Springs in your Rear Suspension

Things at the rear are a little different though. With our front end telescopic forks the wheel movement and the spring movement are linear and there's a 1:1 correspondence between them, rear suspension tends to be a bit different.

Firstly, the rear wheel is moving in an arc, but for most purposes we can consider that as up-and-down, but quite often the shock absorber, and hence the spring, are leant over and are moving in a different direction from the rear wheel which doesn't bode well for that nice 1:1 correlation between wheel and spring you have with front forks.

The two main problems that rear suspension poses are the relationship between wheel movement and spring movement, and the relationship between the wheel rate and the spring rate. From an engineering point of view, those are actually the same problem, but it makes it easier to explain if I treat it as two problems.
Unless you have a rising-rate linkage type set-up, then irrespective of whether the rear suspension has one or two suspension units, there's going to be a moving shock mount, and a fixed shock mount. The problem s that as the wheel won't move in the same direction as the shock is compressed, so the shock will have to 'swing' back and forth in an arc, which means that for each successive millimeter the wheel moves in its travel, the shock is compressed by a smaller and smaller amount. Looks cool, gets softer as it moves.

This also means that a shock with 102mm travel may well let the rear wheel move a lot further that 102mm, which is going to get embarrassing if the rear wheel moves further than the amount of ground clearance. You can work all this out using a lot of complicated trigonometry, but on the other hand you can just measure the distance from the pivot to the shock mounts, the length of the shock and its travel, and draw it all out if you're thinking of having radically canted shock absorbers. If that seems like too much trouble, then making sure that the shock mounts are equidistant from the swingarm pivot is simple and not going to throw up any nasties.

This article is a sort of follow-up from The Front Fork Springs.
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