Instant expert: PROGRESSIVITY
What is it?
A bike’s leverage ratio is how far the rear axle moves for every millimetre the shock shaft moves. ‘Linear’ suspension set-ups maintain the same leverage ratio throughout their travel, so the effective spring rate of the bike (how hard it is to compress the suspension at any point in its stroke) is dictated entirely by the spring rate of the shock. On ‘progressive’ frame designs, the leverage ratio decreases as the suspension moves through its travel, making it progressively harder to compress the shock, irrespective of the spring rate of the shock itself. This ‘ramping up’ towards the end of the stroke is known as ‘progressivity’.
What affects it?
Not only do suspension systems have differing amounts of progressivity built in, but rear shocks do too. Coil-sprung shocks are naturally linear, while air springs are naturally progressive and can be made even more so. So, a bike with a linear suspension design, such as a singlepivot, can be made progressive by using an air shock. And bikes designed to be compatible with coil shocks tend to have progressive linkages.
What effects does it have?
On a linear bike, the suspension doesn’t ramp up towards the end of its stroke. This can lead to harsh bottom-outs unless you increase the spring rate (by upping the air pressure or fitting a heavier coil), which reduces small-bump sensitivity and grip. On a progressive bike, the firmer end to the stroke means you can maintain the same spring rate without bottoming out – or even reduce the spring rate, for a softer feel earlier in the travel without bottoming out any more than a linear bike.