To fully utilize the BBSS...
To fully utilize the BBSS setups, you need to prepare your car in the shop before you go to the track. The upper arm angles will need to be reduced to lessen the camber change from the increased dive the car will experience. Don't forget to recalculate your moment center location and make sure it's in the correct location.
Contrary to the conventional unbalanced situation, with the BBSS unbalanced syndrome, the RR tire does not have to turn the car. It only needs to resist the centripetal loads and keep the rear of the car on the track. It carries a heavier load to help it do that. But, the RR tire will be working harder in the acceleration phase because it will carry most of the rear loading of the car.
Since the BBSS setup is unbalanced with the front tires being more equally loaded side to side than the rear, the front develops more grip. So, there is a need to increase the crossweight percent to tighten the car to a more neutral handling condition. And that is exactly what we find when we do a back-to-back test.
Keeping in mind that there are many variations of the BBSS setups, let's take a look at a common configuration for the straight rail Late Model cars that usually run the touring series. We will offer general directions, so don't run out and put this in your car. Every car is a little different and a slow approach to the transition will keep you from getting in trouble.
This shock graph shows the...
This shock graph shows the rebound adjustment range of our hlins Left Front shock and how much adjustment is available from a double adjustable shock. You should be able to completely tune your shocks to the different BBSS setup. Note how the extreme cut-off in rebound affects the compression side in the lower two rebound lines.
The sway bar size runs from a low of 1.375 inch diameter medium wall thickness through a 1.50 inch bar and all of the way to 2.0 inches and more. For most Late Model cars, 1.50 to 1.75 is more common. Some very successful teams have backed off the very large bars and are now running a 1.375 inch bar with either medium or thick walls.
Front spring rates for the BBSS setups vary from a low range of 125 lb/in springs up to 225 and 250 lb/in springs. Again, if you're in the 200 lb/in range, you're leaning more to the soft conventional setup configuration, which is gaining popularity. The RR spring rate is usually increased over conventional rates from 100 to 300 lb/in. This means you would run a minimum of a 250 lb/in spring all of the way up to and beyond a 400 lb/in spring.
The crossweight must be increased along with these changes. Typical increases are from 2 to 4 percent of total weight. It's often better to begin with the lower crossweight range (for a 50 percent front percent car, it's around 51.5 percent cross with a conventional setup) normally used with stiffer springs and smaller sway bars and then add 3 percent or so until the car is neutral in handling.
The same shock is shown in...
The same shock is shown in compression adjustment. The rebound settings are the same for every compression adjustment, but those still affect the rebound side.
Since the rear roll angle is a lot less than the front, we also want to lower the Panhard/J-bar about as low as it will go. With most chassis designs, we are limited to going down to 8-9 inches off the ground. Go there!
The RR shock will travel about half as much or less with the BBSS stiff spring in the car, so you need to adjust your RR trailing arm angle so that there won't be any rear steer to the right in the turns. With normal travel of 3.5 to 4 inches for the conventional setups, we usually use around 1.5-2 degrees of trailing arm angle in the right trailing arm. When installing the larger spring in the RR on the BBSS setups, reduce that to half, or 0.75 to 1 degree of angle-front high, of course.
One of the biggest changes that must accompany the BBSS setups is to your shock rates, both compression and rebound. The compression settings generally go up at the RF with the LF compression going down.
The RR will need a little more rebound to control that stiff spring. It's very helpful in the tuning stages of the conversion to BBSS that you use adjustable shocks, preferably double adjustable.