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Shock Absorber Design: Monotube Versus Twintube and the Benefits of Each :: October 2nd, 2017

While there are many differences between the construction of a twintube and a monotube shock, the function is essentially the same. Both are designed to control the movement of the springs and keep tires in contact with the ground, and each have their respective benefits.

A common misconception is that a twintube shock is not gas-pressurized. While this was the case for many years, twintube shock technology has evolved and gas-pressure is now a standard in shock absorber design. Below we’ll take a look at the differences between monotube and twintube gas-pressure shock absorber designs and the benefits of each.

Monotube Shock Absorbers

What is a monotube shock absorber?
A monotube shock absorber is constructed using a single tube filled with oil and gas separated by a floating dividing piston. This prevents the negative effects of cavitation such as performance loss and suspension wear. As the piston rod and piston cycle through the hydraulic oil, the gas keeps the oil pressurized to prevent it from mixing with air and foaming up (cavitation).

How does a monotube shock absorber function?
As the tire and wheel move up and down on a vehicle, the piston inside the monotube shock absorber cycles through hydraulic fluid. As the piston moves up and down through the column of oil, it creates pressure differentials. These “pressure differentials” can be controlled by modifying the characteristics on the piston by adding or removing metal shims (valve plates), resulting in an increase or decrease in compression and rebound damping forces.

Gas Pressure Principle (monotube):
As a shock piston cycles through hydraulic oil, cavitation (the mixing of air and oil) will naturally occur. By applying gas pressure to the oil column, cavitation is prevented.

Benefits of a Monotube:
– Ability to run higher gas pressure (200-360psi) and less prone to cavitation
– Greater heat dissipation
– Larger oil capacity and piston diameter for precise and consistent damping force
– Shocks can be installed at any angle

Twintube Shock Absorbers

What is a twintube shock absorber?
A twintube shock absorber has a main outer tube and a secondary inner tube. The piston rod and piston cycle through hydraulic oil in the smaller inner tube of the shock, pushing oil into the outer tube.

How does a twintube shock absorber function?
As the tire and wheel move up and down on a vehicle, the piston cycles through the hydraulic fluid in the inner tube, pushing the oil into the outer tube. As the piston moves up and down through the column of oil (rebound and compression), it creates pressure differentials. These “pressure differentials” can be modified by changing the characteristics on the piston via adding or removing metal shims or plates to either side of the piston. This modifies how much oil flows each way through the piston, thus stiffening or softening the damping pressure.

Gas Pressure Principle (twintube):
As the piston cycles through the hydraulic fluid in the inner tube and forces oil into the outer tube, gas pressure in the outer tube reduces the likelihood of oil mixing with air (cavitation). When oil and air mix (cavitate), the oil becomes foamy and loses viscosity. This allows the piston and piston rod to move more freely through the tube, reducing control for the driver.

Benefits of a Twintube:
– Outer tube provides protection to internal components
– Cost effective design and manufacturing process
– Lower gas pressure creates a more comfortable ride
– Increased stroke due to gas being contained between tubes versus chamber (as compared to non-reservoir monotube shocks)

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