In between the first and second autocross I made the first suspension change: stiffer bump stops.

The difference was amazing. Ten seconds into the first run I knew that, if I continued to develop the car (and the driver) it could be competitive in C-Street against the ND2 Miatas. So. Much. Grip!

The new bump stops made a huge difference. Less roll, faster response, balance that moved toward oversteer and the biggest thing: more grip. The extra grip comes from not losing as much camber because the car rolls less. A major weakness of this car is insufficient static negative camber in the front. Well, if you don’t lose as much then you don’t need as much and peak grip is increased.

The car was easy to handle. It’s somewhat bouncy now, especially on this bumpy site, since the stock damping is now a bit insufficient for the new spring rate. I was 2nd of 11 in C-Street against the same crowd where I was 5th of 10 last time. And I was 7th pax overall of 109, a huge move up from 22nd in the first event.

I think I was a little conservative on the bumpstops. The plan is to go up one durometer step and add an adjustable front bar. The Tarett hollow 5-hole front bar has already arrived.

I’ll show you what I did.

In the picture above you see the stock front bump stop on the left. The rules states that any alternate design cannot increase the length of the strut at the point of contact. Since I’m still using the stock strut that means that the height of the new bump stops cannot be more than the height of the old.* They can be shorter, however.

These donut-type bump stops were bought from ReSuspension. They sell a wide variety of types and stiffnesses. I like these and used this type on my 944 back in the day because the shape creates an initial soft-ramp of the spring rate as they flatten out. Since then I’ve used others that are pure cylinders, but a different material, and they seem to work equally well.

In the above picture the new bump stop vs the stock Porsche for the rear is shown. Two of the donuts as-molded were too tall, so I had to trim them.

The holes in all of these is 0.625″ which is smaller than both the front and rear shaft diameters, and the diameters are not equal front and rear. So, I had to open the center holes to get them to slide onto the shafts.

ReSuspension provides technical data on these donuts for a single and double and triple stacks. I use this information to calculate the resultant total spring rates front and rear. It’s possible to mix and match various durometers to get a range of spring rates. However, the trimming affects the spring rate that these stacks actually produce. Determining how much will require direct measurement. For instance, shortening the rears should have made them exhibit a stiffer spring rate, based on how much I cut off something like 15%. As I go further I plan to figure out a way to measure the rates. All that’s really needed is a press, calipers, and a load sensor of some type. (Bathroom scales?)

Next event I’ll have a similar arrangement except with donuts one full durometer-step stiffer. The calculations indicate that this will push the balance even further into oversteer. I hope to counter this with the new front bar, which I also hope will tend to keep the inside rear tire down better. In this event I had issues with spinning the inside rear due to the lack of an LSD.

*For any aftermarket strut the allowable height of the bump stop could be more or less, depending upon the design. The supplied bump stop might not be legal. In fact, the allowable bump stop height for an aftermarket strut almost certainly will not be the same as stock or what is supplied. You must understand the geometry, measure precisely and determine what’s what. No supplier has any obligation to meet SCCA rules. I’ve already run into an issue with Bilstin B6 struts that I’ve purchased. By my measurements the as-delivered fronts are not legal in SCCA Street-class.