monotube

Monotube is a term used in engineering to describe a design that uses a single cylindrical tube as the primary enclosure for a given function. The most common association is with monotube shock absorbers in automotive suspension systems, where a single cylinder contains the damping piston, hydraulic fluid, and, in many designs, a charge of nitrogen gas.

In a monotube shock absorber, the damping action occurs as the piston moves through the oil inside the main cylinder. Gas pressure is used to separate the gas (often nitrogen) from the oil, or to form a floating piston that separates the gas and fluid. This arrangement helps control pressure, reduce cavitation, and manage heat generated during damping. Compared with twin-tube dampers, monotube designs typically offer better heat dissipation, more consistent damping under heavy use, and a firmer feel, though they can be pricier and sometimes ride firmer on regular roads.

There are design variants within monotube concepts. External-reservoir monotube shocks include a separate reservoir attached to the main body to increase oil capacity and improve fade resistance; internal-reservoir or integrated designs use wider main cylinders and internal baffling to achieve similar effects. Monotube configurations are also used in other hydraulic and gas-dpring applications where a single-tube enclosure provides a simple, efficient damping or actuation path.

In summary, monotube denotes a single-tube design used in various devices, with automotive monotube shock absorbers being the most widely recognized example. See also twin-tube shock absorber for a contrasting approach.