A swingarm (originally known as a swing fork) is the main component of the rear suspension of most modern motorcycles and ATVs. It is used to hold the rear axle firmly while pivoting vertically to allow the suspension to absorb bumps in the road.
Originally there was no rear suspension - the frame design being a stronger version of a bicycle frame. Many types of suspension were tried including Indian's leaf spring suspended swingarm, and Matchless's cantilevered coiled spring swingarm. Immediately prior to and after WWII the "plunger" system 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 made them more robust than telescopic forks. The swingarm has also been used for the front suspension of scooters. In this case it aids in simplifying maintenance.
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 swing arm movement to compress shock absorber/s generally mounted in front of the swingarm. The HRD-Vincent 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.
Single-sided swingarms allow the rear wheel to be mounted like those of an automobile.
Parallelogram Suspension was first introduced commercially in 1985 on the Magni "Le Mans". Magni called the system Parallelogrammo. This new suspension system eliminated the torque reactions normally associated with shaft drive motorcycles and enabled the bike to perform similarly to a chain-drive unit. Similar systems have been developed by other manufacturers.
Paralever is BMW's version of the system. It allows the driveshaft to pivot along the same axis as the sprung rear frame due to the addition of second link between the rear drive and transmission. Paralever was originally introduced in 1988 R80GS and R100GS motorcycles and provides a reduction in the phenomenon known as "shaft jacking" where the rear of the motorcycle would lift skyward under certain riding conditions.
Moto Guzzi has introduced a variant of the system it named the Compact Reactive Drive Shaft system (patented and named Ca.R.C.). The main difference is that the driveshaft is free to float into its structure providing much softer feedback from transmission. Additionally, the upper arm of the Ca.R.C. is not part of the structure but just a guide to close the geometry of the suspension (it means that, differently from BMW version, the suspension will work also with a broken upper arm).