A torsion axle is a form of combined axle and suspension unit where the deflection of the suspension is provided by the torsion, or twisting, of the axle bar within a steel sleeve that is lined with rubber compounds to provide rotational resistance. The dynamics of the torsion axle are founded in the traditional use of torsion bar suspension. In systems featuring torsion bar suspension, however, vertical movement of the wheel causes the actual bar itself to twist rather than rotate in a resistive sleeve.
Historically, torsion bar suspension has been used in a variety of vehicles, trailers, and military equipment. The use of this type of suspension started to become common through the mid to late 1930s, but it has declined in popularity since the late 1980s. The benefits of this type of suspension include durability and easy adjustment of ride height and spring rate. The lack of the adaptability in spring rate, as provided by standard coil spring suspension systems, has been known to lead to hard, bumpy rides.
Differing slightly in design, torsion axle systems are mostly found on towing trailers due to the free-wheeling nature of trailer wheels. In a torsion axle configuration, the two adjacent wheels are fixed to the end of right-angled stub sections located at either end of an interconnecting straight metal bar. The connecting bar is manufactured in a variety of cross-sectional profiles depending on the company producing the system.
As the trailer passes over bumps or rough ground, vertical movement in the wheel produces a rotational effect in the interconnecting bar. To create a suspension effect, the straight bar passes through a sleeve that is firmly fixed to the chassis of the vehicle or trailer. Torsion resistance is provided by a series of heavy-duty rubber sections that run through this sleeve. When the bar rotates in the sleeve, the rubber sections are compressed, damping the rotation and returning the wheels to their set ride height.
The form of torsion axle described above provides nonindependent suspension characteristics as both wheels are fixed to the same rotating bar. This means that, if one wheel experiences vertical movement, the opposite wheel will also experience a percentage of that vertical movement. The amount of movement experienced by the opposite wheel is determined by the torsion resistance in the connecting bar itself; if the bar has a low torsion resistance, then the opposite wheel may experience no movement whatsoever. Torsion axles systems for trailers are also available with independent suspension, where the bars connecting each wheel are separate and terminate within the sleeve.