In the last Making it Stick segment, we covered the basics of suspension dampers and the inner workings of common twin-tube designs. In this installment, we'll continue our discussion by digging into the more advanced monotube dampers and their state-of-the-art variants.
Monotube Characteristics
As the name implies, monotube dampers contain the piston and rod within one large-diameter tube. A floating piston at the bottom of the damper provides a physical barrier between the damper's oil and the high-pressure nitrogen gas needed for shaft displacement. Monotube dampers run pressures upwards of 300 psi. This high internal pressure inside the gas chamber means the damper oil is less likely to bubble, or cavitate, as it travels through the valves at high speeds.
Monotube dampers use a deflected disc-metering valve on each side of the piston to control both compression and rebound damping. Each valve is composed of a stack of springy steel disks, or shims, called a Christmas tree, that completely or partially blocks the orifices that allow oil to pass through the piston. The discs serve as one-way check valves that prevent oil from flowing backwards through the piston orifice and also meter the flow of oil through the orifices. The Christmas tree on the rod side of the piston controls the compression damping while the tree on the other side of the piston controls the rebound damping. When the oil wants to pass through the orifices, it must bend the disc up and out of the way to pass, much like a reed valve in a two-stroke motorcycle engine.
To tune a monotube shock, the deflection characteristics of the discs and the oil flow through the valve can be tuned by changing disc diameter, thickness, and/or by stacking several discs of varying thickness and diameters. Sometimes the discs have notches cut into them to allow the oil to bleed past and alter the shape of the damping curve. This flexibility allows for very fine-tuning of the damping force.
The characteristics of the damping force curve can also be controlled by the amount of preload on the Christmas tree. The shape of the valve-seating surface of the damper's piston usually controls the Christmas tree preload. The more concave the seat is, the more the Christmas tree is preloaded. More preload tends to increase the amount of damping at low piston speeds like those created by body motion. The size and shape of the orifices in the piston also affect the shape of the damping curve.
Monotube Advantages
Unlike twin-tube dampers, the gas chamber in a monotube damper is separated from the oil by a physical barrier, the floating piston. This separation means that the high-pressure nitrogen gas does not mix with the oil, and thus does not cause inconsistent damping force. This separation also means monotubes can be run at any angle, even upside down, which is advantageous because it reduces un-sprung weight by shifting the lighter end with the shaft to the moving suspension and keeping the heavier damper body attached to the chassis. Inverted-shaft MacPherson struts are often used in rally racing for this reason, and because they are harder to bend.
Because most monotube dampers have all the compression and rebound valving in the piston, monotube pistons are much bigger than those of twin-tube dampers. A larger piston displaces more oil, creating more flow through the valves than the smaller twin-tube piston; this makes monotubes much more sensitive to small suspension movements. This high flow in turn makes for more repeatable and accurate control of the damping force.
The deflected-disc valve systems found in monotube dampers are more precise, repeatable, and tunable than the system of check valves, springs, and orifices found in twin-tube dampers (although a few twin-tube designs do have deflected-disc valves in the piston).
Monotubes also run much cooler than twin-tube designs because there is no outer tube and extra layer of insulating gas to block internally generated heat. For these reasons, monotubes are the choice for all high-end street and racing dampers.