A swingarm is a part of a motorcycle or bicycle frame that attaches the rear wheel to the main frame while still allowing the rear wheel to pivot up and down when a shock or bump is incurred. Several swingarm designs exist, and the single-sided swingarm is commonly used today on motorcycles. Earlier versions featured a bar on each side of the wheel that attached to an axle running through the wheel at one end of the arms, and attached to the frame at the other end by an axle run through bearings, bushings, or both.
Early motorcycles did not have rear suspension at all. The rigid frames would have to absorb the shock from road vibrations and other types of shocks, which meant frames frequently broke. The swingarm made shock absorption more efficient, though early models were somewhat clunky and prone to failure. Plunger systems allowed the rear axle to travel up and down two vertically-mounted posts, and while this system worked, it was not always the smoothest shock absorption system. The rebound after a shock was absorbed could alter the way the bike handled.
The swinging fork was another type of swingarm that improved on the plunger design. The parallel bars that attached to the main triangle at one end and the axle at the other were attached to a pair of shocks, one on each side of the bike. The shock ran from the swingarm to the seat rail of the bike, providing a near-vertical shock absorption system. This allowed for more plush shock absorption, but it also added weight to the system.
A cantilever version of the swinging fork became common as well. This variant used the same parallel bar idea as the original swinging fork, but the shocks were not mounted between the swingarm and the seat rail; instead, the shock was mounted toward the front of the swingarm and attached at the other end to the main triangle of the frame. This allowed for the use of one shock instead of two, and it improved the motion in which swingarms could travel, thereby improving handling of the motorcycle. From this design, the single-sided swingarm was born. This design works similarly to the cantilever shock system, but instead of using two parallel bars to connect the rear wheel to the frame, only one bar is used, and it is mounted on only one side of the wheel.
