Motorcycle Rear Suspension Systems
The rear suspension has had but two major changes in its evolution, but it turns out that the latter form has a myriad different forms.
Hard Tail & Friends
The hard-tail or rigid rear suspension was characteristic of early motorcycles that relied on the cushioning effect of the rear tire and the seat springs and the rider's bleeding kidneys to absorb road shock.
The first popular attempt at rear suspension was called the "plunger" system. This set-up used a set of small spring-shock absorbing devices between the rear wheel and the rigid rear section of the machine. Limited rear-wheel vertical travel helped to cushion road shock without resorting to excessively large balloon tires. This plunger system was the forerunner of modern swing arm rear suspension. Fortunately no motorcycle manufacturer these days is crazy enough to manufacture this very primitive form of rear suspension.
While the adoption of front suspension systems occurred just prior to World War I on pretty well all motorcycles manufactured at the time, several motorcycle manufacturers were holdouts and did not adopt rear suspension systems until well after World War II was through. However, rear suspension motorcycles were available on the market for a couple of years before World War I. The 1913 Indian Single, for example, had a swingarm which was attached to a leaf spring (which made it look like the side of an old truck once the fender had fallen off).
One of the reasons why rear suspension systems were adopted rather late by some manufacturers is due to the belief at that time that hard tails handled on smooth asphalt better than sprung rear suspension motorcycles. That was true only if you consider the state of the art in the Great Depression. Some of those rear ends had springing that resembled a trampoline, so on smooth roads, I'd probably opt for a hard tail too. However, since there were very few smooth roads in those days, any suspension had to be better than no suspension at all! I've ridden hard tails and all I can say is OUCH!
Swingarm Rear Suspension
The swingarm rear suspension type is used almost universally today, so our discussion of rear suspensions is limited to this design. It consists of a pivoting fork, or swing arm, attached to the lower section of the frame just behind the transmission area, and a spring-shock absorber unit that vertically supports the top rear section of the frame. There are literally as many variations as there are modern motorcycles. Some have two spring shock absorbers one per side mounted from under the center-rear side seat to the swing arm, some have a single spring shock absorber (monoshock) mounted far forward, right behind the engine, and some bikes have just a single arm swingarm which makes the bike look very strange from that one side but certainly does make changing that rear tire easier.
There are some bikes which have the shaft drive acting as one side of the swingarm, and there are some motorcycles where the shaft drive is the only swingarm that there is. Many scooters, even the ones with engines more than large enough to be listed as true motorcycle powerplants, which mount the engine on the swingarm itself. So much for unsprung weight!