Suspension school · Progression & Volume

Matching Progression to Your Frame

Leverage curves, linkage progression and why the same parts feel different on different eMTBs

The rear shock never meets the trail directly. The linkage sits between wheel and shock, and its leverage ratio, how many millimetres the wheel moves for each millimetre of shock stroke, changes as the bike compresses. Plot that change and you have the leverage curve, the frame's own contribution to progression. Ignore it and you can chase your tail for months with tokens and pressures.

Reading a leverage curve

A bike that starts at a 3.0 leverage ratio and falls to 2.4 at bottom-out has 20 percent progression. The falling ratio means the wheel needs progressively more force to compress the shock the same amount, so the frame itself behaves like a progressive spring, before the shock adds anything. Manufacturers increasingly publish these curves, and reviews often quote the percentage. It is the single most useful number for deciding how to spring the back of your bike.

High vs low progression frames

• Low progression, under about 15 percent: the frame adds little ramp, so the shock must supply it. Air with spacers is the natural fit, coil only with HBO or a progressive spring
• Mid progression, roughly 15 to 25 percent: the flexible middle ground. Air and coil both work, and tuning is about taste
• High progression, above about 25 percent: the frame supplies plenty of ramp. Run air shocks with few or no spacers, and enjoy coil, because this is what the frame was drawn for

The symptoms of a mismatch are recognisable. An air shock stuffed with spacers in a high-progression frame never reaches the last quarter of its stroke, and the ride feels dead. A coil in a low-progression frame blows through to hard bottom-outs on hits the fork shrugs off. In both cases riders tend to blame damping, and no amount of clicking fixes a spring-shape problem.

Why the same fork feels different on different eMTBs

Forks have no linkage, yet the identical fork with identical settings still rides differently from one eMTB to the next. Geometry decides how much of the system's mass presses on the front wheel. A slack 63.5 degree head angle and long front centre push the front axle away from you and unweight the fork on steep terrain, while a steeper, shorter bike drives more weight through it everywhere. Then the eMTB numbers stack on top: the motor and battery add mass low and central, total system weight climbs past 100kg with many riders aboard, and heavy braking on a 25kg bike loads the fork harder and for longer than on an analogue bike.

The result is that a fork set up beautifully on one bike can dive and bottom on another. Trust the O-ring on the bike you actually own, not settings copied from a friend or a forum thread about a different frame. When you swap a fork between bikes, or replace an analogue bike with an eMTB, re-run the whole diagnosis, and expect the eMTB to want roughly one more token or a firmer ramp-chamber pressure than the bike it replaced.

Finding your frame's numbers

• Check the manufacturer's site for a published leverage or progression figure, often in the frame kinematics or FAQ section
• Look at what the brand ships and endorses: a frame offered with a coil shock from the factory is progressive enough for coil
• Search reviews for the progression percentage, which reputable tests usually quote
• If nothing is published, let the shock tell you: count how many spacers you need for a healthy bottom-out pattern, and that is your frame's character in practice

The goal is a sensible division of labour. Let the frame provide the progression it was designed to provide, use the shock's volume or ramp tuning to trim the last 10 or 20 percent of the behaviour, and keep damping for controlling speed, not for papering over the wrong spring shape.