Suspension school · Progression & Volume

Volume Spacers in Practice

When to add, when to remove, and how to diagnose it with the O-ring

A volume spacer is a plastic block that takes up space inside the positive air chamber. Fitting one does not change the air pressure you pump in, so the fork or shock feels almost identical at sag and through the mid-stroke. What changes is the end of the travel. With less volume to compress into, pressure builds faster deep in the stroke, and the last third of the travel firms up.

Sag stays the same

Sag lives in the first part of the stroke, where the chamber is still large and a spacer's few cubic centimetres barely register. That separation is the whole point of the system. Pressure sets sag and ride height. Spacers set bottom-out resistance. You can tune one without disturbing the other, which means you should never be adding air pressure just to stop bottom-outs, and never pulling spacers to fix a sag problem.

The two do work as a pair, though. Adding a spacer often lets you drop pressure by around 3 to 5psi in a fork, gaining small-bump grip while the extra ramp-up keeps the big hits covered. That trade is one of the most useful tunes in mountain biking.

The O-ring diagnosis

The rubber O-ring on the stanchion or shock shaft is your evidence. Push it down against the wiper seal at the top of a descent you know well, ride the descent properly, then read it at the bottom. Do this over several rides before touching anything, because one ride is an anecdote.

• Ring reaches the end regularly and you feel a harsh metallic thud: not enough progression, add a spacer
• Ring reaches the end once or twice a ride with no harshness: perfect, leave it alone
• Ring never gets past about 80 to 85 percent even on your biggest hits: too much progression or too much pressure, remove a spacer first if sag is correct
• Ring shows full travel but you never felt it: also healthy, a well-tuned bottom-out is soft enough to miss

Using full travel once or twice a ride is the healthy pattern. Travel you never use is travel you paid for and are not getting, and it usually means the ride is firmer everywhere than it needs to be.

When to add a spacer

• You bottom out harshly on big hits even though sag is set correctly
• You want to run lower pressure for grip but the fork already uses all its travel
• Your riding includes drops, jumps, or steep terrain with compressions
• You weigh 95kg or more kitted up, where factory volumes are usually optimistic
• You race, where fatigue and speed produce bigger hits later in the day

When to remove a spacer

• You cannot reach roughly 90 percent of travel on the biggest hit of a real descent
• The mid and end stroke feel harsh, like the fork hits a wall part-way down
• Your hands and forearms pump up on fast rough sections while the O-ring shows unused travel
• You mostly ride smoother trails without big single impacts

One at a time

Change one spacer, reset sag to confirm nothing drifted, then ride the same trail you used for the diagnosis. Each spacer is a clearly noticeable step, so there is no reason to jump two at once. Keep a note of what is installed. Six months from now, when the bike feels different, that note is worth more than any setup chart.

The eMTB starting point

With 20 to 27kg of bike underneath you, bottom-out loads are simply higher than on an analogue bike. Start at the factory count for your fork and travel, but expect to end up one spacer above where the same fork would sit on a lighter bike. Riders at 95kg and above, and anyone with a race bias, should treat one extra spacer as the default starting point rather than the exception. Fox acknowledges this directly by shipping its E-Bike+ air springs with an extra spacer already installed.