Before you get all giddy and run out to the shop to get started converting your setup to the BBSS, I need to explain one problem associated with those setups. They are very hard to maintain any consistency with. The envelope for being dynamically balanced is narrow. That's not my personal opinion, that is the feedback I get from many racers who attempt to run these setups. Many go back to more conventional setups and get rid of the frustration.
Is Zero Roll Angle Possible? I had someone ask me the other day, "Can we really achieve zero roll angle? Doesn't every car roll to some extent even with a very large sway bar?" The answer is yes and no. It is in the arrangement of springs combined with other settings where we find the answer to his question.
The front can be made to roll very little and with a very high banking angle, near zero. We did just that way back in the late '90s at Daytona where we achieved a half degree of roll, front and rear. Then NASCAR instituted a minimum rear spring rate and we lost the method. A stock car with a 1.75 inch diameter front sway bar will roll approximately 1.3 degrees up front with 1.7 g's of lateral force and running on a 12-degree banked track. Even when the Right Front rests on a bump rubber with a spring rate of 750 pounds/inch, the roll angle only goes down to 1.0.
The trick is to utilize the banking and rear spring split to cause the rear to want to roll left the same amount. Yes, I said roll left. How do we do that? Simple, we install a much stiffer RR spring combined with a softer LR spring. This creates a compression in the LR spring that is more than the compression in the RR spring and the result is a negative roll angle, or roll to the left that is greater than the overturning moment roll angle, to create the negative roll angle once thought impossible in a stock car.
We need to know the spacing between coils on our springs related to the amount of wheel/sp
We recently observed a spring going into coil bind on a NASCAR truck using a Mittler Pull
We tested four different bump rubbers (those intended to be placed onto the shock shaft an
Explanation of Negative Roll Angle Negative roll angles are possible with banked tracks because the forces that influence the roll of the car in the turns (gravity and the lateral force) combine in a magnitude and direction of force that is down and to the right of the CG. This direction passes through the track surface between the rear tires creating a force pulling down on the rear of the car.
When we pull down on a suspension that has a large spring split, RR stiffer, we create a roll angle to the left. This combines with the natural roll angle to create a net roll of less than zero degrees. So, if our front end is rolling 1.2 degrees positive and the rear is rolling 1.2 degrees negative, then the result or average is zero degrees. And that is the magic of the BBSS setups.
The trick is in knowing or predicting when we have achieved equal and opposite roll angles so that our car has the desired zero roll angle. We can actually overdo the process and put too much load on the LF tire creating a serious imbalance in tire loading. The perfect setups with zero roll creates equal loading on opposing pairs of tires, LF to RR, and RF to LR.
A More Precise Example Let's use for example a typical Midwest Late Model straight rail car. We install a 1.75 inch (1 5/8 inch) diameter bar with a 1.25 hole for a wall thickness of 0.25 inch. We also install spring rates of 150 across the front with a bump rubber at the RF, a 150 LR spring and a 325 RR spring. The front roll center's dynamic location is 1.00 inch off the ground and 2.5 inches left of centerline after dive and roll, or dive only in this example. I won't tell you the Panhard bar height, but it is low.
When we model this car on a 12-degree banked track with a g-force of 1.7, the desired front roll angle, or what it would achieve if not influenced by the rear, is 0.881 degrees. With the rear spring split we see a negative roll angle of (-)0.901. Together we see where the car, if the chassis is sufficiently rigid, will attain a net roll of zero, or very close to it.