The Base of the P/D 5th Coil Shock is a Non-Adjustable Small Body Twin Tube 1000 Series Shock.
The compromise of choosing the correct spring for off the corner traction or straight-away traction is over. With the Progressively/Digressive fifth coil assembly you can create the best of both worlds. The P/D unit mounts in the same positions as the conventional fifth coil assembly.
The Problem: A stiff fifth coil springs provides instant traction when the driver starts to pick-up the throttle off the turn, but loses traction halfway down the straight-away, because the rear tires break loose from too much spring rate. A soft fifth coil spring does not hook the rear tires to the ground instantly off the turn, but does provide good traction down the straight-away.
The Cure: The P/D was designed to provide better forward traction off the turn and all the way down the straight-away. The P/D assembly has maximum spring rate the 1st inch of travel, and from 1 inch to 4 inches of travel the spring rate decreases to provide a softer total spring rate. For example a standard 400.b. fifth coil with 3 inches of travel would have a total rate of 12oolbs. The P/D assembly using a 400/300 spring combination with 3 inches of travel would have a total rate of 926lbs.
The P/D assembly is a combination of two springs with a slider assembly held together with a small diameter shock in the center. For the first inch of travel, the fifth coil assembly primarily uses the long spring. After that the additional travel uses the combination of both springs providing the desired spring rate. A small diameter shock in the center of this assembly dampens the spring to control any undesired oscillation.
The Disclaimer: The P/D fifth coil assembly is not a cure for all ill-handling race cars. It will increase forward bite over a standard fifth coil assembly, but it will not take a race car that has poor forward bite because of a bad rear suspensions combination and make a winner out of that car.