As our motor progresses at the machine shop, we’ll take stock of our suspension setup. The next few installments will detail our baseline which is the result of several years of competition in the CSP category. As we do so, we’ll discuss areas where we expect to make changes.
First up are springs. The primary job of springs in a production-based race car is to define the range of motion of the suspension. Stiffer springs allow for lower ride heights without bottoming. The first year we ran the car in CSP trim, we settled on rates of 550 lbs in the front and 300 lbs in the rear. With this, we could lower the car’s cg quite significantly, lessening the weight transfer from inside tire to outside in a turn. This resulted in more efficient use of our tires and higher cornering forces.
Unfortunately, we also had some inconsistent handling. This was especially true on bumpier surfaces. We did a complete analysis of camber and toe change versus ride height by using an alignment rack and varying the ride height. The results showed severe bump steer and roll steer within the range of motion of our lowered ride height. The Miata has a wonderfully benign design when used within its normal range of motion, but becomes treacherous when lowered too far. Unfortunately, the SP (and ST) rules limit our ability to fix this through traditional geometry mods.
Our band-aid solution was to run much stiffer the second year (700/450) to reduce our range of motion, and thus limit the amount of toe change. The car became much more stable and predictable. We were also able to run a bit lower on ride height, which improved our cornering grip even more. This will be our starting point for STS2 but we have some concern that it may be too much for our street tires. The super-stiff sidewall of the Hoosiers used in SP allows the springs to do their job, but an ST tire has a much softer sidewall. The softer ST tire then becomes the primary spring force when the suspension springs have a higher rate than the tire’s rate. This results in a pogo effect since the tire’s spring force is not properly damped as is that of a suspension spring. We’ll experiment with this.
Another point to mention on springs is weight savings. While long springs will work, you really only need springs that will just fully unload at max dynamic suspension droop, and will not coil bind at full compression. To that end, a 6″ spring is long enough for a Miata. Its lighter than the 7″ and 8″ springs that many people run. Along the same lines, we run 2.25″ diameter springs instead of the more common 2.5″ just for the weight savings. A look at the online Eibach catalog will give a good idea of spring weights. Its not a lot, but every bit counts.
The picture below shows our complete coilover setup. The shock is a Koni 28-series monotube race shock which we will discuss in detail as a future topic. Note the use of later-model (99+) Miata upper spring perches which provide three important advantages over the original 90-97 setup: 1) allows an additional .75″ of suspension travel in compression, 2) eliminates the undamped rubber in the original mount, and 3) saves weight. Shock mounting bushings were fabricated by sawing a pair of urethane sway bar bushings in half. The bottom washer is also from a 99+ Miata. This combination effectively reduces bushing compliance yet still provides for the angular motion needed to reduce sideloads on the shock shaft as the suspension moves. The bump stop is cut down to the size needed to just touch as the suspension contacts the unibody giving us maximum use of the designed suspension travel.
- Koni 2812 shock, Hypercoil spring, 99+ Miata upper mount, custom bushing & bump stop
Next time we look at sway bars and alignment.
–Andy
