Swingarm
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A swingarm 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.
Prior to the rear swingarm the most common suspension was first none - the frame design being a stronger version of a bicycle frame - then "plunger" in which the axle moved up and down two vertical posts. 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:
Twin-shock - 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. This makes wheel maintenance simpler since removal involves the loosening of a single nut, sliding the wheel forward to slacken and uncouple the drivechain, and then pulling out the axle shaft. With their S-shaped contour, these swingarms need to be much stiffer than the double-sided versions to accommodate the new torsional forces incurred by holding onto the wheel by just one side. Having a single mounting point also guarantees proper wheel alignment. The single-sided swingarm are also found on small machines where a robust chain case acts as the swingarm connecting engine and rear wheel.
Ducati has created several models featuring these swingarms but have chosen to return to double-sided versions for their most recent models. BMW has long been an advocate and many BMW designs incorporate shaft-drive rather than chain drive.
The quest for additional performance through weight reduction has seen the departure of the center stand that can be used to raise the rear wheel off the ground. Lacking this built-in facility, quick rear wheel removal becomes moot without a wheel stand.
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.
