Swingarm
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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 then telescopic forks. The swingarm has also been used for the front suspension of scooters. In this case it aids in simplifying maintenance.
[edit] Swingarm Types
Swingarms have come in several forms over the years:
Twinshock - 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.
Monoshock - came about in the mid-eighties and utilized a single shock mounted behind or under the engine and linked to the swingarm with additional pivots. This arrangement allowed for greater suspension travel and eliminates the need to synchronize shock absorbers.
Single-sided swingarms allow the rear wheel to be mounted like those of an automobile.
Paralever is BMW's rear suspension technology that 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 BMW Paralever it named the Compact Reactive Drive Shaft system (patented and named Ca.R.C.). The main difference is that the cardan 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 project, the suspension will work also with a broken upper arm).
It is believed that the term Paralever was developed due to the appearance of a parallelogram between the four items making up the rear suspension. (rear drive, drive shaft, transmission, and lower or upper brace). Other motorcycle manufacturers have started to adopt this approach.
Drag racing motorcycles will often use longer swingarms to keep their center of gravity, i.e. the engine, as forward as possible to reduce the tendency to wheelie at the start.
