Suspension school · Suspension Basics

Air Pressure: Doing It Properly

Pump technique, the equalisation ritual, and the pump hiss myth put to bed

Air pressure sets your spring rate, which is how firmly the suspension resists compressing. It is the single most powerful adjustment on the bike, and unlike a coil spring it costs nothing to change. All you need is a shock pump and a few unhurried minutes.

Start from a chart, not a guess

Every manufacturer publishes pressure charts by rider weight, usually printed on the fork leg or the shock body and always on their website. They are decent starting points, typically within 10psi of where you will end up. The calculator on this page starts from those same manufacturer charts, then adjusts for your weight with kit, your bike, how you ride and where you ride, so it lands you closer still. Either way, the chart gets you into the neighbourhood and the sag measurement gets you to the door.

Use the right pump, the right way

• Use a proper shock pump. They read to 300psi or so on a fine gauge and move a tiny volume per stroke. A track pump or a compressor is far too crude for the job and can damage seals.
• Thread the pump on smoothly until the gauge registers, then stop turning. Overtightening chews up the valve seal.
• Stick with the same pump. Gauges vary by a few psi between pumps, and consistency matters more than absolute accuracy.
• Check pressure with the bike unweighted and the suspension at rest, ideally at room temperature.

The equalisation ritual

Modern air forks have two chambers: the positive one you pump up, and a negative chamber on the other side of the piston that softens the very start of the travel. They balance themselves through a tiny transfer port, but the port only lines up at one point in the travel. So when you are airing a fork up from empty, or making a big change, add about 50psi at a time and slowly cycle the fork through a few deep compressions before adding the next 50. That lets the negative chamber catch up through the port.

Skip the ritual and the chambers end up out of balance. The classic symptoms are a fork that sits strangely high and feels harsh off the top, a fork that sucks itself down into its travel, or a pressure reading that looks right but rides wrong. Shocks appreciate the same treatment after big changes: pressurise in stages and cycle slowly between them.

The pump hiss myth

Unthread a shock pump and it hisses, and half the internet will tell you that hiss is your carefully set pressure escaping. It is not. The hiss is the air in the pump hose emptying, and it happens after the valve has already sealed. You lose nothing when you take the pump off.

The air you actually lose moves when you put the pump on, because the empty hose has to fill from the spring. On a fork the chamber is big enough that it barely matters. On a shock, with its small can and high pressure, attaching the pump can drop the true pressure by several psi. The practical fix is simple: always measure the same way with the same pump, and treat the number as your personal reference rather than gospel. Checking and topping up in the same visit corrects the loss automatically.

Temperature moves your pressure

Air pressure tracks temperature. As a rule of thumb, a fork set to 80psi in a 20 degree garage will be down roughly 3psi for every 10 degrees the temperature falls, so a frosty morning can quietly take 5 or 6psi out of it. Cold also stiffens seals and thickens damper oil, which is why the same bike feels firmer and slower in January. Set up in conditions close to the ones you ride in, and expect to tweak with the seasons.

Heat works the other way. A long sustained descent warms the damper and nudges pressure up a touch. That is normal and nothing needs doing mid-ride; just do not be surprised if a big mountain day reads slightly different at the bottom.

eMTB pressures

A 20 to 27kg bike needs more spring than a 14kg one, which is why RockShox prints 'for eBike add 10psi' on its fork legs and Fox publishes separate E-Bike+ charts, roughly 9psi up on a 38. The rear takes the biggest share of the extra load because the motor and battery sit low and central. If your sag says you need more pressure than the analogue chart suggests, believe the sag.

Reading the signs

• Too much pressure: harsh through the hands, tyres skipping instead of tracking, the bike riding high, the last 20% of travel never touched.
• Too little: diving under braking, a vague wallowing feel mid-corner, hard clunking bottom-outs, pedals clipping the ground more than usual.

When you adjust, move in small steps: around 5psi at the fork, 5 to 10psi at the shock. Then re-check sag and ride the same piece of trail before deciding anything.