The pull-bar third link acts...
The pull-bar third link acts much the same as the torque arm by extending under acceleration, which serves to soften the application of torque to the rear wheels. The rotation of the rear end can be utilized to produce several effects such as introducing rear steer and increasing the crossweight percentage while under acceleration.
When we are at mid-turn, the lateral forces will be resisted by the tires at the contact patch and all four tire contact patches will be 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 at that same time, we can help the rear tires maintain their attachment to the racing surface.
The traction circle theory of tire technology tells us there is only so much traction available from a particular tire and its contact patch, the direction of the forces doesn't matter. The actual number in pounds of resistance is based on the size of the tire contact patch, the adhesion properties of the compound itself, the amount of weight on the tire and the tire slip angle, or angle of attack relative to the direction of travel of the car.
The tire needs to be able to transition from the one direction of resistance (lateral, which is the resisting of 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 surface of the track.
If this transition happens too quickly, the tire is "shocked" and will most likely break loose. This is very 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.
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. As the driveshaft rotates, the pinion gear (attached to the driveshaft through the universal joints) engages the ring gear (attached to the axles through the differential) and tries to climb it. This applies a force that will try to rotate the rear end in a counterclockwise direction when viewed from the right side.
The pull-bar or lift arm can absorb some of the torque going to the tires during initial application of power. By being able to move, these devices will 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 in the track surface mean more spring rate is needed in the devices. Slicker track conditions require less spring rate and more travel for increased torque absorption.
Anti-squat is a geometric suspension design that utilizes the torque that is 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 so that 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 can exert an upward vertical force that resists the squat created by weight being transferred to the rear under acceleration.
Anti-squat enhances rear traction in two ways. First, it helps keep the rear of the car higher (as well as the center of gravity). It also keeps the rear spoiler higher because the rear of the car is higher.
Weight transfer is directly related to the height of the center of gravity (CG). The higher it is, the more weight transfer we have to the rear under acceleration. So, a higher CG promotes traction as more weight is transferred while under acceleration. Along with that, a higher rear spoiler catches more air and produces more aero downforce for added grip at the rear tires.