Suspension school · Progression & Volume

Coil Springs and Progression

Linear springs, progressive coils and hydraulic bottom-out on heavy bikes

A coil spring is the benchmark for suppleness. There is no air seal friction, no pressure ramp from heat on long descents, and the rate you buy is the rate you get, lap after lap. On an eMTB, where motor power makes technical climbing traction genuinely useful, that constant grip is worth a lot. The catch is the shape of the curve. A coil is almost perfectly linear, and linear has a weakness at the bottom of the stroke.

The linear problem on a heavy bike

With a linear spring, the last millimetre of travel resists no harder than the first. All bottom-out control has to come from somewhere else, either the frame's leverage curve or the damper. Put a coil shock on a frame with little built-in progression, add the mass of a 20 to 27kg eMTB, and hard bottom-outs arrive quickly. This is why the coil question is really a frame question, covered in the next article, and why the industry has produced two good workarounds.

Workaround one: progressive coil springs

Several spring makers wind coils with variable spacing or dual rates, so the spring stiffens as it compresses, typically ramping its rate by roughly 15 to 35 percent by the end of the stroke depending on the model. A progressive coil keeps the sensitivity of steel off the top while adding a measure of the end-stroke support an air spring gives you. It is a partial conversion rather than a full one, because the ramp is fixed at manufacture. You cannot tune it the way you can add a token or 10psi of ramp pressure, only swap the spring.

Workaround two: hydraulic bottom-out

Hydraulic bottom-out (HBO) attacks the problem through the damper instead of the spring. RockShox's implementation on the Vivid and Super Deluxe is a separate damping circuit that engages in the last 20 percent of the travel, with a five-position adjuster to set how firmly it cushions the final movement. Full travel is still available, but the shock arrives there slowly and quietly. A bottom-out becomes a dull thud rather than a clank.

HBO and spring progression are not the same thing, and the difference matters. Spring progression is position-sensitive, pushing back whenever the shock is deep in its travel, however slowly it got there. HBO is speed-sensitive, resisting hard on a fast flat landing but letting a slow g-out settle deep without harshness. For a heavy bike that is a genuinely useful behaviour, because it catches the violent events while leaving the spring free to stay soft and linear the rest of the time.

When coil suits an eMTB

• Your frame has meaningful leverage progression, roughly 20 percent or more, so the linkage supplies the ramp the spring lacks
• You value traction first: technical climbs under motor power, wet roots, off-camber grip
• You ride long, sustained descents where air-spring consistency fades and coil does not
• Your shock has HBO or you fit a progressive spring, covering the bottom-out gap
• The weight penalty of roughly 300 to 500g is irrelevant to you, and on a 24kg bike it usually is

When coil is the wrong answer

• Your frame is close to linear and your shock has no bottom-out circuit
• You jump, drop and ride bike park regularly and hate the sound of a hard bottom-out
• You want one bike to cover wildly different terrain, where air's tunability wins
• You are between spring rates and the frame's progression cannot cover the gap

One last note on rates. Coil springs come in fixed steps, usually 25lb apart, and an eMTB's extra mass means you will often sit between sizes that would be obvious on a lighter bike. If sag is right at the top of the stroke but the bike rides deep, the frame and spring rate are having an argument that no clicker will settle. Get the rate right first, then tune.