A Warning: Please remember, this is not your car. The other measurements, weight location and amounts, and more mean that you can't just install these springs and sway bar and expect it to work for you. This is just an example of how it can work. But know that it is being made to work in this very same way for some very successful teams, not only in short track, but at the top level (money wise, not necessarily interest wise) of circle track racing.
Coil Bind versus Bump Rubber In our example above, we used a bump rubber that rated at 600 pounds with the amount of loading compression, or shock travel our RF suspension would experience at a particular track. Suppose we went to a different track where the shock travel would be greater and where the RF bump rubber were compressed more so to a rate of 1,350. The front roll angle would be reduced.
When the front roll angle is reduced, in this case to 0.664, then we no longer have zero roll angle for the whole car, we have a negative roll angle and an imbalance. The higher rear roll angle, in the negative direction, will force more loading onto the LF tire and make it work too hard comparatively. If we change our rear roll angle to (-)0.660 by installing a 300 RR spring and raising the Panhard bar a bit, we will again have a zero roll angle.
When we get to this point, we see where small changes in spring rates and Panhard bar heights make relatively big differences in roll angles. It is this difficulty searching for the right roll angles front and rear that makes maintaining the BBSS setups so difficult. Yes, when they are right, they are fast on tracks that suit them. But when they are off a bit, they are bad. Consistency seems to be a problem with teams that travel to different tracks where they must search for the correct combination.
When we coil bind, we limit that corner's ability to be adjusted in spring rate. If our suspension stiffness goes to infinity, or for example's sake, 3,000 pounds/inch of rate, our sample car would have a front roll of 0.398 if the RF only is in coil bind, and 1.090 if the LF only goes into coil bind. That's a huge difference. What if both front springs go into coil bind at some point on the track and alternate back and forth, then our roll angle up front would necessarily alternate between the 0.398 and 1.090 roll angles. With both front springs in coil bind, the front roll angle would be 0.340.
The yellow rubber stayed fairly...
The yellow rubber stayed fairly soft up to about 1.00 inch of travel and then the rate rose quickly to 744 pounds/inch at 1.50 inches of travel.
The black rubber was actually...
The black rubber was actually the stiffest at 1.50 inches of travel, but softer than the blue rubber below that range of travel. We need to think out how hard, or how much, we will be into our bump rubber and know the rate that the rubber will be adding to our spring rate.The black rubber was actually the stiffest at 1.50 inches of travel, but softer than the blue rubber below that range of travel. We need to think out how hard, or how much, we will be into our bump rubber and know the rate that the rubber will be adding to our spring rate.
The blue rubber was the stiffest...
The blue rubber was the stiffest overall of the four. From 0.75 inch up to 1.25 inches it was significantly stiffer that the other three. This would be the favorite bump rubber to use if you are planning on sitting hard on the bumpstop.
The Necessity of Knowing From these examples, we now understand the importance of knowing just which corner is in coil bind or on the bump rubbers and maybe more importantly, how much. A half inch into a bump rubber might create a 115-pound rate and 1 1/2 inches into it would create a 1,550-pound rate, based on bump rubber rating I did recently.
Variable-rate springs are very unpredictable because the roll angles are constantly changing and we can't count on a known roll angle to let us design our setup. The dynamic balance as well as the handling will be all over the place. You almost have to go all the way and squash a bump rubber to maximum rate, or 1,500-plus pounds to be able to plan out a setup.
Wavering between 1/2 inch of travel and 1 inch of travel causes way too much variation in spring rate. Coil binding is one way of going directly to a known spring rate and staying there. The problem is, if the track is too rough, the coil bound corner will bounce and loose contact with the track, and with that, all traction will be lost too.
Rating Bump Rubbers I recently rated a selection of fullsized bump rubbers. These are the units that go onto the shock shaft and are compressed when the shock compresses sufficiently. They are color coded for density and here are the rates. We had, in order of stiffness, first being softest, Red, Yellow, Black, and Blue rubbers.
The Red rubber rate went from 45 at 0.25 inch to 116 at 1.00 inch, 276 at 1.50 inches, and a high of 516 at 1.75 inches. The Yellow rubber rate was 90 at 0.25 inch, 132 at 1.00 inch and 744 at 1.50 inches. The Black rubber rose to 159 at 0.25 inch, 233 at 1.00 inch, 425 at 1.25 inches and 1,548 at 1.50 inches. The stiffest rubber was the Blue one, and it was rated at 248 at 0.25 inch, 350 at 1.00 inch, 634 at 1.25 inches and 1,240 at 1.50 inches.