Rear suspensions can also benefit from adjustable shocks. While single-adjustable shocks are better than nonadjustables, a double-adjustable shock that allows independent tuning of the compression and extension is the best way to go. For example, with a greasy or bald starting line, you might try a softer extension setting to allow more rear body rise to soften the hit on the tires to prevent tire spin. Conversely, a track with teeth usually will respond to a stiffer extension setting to apply more torque to the tire rather than wasting that power on body rise.
Koni, QA1, and Chris Alston's new Vari-Shocks are examples of single- and double-adjustable shocks that allow the racer to fine-tune the suspension to optimize traction based on existing conditions. Obviously, double-adjustable shocks are considerably more expensive than single adjustables, which cost more than non-adjustable shocks. How much control do you want?
Coil SpringsSprings do much more than just keep the fenders off the tires. Stored energy in the front springs can be used to help weight transfer in slower cars and can be dialed in to help calm down a quicker car. Moroso and Competition Engineering make very tall coil springs for early A- and F-body Chevys, Fox-bodied Mustangs, and several others. These springs come in various rates depending upon front-end weight of the car. The only way to know for sure is to weigh the entire car on a scale and then weigh the front and rear halves separately. Front-end weight will generally measure between 52 and 58 percent of total weight.
For A-body GMs and Fox-body Mustangs, it's possible to band-aid a stock rear suspension with an Air Lift bag in the right rear to counteract the tendency for the rear axle to lift the right-rear tire under acceleration. Placing 10 to 15 psi of air pressure in the right rear bag placed inside the spring will plant both rear tires. Moroso makes rear coil springs that offer different spring rates with the right rear spring stronger than the left, eliminating the need for the air bag. Combined with adjustable shocks, these changes can create more consistent launches.
Another important tuning device is a rear-mounted aftermarket antiroll bar. Unlike the factory bars that bolt to the rear control arms, companies such as Competition Engineering and Dick Miller Racing sell a universal weld-in unit with frame mounts that extend links to the rear axle. The links use threaded rods and spherical joints that can be used to adjust preload. While preload is usually added to the right rear to compensate for rear axle lift, it's also possible to add or remove preload from both sides to fine-tune the rear suspension.
The Search For Instant CenterCars with coil-spring rear suspension are not always immune from wheelhop. For those A-body GM and Fox Mustangs that suffer from a bad case of bouncing rear tires on launch, there are some simple fixes. The easiest and quickest way is to adjust the rear ride height until the lower rear control arms are parallel to the ground. Some car crafters have been known to jack the back of their cars up with air shocks to fit a big set of tires. This not only changes the instant center but usually overly stiffens the rear suspension which can cause that evil hop. The smart move is to keep the ride height near level.
Coil-spring cars also have a tendency to squat in the rear under launch. While this may look like the car is really planning the rear tires, the reality is that when the body squats, the rear tires are momentarily unloaded. To fully plant the rear tires, a small amount of body rise is necessary, evidenced by an increase in the distance from the axle centerline to the rear fender lip under launch. For third-generation Camaro and Firebirds, Spohn Performance makes a weld-on relocation bracket for rear lower control arms to help eliminate squat, especially for lowered cars.