Before we go into digressive shock absorbers, lets take a look at rally cars. You have probably seen videos of rally cars powering their way across rough undulating terrain and yet still keep on track. Rally cars have been called F1 cars in disguise. Compared to street cars, most street cars would have gone out of control unless driven slowly across bumps. You will know what I mean if you have driven over speed bumps too fast. You get the “crash, boom, bang” feeling.
One of the reason rally cars are able to do what they do, is a specially tuned suspension. The other reason is they use shock absorbers with digressive valving. Digressive valving is a revolutionary way of valving shocks that is entirely different than in usual shock absorber design. Traditionally shocks have more damping force at higher shaft velocities, and that’s why they are called progressive shock absorbers. These shock absorber design have multi-stage valving to cater to various types of road conditions. Despite of these multi-stage valving, they are quite different from digressive valving, which is opposite of how shocks have been traditionally valved.
A digressive shock absorber has stiff, progressive damping at low shaft speeds (less than 3 inches per second). This helps to control car body motion like roll, dive and squat. At this point, it functions like a typical stiff racing shock absorber.
At higher shaft speeds, between 3 to 10 inch per second the damping force is more or less constant despite the rapid change in shaft velocity! The result is the suspension is kept supple in response to bumps and irregularities in the road, therefore traction is maintained on bumpy roads by reducing suspension hop, yet provides a firm ride, even at the high damping rates needed for body motion control. First used with success in off-road racing, digressive shock absorbers have only recently made their way into the on-road racing area.
In the past, digressive shock absorbers are only available from high end shocks manufacturers like Bilstein, Ground Control, Koni, JFZ and Penske. Now digressive shock absorbers are used in family sedans. Even the 2007 Nissan Sentra has digressive shock absorbers. There will be more after market replacement for digressive shock absorbers. Stay tuned for more.
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