A swingarm, or "swinging arm" (UK), originally known as a swing fork or pivoted 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.

A motorcycle's suspension serves a dual role, providing the rider with control of steering and braking while also absorbing the road conditions to give a more comfortable ride. The suspension components consist of fork tubes on the front of the bike and a swing arm in the rear. The swing arm is the main component of the rear suspension and also provides a base for the rear axle to be mounted.

The swing arm is joined to the motorcycle at a higher pivot point than where the rear axle is connected. This works to prevent squat in the tail of the bike when you accelerate and helps to provide adequate spacing for the shocks to function. When the rear brakes are applied, the swing arm is pulled level with the road. This lowers the pivot point where the swing arm joins the bike frame and lengthens the wheelbase at the same time, making the bike more stable and easier to control.

There are two types of swing arms found on most bikes. Typically, most bikes have what is referred to as a monoshock regular swing arm. In this design, a coilover shock is joined to a linkage that is connected to the bike frame and the H-shaped swing arm itself. A newer version is the single-sided swing arm. This type is similar to the H-shaped swing arm in function and design, except that one side has been removed so a tire can be easily changed.

How the rear suspension on a motorcycle evolved

"The days before suspension - only a spring-loaded saddle"

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.

                                                                                                       "Plunger"

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.