From the outside, most shock absorbers and struts look pretty much alike: a round steel tube that telescopes up and down with bushings or fittings on both ends. But inside there can be significant design differences that affect not only the ride control characteristics and performance of the damper, but also its cost. So let's take a closer look at the “monotube” design.

In a conventional twin-tube shock absorber, the inner piston chamber is surrounded by an outer tube that acts as the fluid reservoir. As the shock pumps up and down, the action of the piston forces the hydraulic oil inside to flow back and forth through valving in the bottom of the shock into the outer fluid reservoir. In a monotube shock, there is no outer fluid reservoir. All the fluid remains in the piston chamber and a floating piston separates the fluid from a high pressure gas charge.

As the piston moves down, the fluid pushes against the floating piston and compresses the gas charge underneath it. The gas is actually nitrogen (air with oxygen and moisture removed). This creates a sort of “air spring” effect that...