It's hard to argue with a brutally quick car. To be quick, you must be able to convert all the power that the vehicle makes into traction that will accelerate the car forward as quickly as possible.
Horsepower and torque numbers are great for impressing your car buddies. Even rear-wheel dynos are fun, because big numbers at the rear wheels will inspire even the most jaded enthusiast. But the final arbiter isn't flywheel torque, rear-wheel horsepower, or even the number of solenoids under your hood. The ultimate deal is how well your car can apply all this power to the ground. If you want to create shock and awe, hook all that torque through the two small patches where the rear tires meet the road, and the dragstrip groupies will beat a path to your garage door.
Different Styles
When it comes to rear suspensions, there are many different ways to make it happen. We'll deal with solid, live-rear-axle applications going in a straight line for this story and leave the corner-turning tricks for another time. We'll specifically look at leaf springs, four-links, and torque-arms, which are the most popular systems for high-performance street machines.
Leaf Springs
Leaf springs are the simplest form of rear suspension since they both locate the rear axle and suspend vehicle weight. The idea dates back to a time just after the invention of the wheel. Leaf springs are both heavy and also prone to wrap-up under high torque loads, which wasn't a problem for our ancestors in Conestoga wagons. Spring wrap-up occurs when the leading end of the leaf spring bends sufficiently to bind the rear suspension, at which point it bounces the tire and wheel off the ground, causing wheelhop. This is an extremely violent torque reaction that can be easily cured with traction bars that stiffen the front spring section. Unfortunately, slapper traction bars also contribute to rear-suspension bind. The most popular leaf-spring traction devices are CalTracs or Competition Engineering's Slide-A-Link bars, which act as a lower control arm to prevent spring wrap-up while eliminating the bind.
Factory Four-Link
This design exchanges heavy leaf springs for much lighter and more compact coil springs. However, this requires control or trailing arms to connect the rear axle to the frame. Factory four-links use a pair of lower control arms matched with a pair of uppers that are angled outward. This angling of the upper arms locates the rear axle laterally to eliminate the need for a Panhard bar. This is the rear suspension used in all GM A-bodies like the Chevelle and also the Fox and SN-95 Mustangs. This rear suspension operates roughly similar to a drag race four-link, where the upper and lower control arms are parallel to the framerails. We'll get into more depth with the drag race four-link shortly.
Torque Arm
The torque-arm suspension is the latest factory version of rear suspension evolution. The torque arm replaces the two upper control arms and is used to accommodate the application of power through the rear axle and to locate the rear axle. This requires the help of a Panhard bar to locate the rear axle laterally. One advantage to eliminating the two upper control arms is gaining valuable space between the rear axle and the body.