What types of shock absorbers do bikes use?
Answered by Gareth Evans, Reactive Suspension There are three main types. The simplest is an emulsion shock which has a piston and shims to generate damping on the end of the shaft and a volume of pressurised nitrogen and damping oil inside the shock body. Riding the bike moves the shock up and down, mixing the oil and nitrogen to form an emulsion. This emulsion has less viscosity than oil alone, so there can be an inconsistent damping action.
If the shock has an adjuster it will generally be a rebound adjuster, however with the adjuster nearly closed it will act as a combined compression and rebound adjuster.
A more complex approach is to keep the nitrogen and oil separate using a ‘floating piston’. Keeping the oil and gas separate offers more consistent performance. A basic floating piston shock will still only have single damping adjustment.
Next up is a floating piston shock with a separate reservoir for the nitrogen, either piggy-backing on the shock body or remotely at the end of a hose. The extra volume of the shock helps to keep the shock cooler.
Having a separate reservoir allows the oil flowing to it to be metered, so you can have separate rebound and compression adjustment. This can also separate that compression adjustment further into just lowspeed, or low and high-speed.
Finally, there’s the twin-tube shock design. Instead of a piston running up and down inside the shock body with a shim stack to control the damping, there is a solid plunger. The shock body has two tubes, one inside the other, with bypass holes at each end. On compression the plunger pushes oil out of the inner tube into a valve housing on the reservoir. The valve housing has fixed compression and rebound pistons inside it. The oil flows through the compression piston, bypasses the rebound piston and flows around the outside of the inner tube to the other side of the plunger. On the rebound stroke as the shaft is pulled out of the body, the plunger pulls the oil out of the centre tube, into the valve housing, and through the rebound piston. The oil bypasses the compression piston and flows to the other side.
On a single-tube shock a piston moving through oil generates damping. On the twin-tube oil is moved through the piston to generate damping.
Not all bike shocks are created equal