We'll use this four-link illustration to show how power is applied through the bars. As torque is applied to the rear axle, the upper bars go into tension, while the lower bars experience compression. This is why leaf springs bend, because massive power is applied attempting to compress the spring lengthwise. | 
The problem with leaf springs is that torque tends to bend the forward leaf. When the leaf bends enough, it binds and then bounces the rear tire off the ground. Adding slapper bars will work, but the best route is Competition Engineering's Slide-A-Link bars. The Slide-A-Link mounts below the spring, placing the spring under tension and the lower link under compression during acceleration. |
A ladder-bar system is a simpler version of a four-link with the instant center located at the fixed pivot point of the bar. You can raise or lower the IC, but you cannot change its fixed length. | 
This is a screen capture of Kevin Gertgen's 4 Link program. The 100 percent antisquat line extends from the rear tire contact patch forward to a point created by the intersection of the horizontal line from the center of gravity to the front axle centerline. The instant center is point D, which is positioned above the 100 percent antisquat line in this particular situation. |
This is a factory-style four-link rear suspension for a '66 Chevelle with a Currie 9-inch and Hotchkis adjustable upper control arms. The Fox Mustang and SN-95 platforms employ a similar trailing-arm arrangement. Note how the upper control arms angle outboard. This triangulation locates the rear axle side to side, eliminating the need for a Panhard bar. | 
There's more to rear suspensions than just instant centers and antisquat lines. Springs and shocks also play a big part in overall chassis tuning. Adjustable shocks allow you to change the rate of suspension compression or extension, and that can make a big difference in traction. |
The 4 Link program will also plot all the possible IC positions based on the locations of the holes for the upper and lower trailing arms. As you can see from this screen capture, there are literally dozens of IC locations. Generally, there will only be three or four that will be useful positions for any particular car. | 
The torque arm became popular with the '82 Camaro. While it may appear that the front attachment point is the instant center (IC), the reality is that a torque arm's IC is actually the intersection point of the lower control arms with the torque arm. |
There are several ways to change the IC location on a Chevelle or Fox Mustang rear suspension. For example, Lakewood and Art Morrison both make a relocation kit that raises the location of the upper rear control arm to shorten the IC... | 
...Another option is to lower the lower rear control arm location, which is what the SSM Auto Lift bars accomplish. |
Pinion angle is important to rear suspension efficiency, but don't expect to see huge gains in e.t. based on a 1- or 2-degree change. The idea is to minimize driveline angle under acceleration to reduce power loss through the U-joints. A chassis dyno may be of assistance here in optimizing pinion angle. | 
We are seeing an increasing number of production-style four-link-rear-suspension cars using frame-mounted antiroll bars to counteract torque. The bar is attached to the rear axle and mounted on both ends to an existing crossmember. When the rear axle begins to lift the right rear, this twists the bar. This is Gary Penn's new Cheap Street car. |