Swingarm

A swingarm (or swinging arm), originally known as a swing fork or pivoted fork, is a single or double sided mechanical device which attaches the rear wheel of a motorcycle to its body, allowing it to pivot vertically. The main component of the rear suspension of most modern motorbikes and ATVs, it holds the rear axle firmly, while pivoting to absorb bumps and suspension loads induced by the rider, acceleration, and braking.

Originally motorcycles had no rear suspension, as their frames were little more than stronger versions of the classic diamond frame of a bicycle. Many types of suspension were tried, including Indian's leaf spring suspended swingarm, and Matchless's cantilevered coiled-spring swingarm. Immediately before and after World War II, the plunger suspension, in which the axle moved up and down two vertical posts, became commonplace. In the latter, the movement in each direction was against coiled springs.

Some manufacturers, such as Greeves, used swingarm designs for the front forks, which were more robust than telescopic forks. In particular, sidecar motocross outfits frequently use swingarm front forks. The swingarm has also been used for the front suspension of scooters. In this case it aids in simplifying maintenance. In motorcycles with shaft drive, such as the Yamaha XJ650 Maxim, the shaft housing forms the left side swingarm.

Swingarms can be single or double sided, and have appeared in a variety of geometries. Many single-sided also incorporate the driveshaft.

Swinging fork - the original version consisting of a pair of parallel pipes holding the rear axle at one end and pivoting at the other. A pair of shock absorbers are mounted just before the rear axle and attached to the frame, below the seat rail.

Cantilever - An extension of the swinging fork where a triangulated frame transfers swingarm movement to compress shock absorber/s generally mounted in front of the swingarm. The HRD-Vincent Motorcycle is a famous early form of this type of swingarm, though Matchless used it earlier, and Yamaha subsequently. The Harley-Davidson Softail is another form of this swingarm, though working in reverse, with the shock absorbers being extended rather than compressed.

Parallelogram - Was first introduced commercially in 1985 on the Magni "Le Mans". Magni called the system Parallelogrammo. Various parallelogram systems have been developed by other manufacturers.

Whereas downward generated mechanical forces would cause a chain-driven bike to "squat" at the rear under acceleration, the torque reaction of a shaft drive machine would induce the opposite, causing the seat (and rider) to rise upwards, a phenomenon known as "shaft-jacking". This anti-intuitive sensation can be disconcerting to riders, which parallelogram suspensions seek to neutralize.