This is a cutaway of a Bilstien monotube shock. The larger piston of this design offers a
Shock absorbers fulfill a vital function, and if you’ve ever driven a truck with worn out shocks you know what it is—they dampen, or control both the compression and rebound of the springs. With the coils on twin I-beams, the shocks are particularly critical to control suspension travel and minimize the camber change that takes place as the tires move up and down.
To understand how a shock absorber works, picture a piston with holes in it pushing against oil in a cylinder. As a liquid won’t compress, the oil will be forced through the holes slowing down the movement of the piston, the number and size of the holes will dictate how much oil can flow and how fast the piston can move. In a shock absorber this happens on both compression and extension, or rebound. The holes, or orifices, in the piston are directional, which allow them to be sized differently so there may be more resistance in one direction than the other. In addition, modern shock absorbers are velocity sensitive, which means the faster the suspension moves, the more resistance the shock absorber provides.
Because there is some air inside a shock absorber, rapid movement of the piston can cause foam to form, which results in considerably reduced damping on compression and rebound. To combat this many shocks use pressurized gas, which stops foam from forming and results in a shock absorber with consistent performance. There are two basic types of gas shocks, the mono-tube high-pressure style and twin-tube low-pressure shocks.
Bilstien shocks come in a variety of configurations: Left to right are pin and T-bar; loop
As its name implies, twin-tube shocks have two chambers: an inner and an outer tube. In this style shock, the piston moves up and down in the inner tube in the main supply of oil, the outer chamber contains additional oil and a low-pressure nitrogen gas charge of 100–150 psi.
Twin-tube shocks are an improvement over totally hydraulic style, but they are still vulnerable to foaming and fade as the oil and gas mixes in extreme conditions. In our opinion, shocks on a twin I-beam Ford are operating in extreme conditions, which is why we opted to install Bilstein monotube gas shocks on the Hot Rod Hauler.
Bilstein’s monotube construction has a number of advantages over the twin-tube style. In a conventional twin-tube design, the majority of the oil is trapped inside the inner tube where the considerable amount of heat generated during operation is trapped, which leads to fade and markedly reduced effectiveness. With Bilstein’s monotube construction, the heat from the oil is transferred to the exterior of the shock body where it can be dissipated.
Below the piston that is attached to the rod, which is called the floater piston, can be s
The monotube design has other advantages. Compared to the average twin-tube design, the Bilstein piston is a staggering 228-percent bigger, which allows more sensitive valving. Inside the tube is a “floating” dividing piston between the high-pressure nitrogen and the oil that eliminates any possibility of foaming and the resultant performance loss.
In some cases, such as our ’73 F-350, there may not be a listing for a replacement shock, but that doesn’t mean you can’t take advantage of Bilstein technology. All you’ll need to know are the top and bottom mounting styles along with the compressed, extended and installed lengths, then contact a Bilstein dealer. For a listing check www.bilstein.com.