The Missing Linkage

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By John Burns

Dear MOby,

Say, what’s all this about linkage-type suspensions and non-link-type ones anyway, and what difference does it make? What’s the difference between a progressive spring and a normal one, and what’s all this rising-rate business?

Suspended Disbelief

Dear Suspended,

The Aprilia Shiver pictured above and lots of other bikes use a plain old non-link type suspension, wherein the swingarm is simply connected to the frame via the rear shock absorber. Its main advantage is that it’s simple, lightweight, has no bearings that need lubricating and doesn’t take up much space. On a bike that doesn’t need a lot of wheel travel, it’s perfectly adequate.

A disadvantage, in the Shiver’s case, is that its spring is just as stiff over small bumps as it is over bigger ones, since it’s a “straight-rate” suspension; the suspension doesn’t get stiffer as the rear wheel moves higher in its travel.

Old-fashioned dual shocks like the ones on this Harley Sportster are also “linkageless,” but H-D gets around the lack of a rising rate by winding these particular shocks with progressive springs, which are softer in the first part of wheel travel to soak up small bumps, and stiffer over larger bumps. The first bit of wheel travel mashes the tighter coils at the top of the spring together so there’s no space left between them, called “coil bind.” Once that happens, there are fewer coils to compress, which effectively makes the spring stiffer. Progressively wound springs are a cheap and easy way to achieve a rising-rate suspension, i.e., the further the suspension is compressed the stiffer it gets.

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