Rear Suspension Tech Guide

Insufficient rear shock travel... 

   

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Insufficient rear shock travel is a common cause of tire spin. Generally, this occurs after the rear suspension has been modified to create more than 100 percent anti-squat (or body rise) under acceleration. The solution is using longer drag racing shocks that do not bottom out before the chassis achieves optimal separation.

Shock Tuning
Just bolting on the best suspension isn't the end of the story. There's still the necessity of tuning the rear suspension. Adjustable shock absorbers are almost a necessity if you are chasing that optimal 60-foot time.

Let's start with some basics. The term shock absorber is really a misnomer. It should be more accurately called a damper, because the device is designed to dampen or regulate spring motion. Shocks are rated by their resistance to motion in compression (bump) and extension (rebound). Most car crafters know that a typical 90/10 drag race front shock is easy to pull apart and very stiff to compress. This design allows the front end to extend easily and then stay up to assist weight transfer. But what you really want is for the front end to rise at the proper rate on the starting line and then quickly settle to keep the nose low at the top end to reduce aerodynamic drag. Equally important is the ability to adjust front and rear shocks to create the effect you desire. Most single-adjustable shocks create changes only in the rebound direction. The more expensive but better approach is to choose double-adjustable shocks that can tune compression and rebound separately.

Let's use a leaf spring Mopar as our tuning example. When the driver hits the throttle, the rear suspension separates, planting the rear tires. But let's say this hit is too harsh, crushing the sidewalls of the tires and causing them to spin. By slowing the rate at which the rear shocks allow the body to rise with a stiffer rebound, the chassis tuner can tweak the rate of torque application to the rear tires. This slows down the application of load, making it easier on the tires, which improves the 60-foot times. On the front end, let's imagine that it actually tops out too quickly, slamming up against the upper bumpstops almost instantly. When this happens, the car will sometimes porpoise, which unloads the rear tires and creates a loss of traction. Stiffening the front shock rebound slows the rate of front end rise, eliminating the porpoise action and generating a quicker run. These are just two simple examples of why it's necessary to tune both ends of the car to optimize traction.