When power is applied to the pinion gear and into the ring gear of the rear axle, the pinion tries to climb the ring gear. When viewed from the front of the car, the clockwise twist of the pinion attempts to lift the right (passenger-side) rear tire off the ground and plant the left (driver-side) tire. This is the natural reaction of all rear axles to torque input. This also explains why drag racers place a certain amount of preload on the right rear tire to counteract this force. An example of this is the use of an airbag over the right rear axle that preloads the chassis to counteract this torque reaction.
At the same time that the axle is attempting to lift the right rear tire, the body is twisting in the opposite direction, which normally results in the body squatting over the right rear. All of this is the reaction to torque input. The more torque you apply or the more gear ratio you use to multiply the torque, the more twisting effort is applied to the chassis. Drag racers and suspension engineers have collaborated to create very specific ways to explain how all this happens and have also come up with ways to manage the power in a systematic fashion.
All vehicles have a specific point around which the entire car will balance called the center of gravity (CG). For most domestic front-engine, rear-drive cars, the CG is generally located forward of the mid-point of the car at around camshaft height off the ground. While all rear suspensions pivot around a given point, this is not necessarily the point at which the rear suspension applies power or lift. Suspension engineers call this lift point the instant center (IC). Different suspensions place this IC at different positions in the car. Because suspension components tend to shift as the body lifts or squats, this position is dynamic, meaning that it moves as the car pitches or rolls. One definition of IC is the unseen center of an arc created by the moving suspension links. The simplest instant center is a drag race ladder bar. The forward mounting point for the ladder bar where it hooks to the chassis also happens to be its instant center. With other rear-suspension designs, the instant center is an imaginary point in space.
Kevin Gertgen's Performance Trends has created a drag race four-link computer simulation program called 4 Link that offers pictures that tell the story much easier. If you look at the illustration, you'll notice a pair of dotted lines that extend from the lines drawn by the two upper control arms and the two lower control arms. The intersection point of those two lines is called the instant center. The 4 Link program allows you to reposition the IC by moving the mounting points of the upper and lower control arms. Also notice the dotted line that extends from the rear-tire contact point forward at an angle. This line intersects a point created by the intersection of the horizontal CG line with a vertical line drawn through the front spindle. This angled line is called the 100 percent antisquat line, or sometimes called the neutral line.
By changing the location of the upper and lower four-link bars, you can move the IC location either above, directly on, or below that 100 percent antisquat line. When the IC is positioned below that 100 percent antisquat line, the rear of the car will squat on acceleration and "hit" the tires relatively softly. When the IC is positioned above the 100 percent antisquat line, the rear of the car will tend to rise on acceleration and "hit" the tires harder. Obviously, if the IC is placed directly on the 100 percent line, the rear will remain neutral.
This explanation holds true for all rear-drive cars, but there is plenty of confusion around the location of the instant center with different suspension systems. For example, with leaf-spring cars, the IC is the front spring eyes, but with ladder bars, the IC is the front pivot point. Factory four-link cars are determined exactly the same way as drag race four-link systems. If you extend imaginary lines forward on a factory four-link rear suspension, the IC generally falls in front of the car, well below the 100 percent antisquat line. This is why all factory four-link cars squat on acceleration. By installing the Lakewood anti-hop bars (for example), this kit raises the rear locating point of the upper control arms roughly 2 inches. This shortens the IC length and also places it above the 100 percent antisquat line, which now helps plant the rear tires.
So, what have we learned here? The main thing to take away from this rear suspension discussion is that there's more to improving traction than just dumping 50 pounds of ballast in the trunk. You can use specific suspension components to help you create optimal traction, but only if you understand how all these components work. This has been a primer intended to introduce you to the ideas around rear suspension science. There are dozens of other variables like weight distribution, engine torque, shock tuning, tire pressures, and of dozens more that contribute to improving traction. That's why this is as much art as it is science. But when you get it to work for you, your car will make you out to be a low-e.t. hero.