picture below shows the parts with the ones on the left having the tube’s mill scale and the barrel‘s weld discoloration re.
I’ve found that when loaded in a Dremel the AUSTOR abrasive wheels shown below do a great job preparing irregular metal surfaces (a tube polisher is obviously the best tool for round tube). The kit below costs less than $15 and contains 60 wheels in multiple grits (e.g., 120, 180, 320, and 400).
As intended, the tube needed to be trimmed and each side was on/off the car several times to get fitment right. I placed a grade-8 washer between the tube nut and the billet upright.
Once everything was installed the open barrel didn’t look finished and it would collect water and dirt which might make its way into the bellcrank’s bearing. I designed and 3D printed a cap. The first version had a mundane flat top so I gave it a profile similar to the dimple dies elsewhere.
I also plan on welding a gusset between the 2” x 2” tube and the vertical billet member.