If the upper link on a three-link...
If the upper link on a three-link rear suspension is angled with the front end lower than the rear end, 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 when accelerating that tries to lift the rear of the car. When decelerating, the opposite occurs and the braking forces try to lift the rear end, causing the car to be loose. Anti-squat should be used in limited amounts.
There is no truth to the theory that the third link produces 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 tradeoff 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 weight comes from weight transfer and/or more aero downforce from a more efficient rear spoiler.
Asphalt cars merely need to maintain a straight ahead attitude when cornering. Dirt cars, on the other hand, must steer the rear end to the right on dry, slick tracks to develop a more sideways attitude that will point the car in the right direction to be better able to get off the corners. In past years, the drivers on dry, slick tracks would need to throw the car sideways in the turns and, in the process, break the rear tires loose in order to point the car. When it was time to get back into the throttle, the tires had already lost traction and spun, producing little forward bite. With the advent of radical rear steer geometry, the cars will now roll over, the rear end will steer to the right to point the car and the rear tires will maintain grip with the track surface and be ready to provide forward bite when the driver gets back into the throttle.
A fairly new concept for added bite on asphalt involves a geometric design that will produce rear steer upon application of power. We can utilize a certain type of rear suspension to create rear steer only under acceleration. In the three-link rear suspension system, if we use a pull-bar, lift arm, or other similar device that will allow the rear end to rotate under acceleration, we can steer the rear end to the left while the car is accelerating.
As the rear end rotates under power with the pinion moving upwards, the whole rear end will move rearwards. If the lower control arms are mounted different distances from the axle, then one side of the rear end will move farther than the other. If the left side trailing arm is mounted lower than the right side trailing arm, then the left rear wheel will move rearward more so than the right rear wheel and this results in rear steer to the left which will tighten a car off the corners.
It might be appropriate to mention another element related to geometry that affects traction off the corners and that is pinion angle. We know that excess pinion angle absorbs some of the engine forces, reducing power to the rear wheels. In the past, we have heard racers say they experienced better bite off the corners when they put more pinion angle in the car.
While it is probably believable that excess pinion angle reduces rear wheel tire spin, it is not because of any mechanical enhancement effect, but rather a reduction in the amount of power that reaches the rear wheels. If we reduce the amount of power that reaches the rear wheels, we also reduce the tendency for the rear wheels to spin. This is not a recommended procedure for adjusting power to the rear wheels.
It is not widely known, but some top dirt racers have adjusted their car's engines to produce less horsepower when slick track conditions would not allow great amounts of torque and horsepower to be put to full use. Using smaller carburetors, adding restrictor plates, unhooking the secondary butterflies, or using electronic traction control that changes the timing or breaks down the ignition on one or more cylinders are all ways that teams have of reducing engine output. We have seen teams change to a smaller engine when they knew they were going to a traditionally dry, slick track. Again, anything we can do to help the rear tires maintain grip at all times will give us a better chance to apply the power available to accelerate the car.