Suspension Setup: Air spring pressure and volume spacers

The stiffness of the spring describes the ratio between a force acting on the spring and its deflection.

It is how elastic is a suspension when you apply a certain force (or weight) to it Stiffness directly impacts riding dynamics and indirectly affects all other adjustment options. Spring stiffness allows you to adjust the sag, which is crucial for riding dynamics.

Sag is the distance your suspension compresses under your weight and gear. Optimal sag enables the suspension to work efficiently in both compression and rebound, making sure your tires stay in contact with the ground when there are holes on the trail. Depending on the use and available travel, riders typically choose a sag between 23 and 33 percent. Generally, the more travel you have, the higher the sag values (DH and bike park riders will often target 28%).

For full-suspension bikes, ensure that the sag of the fork and shock do not differ too much. Otherwise, the bike’s handling might become too front- or rear-biased.

For air suspension (suspensions with an air spring using pressurised air), stiffness can be easily adjusted by changing the air pressure using a special pump for mtb suspensions (often called "shock pump").

Remember to account for your gear's weight, like clothes, shoes, or a backpack. Total weight of equipment and clothing is rarely under 4 kgs, and generally between 5 and 7 kgs.

Tip: Manufacturers like Fox Racing Shox and RockShox provide setup tips for their suspension forks, often found directly on the fork stanchions. This isn’t possible for shocks due to the varying leverage ratios of different rear frames.

One you get the pressure right for the target sag, it is possible that the suspension will still feel too stiff or not reactive enough, this is where you should question how the spring is setup internally and whether the spring chambers should be re-configured to work better with a pressure that make sense for your weight or practice.

Modern air springs are usually based on two chambers:

• the positive chamber
• the negative chamber

The positive chamber is the main air chamber that supports the rider’s weight and handles most of the spring's work, absorbing impacts and bumps while riding.

The negative chamber is a smaller chamber right under it, separated by a mobile piston, that works in conjunction with the positive chamber. The negative chamber balance the force of the positive spring when the fork is in its neutral/unloaded state and has a great influence on small bump reactivity.

Each have a volume which, in relation to the pressure being used, has a great influence on how the suspension will feel. Positive air chamber volumes can generally be changed using volume spacers. Negative air chamber generally have a set volume that can not be changed except when using upgrades like Vorsprung's Lufktappe.