Suspension school · Progression & Volume

What Progression Actually Is

Spring curves explained, and why air ramps up while coil stays straight

A spring curve is a simple graph. Along the bottom is how far the suspension has compressed, up the side is how much force it pushes back with. Every conversation about volume spacers, ramp chambers, coil conversions and bottom-out is really a conversation about the shape of that line.

Linear vs progressive

A linear spring pushes back the same extra amount for every extra millimetre of travel. A coil spring is close to perfectly linear. If the first 10mm takes 30kg of force, the next 10mm takes another 30kg, all the way to the end. A progressive spring gets firmer as it compresses, so each successive millimetre costs more force than the one before it. The deeper you go, the harder it fights back.

Why does the shape matter? Because suspension has three jobs that pull in different directions. Off the top it should be soft, so the wheel tracks small bumps and the tyre keeps gripping. In the middle it should be supportive, so the bike does not wallow when you pump, corner or brake. At the end it should be firm, so a flat landing does not send a metallic clank through the frame. No single straight line does all three well. A curve that starts shallow and steepens towards the end can.

Why air is naturally progressive

An air spring is a sealed chamber of gas. Compress the fork and the chamber gets smaller, which raises the pressure. Halve the volume and the pressure roughly doubles. Early in the stroke the chamber is still large, so each millimetre of travel removes only a small fraction of the volume and the force builds gently. Deep in the stroke the chamber is already small, so the same millimetre removes a much bigger fraction and the force climbs steeply. That is why the last 20 percent of travel on an air fork takes far more force than the first 20 percent, without any extra parts fitted.

This is also why volume is the tuning lever. Make the chamber smaller and the ramp-up starts earlier and climbs harder. Make it larger and the curve flattens towards coil-like behaviour. Volume spacers, ramp chambers and oversized air sleeves are all just ways of changing chamber size.

The negative spring

Modern air springs also contain a negative chamber that pushes the opposite way, helping the fork or shock move off the top against seal friction and the initial air pressure. Most current designs self-equalise through a transfer port as you cycle the suspension. A good negative spring is why a modern large-volume air fork can feel almost coil-like in the first third of its travel while still ramping hard at the end. It shapes the start of the curve, while volume tuning shapes the end.

The frame is part of the spring

At the rear, the wheel never feels the shock directly. The linkage sits in between, and its leverage ratio changes through the travel. A frame whose leverage falls by 25 percent from start to finish adds its own ramp-up on top of whatever the shock provides. A frame that is nearly linear leaves the whole job to the shock. Two identical shocks with identical settings can behave completely differently in two different frames, which is why there is a separate article on matching progression to your linkage.

Why this matters more on an eMTB

A full-power eMTB weighs 20 to 27kg before you add a rider, a pack and a bottle. Every load the suspension sees scales with that mass. Compressions, flat landings, braking bumps at speed and the g-out at the bottom of a steep chute all arrive with more energy than they would on a 15kg analogue bike. The start of the spring curve can stay soft, but the end of the curve has to absorb noticeably more, which is why eMTB setups usually run slightly more progression than the same fork or shock on a lighter bike.

The vocabulary

• Spring rate: how much force the spring adds per millimetre of compression at a given point
• Linear: the rate stays the same all the way through the travel, like a coil
• Progressive: the rate rises as travel is used, like any air spring
• Ramp-up: the steep final section of an air spring curve
• Progression tuning: changing how early and how hard the ramp-up arrives, usually via volume or a ramp chamber
• Leverage curve: the frame's own contribution to what the rear wheel feels