We can use the engine torque...
We can use the engine torque to our advantage by mounting the rear control arms in a certain way. If the upper link on a three-link rear suspension is angled, with the front end lower than the rear end, then the force that tries to rotate the rear end will try to make the third link more horizontal. This applies an upward vertical force to the front of the link that tries to lift the rear of the car. When decelerating, the opposite occurs and the braking forces try to lift the rear end.
We have learned that traction can be better maintained if we decrease the amount of torque from the initial application of power that reaches the rear tire contact patches. Doing this helps the tires adjust to the transition of forces from lateral to longitudinal.
At mid-turn, the lateral forces are resisted by the tires at the contact patch, and all four tire contact patches are at the limit of lateral adhesion if we are going as fast as we can without sliding. In more simple terms, the tires at that point are about to give up and slide. If we can reduce the initial shock transferred to the rear tires through the driveline when applying power, we can help the rear tires maintain their attachment to the racing surface.
We do that by using various mechanical devices that move as the rear end wants to rotate with the application of power. The rear end, when viewed from the left side, desires to spin or rotate clockwise when we gas up the car. A lift arm, pull bar, or similar device will absorb some of the torque caused by the acceleration isolating that force from the tires.
The traction circle theory of tire technology tells us there is only so much traction available from a particular tire and its contact patch, no matter what direction the forces are coming from. The actual net force of resistance is based on the size of the tire contact patch, the adhesion properties of the tire itself versus the track surface properties, the amount of load resting on the tire, and the tire slip angle, or angle of attack relative to the direction of travel of the car.
The angles of the four bars...
The angles of the four bars on a dirt Late Model car determine the attitude of the rear end as the car goes through dive and roll in the turns and off the corners. There is a science to setting these angles for different conditions. Most top teams are getting less and less radical with the trailing arm angles.
The tire needs to transition from one direction of resistance (lateral, which is resisting the centrifugal forces that are at right angles to the direction of travel) to the other (longitudinal or inline acceleration associated with application of power) over a longer period of time in order to maintain grip with the track.
If this transition happens too quickly, the tire is shocked and will most likely break loose. This is obviously detrimental to performance because in order to recover the grip in the rear tires, we must back off the throttle and allow the tires to reattach themselves to the track surface. This takes a lot of time, and we lose a lot of ground in the process.
The pull bar can also absorb some of the torque going to the tires during initial application of power. By being able to move, these devices absorb some of the torque of the motor for a short period of time, usually long enough to allow the tire to adjust to the new direction of force.
We can experiment with different rates of springs and shocks in these systems to adjust to and perfect the traction enhancement for different conditions. Higher amounts of grip on the track surface necessitates more spring rate in the devices. Slicker track conditions require less spring rate and more travel for increased torque absorption.
Antisquat is a geometric suspension design that utilizes the torque transferred to the rear end and tries to rotate the differential. On a three-link car, the third link (upper link mounted above the center of the rear end housing) can be mounted at an angle, with the front mount lower than the rear mount.
We can cause the car to produce...
We can cause the car to produce rear steer to the left to tighten the car while accelerating off the corners for more forward bite by staggering the heights of the trailing links.
When the car is accelerating, the force caused by the pinion gear trying to climb the ring gear causes the link to try to straighten out. Since the rear of the link that is mounted to the rear end cannot move vertically, the front mount will exert an upward vertical force that resists the squatting that comes from added load being transferred to the rear under acceleration.
Antisquat enhances rear traction in two ways: by keeping the rear of the car and the center of gravity (CG) higher, and by keeping the rear spoiler higher and more in the air stream.
Load transfer is directly related to the height of the CG, and the higher it is, the more weight transferred to the rear under acceleration. So a higher CG promotes traction as more weight is transferred. Along with that, a higher rear spoiler catches more air and produces more aero downforce for added grip at the rear tires.
There is no truth to the theory that the third link produces added mechanical downforce on the rear tires through the rear end. Any pressure put on the rear end by virtue of the link wanting to straighten out is offset by the reduced compression in the springs, and the trade-off in total load is even.
For every action, there is an equal and opposite reaction. We cannot pull weight from out of the sky, so as we stated, all added load comes from weight transfer and/or more aero downforce from a more efficient rear spoiler.