Since many cars run a rod-oriented shock, we used the chrome rod-style twin-tube shock to illustrate how the Bilstein monotube differs. Duck cut this piece twice: once to separate the reservoir (with shock eyelets attached) to expose the functional cylinder, and second to separate the functional cylinder and expose the piston.
Due to its construction, a Bilstein shock has no real moving parts (other than the piston and rod assembly itself). There are no springs or check valves to fail. The monotube design also allows Bilstein to separate the gas-charged chamber with a floating piston, a design that lets them load the shock with up to 360 psi of nitrogen-a higher pressure than conventional shocks. The gas chamber's main duty is to stabilize oil. Stabilizing (with pressure) prevents aeration and cavitation. For perspective, the swirly, foamy mass behind a boat is a result of the propeller cavitating the water.
Bilstein's deflective disc valving reduces harshness and offers better body motion control by a series of application-gauged discs. The piston has eight fluid channels that run through the piston itself: four for compression control and four for rebound. The design is referred to as digressive: The piston diverts oil from the outside diameter of one piston side to the inside diameter of the other piston side. On the inside diameter of each side of the piston there's a shim stack with flexible plates.
As the pressure exerted on the shock compresses it, it forces oil through one set of channels from the outside diameter. Those channels lead to the inside diameter of the opposing side where the oil must deflect (bend) thin metal plates to flow. The plates' resistance dictates flow control and the amount of damping. As the shock rebounds, the same thing happens, only through the second set of channels that deflect metal plates on the piston's opposite side. That extra space means Bilstein can use digressive channels to tune both compression and rebound properties independently via deflective disc valving, whereas many can only control one or the other with deflective disc valving. "Deflective valving means Bilstein shocks are speed-sensitive," Duck explained. "The faster the piston moves, the more it forces the oil to deflect the valve plates," which means that whenever suspension speeds go up or road surface goes down, the more the shock works. Along with the solid plates, there's a notched plate in each stack. The notched plates are bypass plates. They're for the quick movements (tar strips, lane marker dots). They work by bypassing smaller amounts of oil, thereby letting the suspension react very quickly so it doesn't transfer that movement to the chassis. "A Bilstein shock can react to something as small as 2 mm (a smidge over 1/16 inch) of shock movement, so the response is instantaneous," which some believe translates into a better ride over small stuff, but full control when the going gets rough.
This is basically all the stuff inside one Bilstein shock, and it's nearly all valving. Oil is channeled past all these plates through the slots, which control the action of the piston (providing a smooth ride).
"Cavitation leads to damping loss. In a shock, the valves are designed to control oil, not air. There will always be air in oil," continued Duck. "When you get people saying, 'It's all right at first, but after a while, the shocks start to fade away,' that's usually cavitation. Pressurizing the oil in a shock prevents that. Very high pressure will eliminate it. People get the idea that high pressure causes harshness, but that's not true."
Bilstein also maintains that the monotube design is an efficient heat diffuser. Since shocks convert kinetic energy to thermal energy, heat is a shock's natural by-product. But "excess heat is an enemy to mechanical things," Duck says. Since a monotube's cylinder is the body itself, the "increased surface area directly on the cylinder translates to greater heat dispersion." And since excess heat causes aeration and excessive wear, "that's a big deal."