Good suspension design has come a long way. Back in the '40s, hot rodders started replacing Houdaille (rotary-type) and friction dampers with tube-type shocks. By the '70s, Jag and Corvette independent units and four-bar suspensions made serious on roads, and by the '80s, the Mustang II and its derivatives took the lead. But hot rodders, whether it were those driving a '32 Ford or a Cobra replica, were not only willing to embrace technology, they pursued it.
But, according to Steve Duck of Race Car Dynamics Suspensions, there's a lot more room to improve when it comes to shock technology. "If you look at some companies' literature, the only thing they have in there about the shock's design is shaft diameter," says Steve. "We're trying to get people to look inside the shock itself. We want to educate people on what makes one shock better than another."
Primarily, what Duck says is that they want people to recognize the difference between garden-variety twin-tube shocks and monotube shocks-the latter being the design Bilstein uses exclusively. In a traditional twin-tube shock (gas-charged included), the shock's body (the visible external cylinder) is a reservoir. The actual damping takes place in a smaller cylinder within the shock's body. What sets a monotube (like Bilstein's) apart is its shock body is the cylinder where the damping takes place. That means that a Bilstein shock maintains a larger piston-to-body ratio (almost 2 inches in their aluminum body shocks) than a conventional twin-tube shock for the same application. And that will give a driver better ability to control the short, quick movements normally associated with some types of hot rods.
For starters, the larger-piston diameter lends itself to better road feel and ride characteristics by offering more space for deflective valving. Deflected valving allows more control while still being able to control dips, sways, and stops. It also allows Bilstein shocks great damping choices to account for ride height, spring rate, and vehicle weight.
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 the suspension speeds go up or the 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.