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 inroads, and by the '80s, the Mustang II and its derivatives took the lead. But hot rodders, whether 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 that 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 its 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.
The disassembled Bilstein illustrates how the shock housing is really the cylinder itself. For reference, the Bilstein shown here is for the same application as the chrome shock. | 
From this angle, the difference in piston size is obvious. The Bilstein's piston area, about 36 mm (1.42 inches) for a steel-bodied shock, is almost twice the size of the twin-tube's one-inch diameter; you can see how that gives them more valving area. Notice how no plates cover the outside diameter. Bilstein uses that area for its oil passages, but we'll get to that later. | 
This is the 46mm (1.81-inch) piston and rod that Bilstein uses for its aluminum and coilover pieces in lieu of the 36mm (1.42-inch) piston from the steel-bodied pieces. The adjustable unit on the right has a 1 1/4-inch piston. The nonadjustable unit on the left has about an inch piston and deflective valving, but only on compression. The clear bag on the lower left is what one of the manufacturers fills with its nitrogen charge. |
Aside from the larger valve diameter, the larger piston gives Bilstein the area to run channels through the piston. On compression, oil flows from side one's OD slots to side two's ID slots where it deflects the plates. On rebound, oil from side two's OD slots flows to side one's ID, where it deflects more plates. Bilstein uses the low-drag Teflon strip (shown next to its piston on the right) as the piston ring. Note the light-colored ring at the shaft's top to protect the piston during hyperextension. | 
While Bilstein uses its German facilities to manufacture parts and OEM shock mass-production, they use the Poway, CA, facility to design, craft, and tune shocks for specialized applications in hot rods and both road- and off-road racing. That's how builders like Ramon Sison can fabricate shocks to fit a car's particular needs, including length, stroke, vehicle weight, and spring rate. | 
Not only does Bilstein's crew hand-assemble each shock, but they also test each one on this dynamometer before it leaves. They've actually had shocks come back after tens of thousands of miles and they're still within their manufacturing specifications-a testament to the high build quality Bilstein has built its reputation on. |