There is a design for a three-link...
There is a design for a three-link rear end that utilizes the motion of a pullbar. The LR spring is mounted in front of the rear end and the RR spring is mounted behind the rear end. When the car accelerates and the rear end rotates as the pullbar extends, the LR spring compresses and the RR spring decompresses. This adds load to the LR wheel and takes load off the RR wheel. This usually causes the rear tires to be more equally loaded, and the car will have more traction to help provide better bite off the corners.
If the car is diving excessively under heavy braking, the RF wheel will lose camber quickly and its tire will loose traction. Antidive effect can help the situation. Although the camber change is quick and the wheel returns to normal camber settings a short time later, once the push starts, it is hard to stop it without slowing down.
We can also use prodive to hasten the movement of the left-front corner on turn entry. Most teams that do this also run the BBSS setups and desire to drop the front end as low as possible and as quickly as possible on turn entry. Using antidive on the RF and prodive on the LF helps square the front end to the racetrack and, along with a softer spring setup, provides an enhanced aero effect.
The RF and LF wheels will always experience changes in camber as the car enters and rolls into the turns. If the front geometry is not designed correctly, then as the car dives and rolls, the RF camber relative to the racing surface will change and the tire will lose grip. In testing conventional setups over a five-year period, it was discovered that most tires want no change in camber relative to the racetrack surface as the car dives and rolls.
The front mounting bracket...
The front mounting bracket on a three-link rear suspension is adjustable to allow the team to change the angle of the trailing arm. The driver can feel small changes to the height of the front of the right trailing arm.
With the advent of the BBSS setups, both front wheels experience a great deal of camber change as the chassis dives 3 inches or more in the turns with an accompanying reduction in chassis roll. Because of restraints in design and the need for geometry controls for moment center placement, we have to live with those properties. Initial camber settings must be revised when changing from conventional to BBSS setups. The LF static camber must be increased quite a bit, and the RF static camber must be decreased.
Every race car needs a certain amount of tire stagger. The rear tire sizes must be different in order to compensate for the turn radius so that the rpm in both rear wheels will be equal. Excess stagger should never be used as a crutch to help make the car turn if it is tight. Doing this will probably make the car loose off the turns while under power.
A pushrod trailing arm is...
A pushrod trailing arm is designed to compress when the car is accelerating. This shortens the link and allows the rear end to steer to the left, which tightens the car considerably. Most teams that find this useful find it necessary to tighten the preload on the rubber biscuit to limit the amount of movement. Small amounts of rear steer go a long way toward tightening the car off the corner.
Some teams refuse to run sufficient stagger to match the radius and banking of the racetrack. I have talked to racers who run on tracks that require 1.75 to 2.00 inches of stagger. They report running about 0.75 inch and complain that the car is tight off the corners. It's no wonder.
Stagger affects handling in each phase of the corner if the car has a locked rear differential. With a Detroit Locker, or similar differential, the axles will unlock going in and through the middle and lock up on exit while under power. So, with the Detroit Locker rear end, stagger should match the radius of the turn where the car is starting to accelerate and the rear end is locked.
The shape of the racetrack can affect how the car is balanced when exiting the turns. If the transition is abrupt and the top of the track drops to match the inside edge elevation, then the RF will follow the drop-off and unload the LR wheel. Shock rebound rates need to be adjusted to allow the LR tire to stay in contact with the racing surface.
A useful design tool is antidive...
A useful design tool is antidive and prodive. Antidive keeps the front suspension from moving too quickly and too far. Prodive causes the front suspension to move more quickly. These two only work while we are braking, and that is exactly when we need them to work. Antidive is accomplished by angling the control arms from a side view so that as the spindle tries to rotate under heavy braking, the motion of the ball joints is in the reverse direction and resists dive. Prodive works in an opposite manner.
If the LR shock has too much rebound, then that tire will lose a lot of load and not be able to provide traction. The RR tire will be the only one trying to accelerate the car, and it might spin. Decrease the amount of rebound in the LR shock and/or soften the rebound in the RF shock to help this situation.
When attempting to tune your car's handling balance at the racetrack, always start with the middle phase of the turns. Run the car at a moderate speed through the middle well below race speed and note how far the steering wheel is turned. Speed up and do a few hot laps, and again note the position of the steering wheel.
If the wheel is turned farther at speed, the car is tight. If it is turned less, it is loose. If you have to steer to the right at mid-turn, bring the car back in. That simple test has helped many teams quickly determine the status of their setups so they will quickly know which direction to go when tuning the mid-turn handling.
Next, tune the entry balance and then tune the exit balance. When all three phases are balanced, work on driver finesse and practice passing maneuvers running high and low off the corners. With the car set up correctly, it is just a matter of experience and a little racing luck that brings that first win. In the next issue of CT, we will delve into the setup problems for a dirt car.
The rear stagger should be matched to the racetrack and not used to correct handling problems. A particular car at each racetrack will require a certain amount of rear stagger. The track radius used to determine stagger matters the most where the car will be accelerating. On some tracks, with cars using Detroit Locker rear differentials, the accelerating portion radius will be larger than the mid-turn radius. Therefore, less stagger should be used so that both rear wheels are turning the same rpm off the corner to avoid wheelspin.