Suspension school · Suspension Basics

Air vs Coil: The Honest Version

What each spring really does well, and why the answer shifts on an eMTB

A spring's job is to hold you up and give the energy of a hit back in a controlled way. Air does it with a sealed chamber of gas, coil does it with a wound steel spring, and both can be excellent. The differences are real, but they are smaller than the forum arguments suggest, and on an eMTB the balance genuinely shifts.

How each one works

An air spring is a piston in a sealed chamber. Compress it and the pressure rises faster and faster, which is why air is naturally progressive: it firms up deep in the travel and resists bottoming out. You set the rate with a pump. A coil spring is rated in pounds per inch: a 450lb spring needs 450 pounds of force to compress it one inch, and the second inch takes roughly the same again. That near-linear behaviour is the source of most of coil's character.

Where air wins

• Rate changes cost nothing. New pack, winter kit, a different bike feel: two minutes with a pump.
• Progression is tuneable with volume spacers, so one spring suits a huge range of frames and riders.
• It is lighter, typically a few hundred grams less than a coil shock and spring.
• The built-in ramp-up gives a safety margin when a landing goes wrong.

Where coil wins

• Less seal friction, so it moves on smaller bumps and the wheel stays glued to chattery ground.
• Consistency. A coil's rate does not care how hot the shock gets or what the weather is doing, so the tenth minute of a descent feels like the first.
• Fewer air seals to wear, so it tends to keep feeling fresh between services.
• A calm, planted mid-stroke that many riders describe as the bike feeling longer-legged than it is.

The catches

• Coil rates come in steps, usually 25lb apart, and finding yours can mean buying a second spring after the first few rides.
• A linear spring in a frame without much built-in progression can blow through its travel. Check the frame is coil-friendly before spending money.
• Air needs its seals serviced more often, and it changes feel a little with temperature.

The eMTB angle

Here is the honest bit. Coil's biggest historical downside is weight, and on a 23kg bike with a motor the extra few hundred grams is close to irrelevant. The motor also means you descend far more metres per ride than your legs alone would earn, and long repeated descents are exactly where coil's heat-proof consistency shows. That is why coil rear shocks are more popular on eMTBs than they ever were on analogue bikes.

The rear of an eMTB also works hardest of all. Motor and battery load the shock, so a spring rate has to account for the bike's mass as well as yours, and a chart written for an analogue bike will suggest a spring that is too light. Expect to run a firmer spring than an equivalent unassisted bike would take, and use an eMTB-aware calculator or a shop that knows the frame.

Up front the picture is different. Coil forks exist but they are rare and heavy, and modern air forks are so good that nearly everyone, eMTB or not, runs air in the fork. In practice the choice is really about the rear shock.

So which one

Air is the sensible default: adjustable, progressive and light enough that you never think about it. Coil earns its keep if you ride long or steep descents regularly, value grip and consistency over fine-tuning, and your frame is happy with a linear spring. On an eMTB the case for a coil shock is stronger than on any other bike, but it is a nice-to-have rather than a requirement, and a well set up air shock gives away very little.