17th Annual Technical Forum: Shocks and Struts

17th Annual Technical Forum: Shocks and Struts

Counterman magazine presents 15 technical and sales topics in an easy-to-read question-and-answer format for the magazine's annual Technical Forum. This article appeared in the August 2009 issue.

Q. What’s the difference between a twin tube shock and a monotube shock?
A. Twin tube shock absorbers and struts have an inner tube for the piston cylinder, and an outer tube that serves as a fluid reservoir. Valves in the piston and in the bottom of the shock control fluid flow as the piston moves up and down. In a monotube shock absorber or strut, there is no outer tube or outer fluid reservoir. All the fluid remains in the piston chamber, and a second floating piston separates the fluid from a high-pressure gas charge in the top of the shock or strut. The pistons are larger than those in a twin tube shock, and the inner surface of the cylinder is polished to provide a good seal for the floating piston.

Twin tube shocks have been used as original equipment on most vehicles since the 1930s. Monotube shocks were not invented until the 1950s, and for many years were used primarily for racing because of their higher cost. The main advantage of the monotube design is that it uses a high-pressure gas charge above the floating piston to control cavitation and prevent the fluid from foaming. “Monotube shocks are often desired when an extra level of ride and handling performance is required,” according to Robert Nossal, Gabriel’s product engineering manager.
In a conventional twin tube shock, the pumping action of the piston creates cavitation that produces bubbles in the hydraulic fluid.

This can churn the fluid into foam, causing a loss of resistance and shock fade under hard use.
In a high-pressure, gas-charged monotube shock or strut, the gas charge pushes the floating piston down on the fluid. This pressurizes the fluid and reduces foaming for more consistent ride control, even under hard use. That’s why monotube shocks have long been a popular upgrade for performance handling. Monotube shocks were widely used in racing in the 1960s and 1970s, and found their way into NASCAR in the 1980s. Since then, monotube shocks have also been used as original equipment on a variety of cars including Mercedes, Audi, Porsche, BMW, Saab and Volvo, 1993 and newer Camaro and Firebird, late model Corvettes and even some pickup trucks.

Gas pressurized twin tube shocks have also been available for many years, but typically use a much lower gas pressure charge to control cavitation and foaming. But for most performance applications, the larger internal piston inside a monotube shock or strut, and the higher-pressure gas charge provide an extra margin of control.

Q. How does valving change the damping characteristics of a shock?
A. When the piston inside a shock pumps up and down, it forces fluid to flow through small orifices in the piston. This creates drag and resistance that slows down the piston and dampens the motions of the suspension.
Spring-loaded valves in the piston and in the bottom of the shock provide additional control. At low piston speeds, the valving is open and provides minimal resistance for a soft, smooth ride.

As piston velocity increases, the valving closes to increase resistance and “stiffen” the shock for better ride control. The valving can also be designed so that it vents pressure and compresses easily when hitting a bump to soften jolts. In some shocks, small bypass slits or grooves in the wall of the piston chamber provide a “comfort zone” where resistance is reduced for a smoother ride. But when the piston moves beyond this zone, resistance goes up for a firmer ride.

Adjustable valving is available on some shocks and struts so the dampers can be tuned to changing driving conditions either manually or electronically. Manually adjustable shocks have a screw or dial to change the internal valving, while electronic shocks typically use a solenoid or small electric motor.

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