The art of shock tuning has long been considered a “black magic.” While this is somewhat true, there is an actual science behind the construction of a valve stack. With roughly 400+ shims in BILSTEIN’s tool box, ranging from 10.5mm to 60mm in diameter, and .10mm to .80mm in thickness, there are an endless number of possibilities when it comes to building the perfect stack for your vehicle. Below, we’ll provide some insight into the different components that make up a proper stack and explain the differences between linear, digressive and compression/rebound only bypass digressive valving setups.
How Does a Valve Stack Function Within a Shock Absorber?
As the tire and wheel move up and down on a vehicle in response to the road surface, the piston inside the monotube shock absorber cycles through hydraulic fluid and creates pressure differentials. These “pressure differentials” can be controlled by introducing a valve stack on either side of the piston. One stack controls rebound and the other controls compression. During compression, the rebound shims block oil flow through the ports in the piston and the compression shims bend open to allow oil flow. During rebound, the opposite effect occurs. Adding or removing metal shims (valve plates) will result in an increase or decrease in compression and rebound damping forces.
A typical valve stack consists of an arrangement of shims, including support, cover, bypass, bleed and preload shims, in a variety of thicknesses and diameters. Each of these shims plays a distinct role in the valve stack setup. BILSTEIN utilizes three main valve stacking combinations: linear, digressive, and compression/ rebound only bypass digressive.
Liner valving is characterized by a high flow rate at low shaft speeds. The oil flow resistance increases as the shaft speed increases. Thus, the faster the shaft speed, the stiffer the shock.
Our linear piston has approximately three times the bleed on the compression side as the rebound side. Bleed refers to the amount of oil flowing through the piston and around the shim stack. The rebound side is usually noted by a small, faint, minus sign (-) in the casting. This means that if the valvings were to be built with the same series of thicknesses and diameters on both sides of the piston, the side with the minus sign will always have the greatest amount of damping force, because it has the least amount of bleed. (The piston can also be inverted to create a low bleed compression, with a high bleed rebound.)
Digressive valving is characterized by a low flow rate at low shaft speeds. This results in more oil flow resistance. The resistance rate increases as shaft speeds increase, but only to a pre-determined level. At that point, the resistance tapers off and as the shaft speed continues to increase, the resistance remains constant. This eliminates unnecessary resistance and provides more control as the vehicle encounters irregularities in the road’s surface.
Compression Only Bypass (C.O.B.) and Rebound Only Bypass (R.O.B.) Digressive Valving
Compression only bypass (C.O.B.) and Rebound only bypass (R.O.B.) digressive valvings are similar to the standard digressive valving, but differ in that they contain a check valve system. The C.O.B. Digressive piston utilizes a check valve, that when installed on the compression side of the piston creates less bleed and therefore more force on the rebound side. When the “C.O.B.” Digressive piston’s check valve is installed on the rebound side, there is less bleed and therefore more force created on the compression side.
How to Determine the Setup That’s Right For You
While there are many variables to consider when tuning your race vehicle, the valving code is arguably one of the most important factors to focus on. For more info on how to get started building your ideal valve stack, take a look at BILSTEIN’s Motorsports Technical Guide, which includes setup recommendations for a variety of motorsport uses. Additionally, BILSTEIN’s Motorsports Technical Team is available at 877-666-7662 to discuss any questions you may have.