Some of the rubbers had a strange tendency to lose rate at a travel range of around 0.25 to 0.75 inch and I'm not sure why that was. It could have been an influence of the shock we were using. All of the rubbers maintained a fairly consistent rate from 0.25 inch to around 1.0 inch and then rose in rate significantly on up to 1.5 inches, which was near full compression.
If you cut your bump rubber down in size, the rate will increase and the distance it rises from full soft to full stiff will be reduced significantly. I tested a Black rubber that had been cut to where only two rings remained from the original four rings. The rates were 210 at 0.100 inch, 150 at 0.200 inch, 160 at 0.300 inch, 250 at 0.400 inch and 420 at 0.500 inch. At 0.5 inch, the rubber was almost fully compressed and further travel would move the rate up beyond 1,000 pounds and to infinity in only the next several 100ths of an inch.
Coil Bind Tips When utilizing coil bind, remember our example car where we went from LF coil bind to RF bind. The roll angles changed a lot from one to the other. The suspension will be solid, or very close to it, with this situation. If the track is the least bit rough or has patches or dips in the asphalt, it may not work.
One tip is to apply a rubber coating to the coils, or just one side of the coils so that when the gap between coils closes, the rubber is compressed and yields a new spring rate that helps to transition from the installed spring rate to full stiff. If you have five coils in your big spring, just a 0.100 inch coating will produce a half inch of compressible rubber.
If you run coilovers and have, say 10 coils, a 0.100 inch coating will give you a total of 1 inch of rubber spacing equal to a 1 inch bump rubber. You can rate the spring with the coating to know the spring rate per distance of travel. Knowing the amount of wheel travel and translating that to shock travel will tell you the spring rate at any amount of bind.
What Does All Of This Mean All of this information is presented to educate you just as it has us so that you can know more about what is happening when you go the BBSS route. It is not intended to coax you in that direction and I think the end result of all of this will be to either dissuade you from attempting these types of setups, or helping you to understand them enough to do it right from the start.
Does this mean I agree with this direction in setup for short track cars, NO. I probably never will encourage racers to do this to a perfectly good car. It is in many ways unnatural. It relies on the stiffness of the chassis to work correctly and if the chassis is not sufficiently stiff, then we introduce another variable into the equation for a higher level of unpredictability.
I don't like variables and I want my setups to be predictable. For those who thrive on trial and error testing, week in and week out, this is your Holy Grail. Have at it and good luck with it. As for me, call me old school, but I'll stay with something more consistent, predictable, and evidenced by the many emails, calls and indications I get across the country, consistency is winning a lot of races.
Conclusion Whether you think my way or not, ultimately does not matter. As a team, you should do what you think is best for your type of racing at your track. If zero roll and running on bump rubbers or in coil bind makes you happy, then by all means, go for it. I have always believed that most racers are not totally happy unless they are making changes and experimenting, or just plain being a scientist in our own little way in our secluded little world.
If you can't be happy at what you do, then quit. If you must experiment, and we know you must, I hope that we have provided enough useful information so that you can either find more success or at least know why you didn't. It's all in the knowing either way.
Note the rates of this black...
Note the rates of this black rubber that was cut in half leaving only two of the original four rings. This looks fairly normal when compared to the other graphs here, but note the distances of travel. They are in 100ths of an inch and only go to 0.500 inch. The other graphs go to 1.50 and 1.75 inches. This half rubber yields a rate of 420 pounds/inch at only 0.5 inch of travel. That's one stiff bump rubber. Because of the short amount of overall shock travel, this one is used on a Grand-Am Daytona Prototype.
In our example BBSS setup,...
In our example BBSS setup, we installed 150 pounds/inch springs on all three corners except the RR where we ran a 325 spring. We were on the RF bump rubber and figured it added 600 pounds/inch of rate to that corner. With the spring split and Panhard bar height, our positive front roll was countered by our negative rear roll to net a zero roll angle.
If we ran harder on the RF...
If we ran harder on the RF bump rubber to yield an added rate of 1,350 pounds/inch. to the 150 pounds/inch installed spring, then we needed to make a RR spring change to 300 and adjust our panhard bar to match the roll angles to again net zero roll. The front got stiffer with less roll and we needed to increase the rear roll (or decrease the negative roll) to net the zero overall roll.