This article was originally published in 2013.
A mountain bike’s suspension will help you navigate the rocks, roots and drops on your favourite run of singletrack—if that suspension is dialed properly, that is. Pneumatic suspension is designed to be easily set up and adjusted, but there are many variables to consider, such as rider weight, trail condition and personal preference. For the most accurate setup, it is best to prepare by donning your typical riding gear, including helmet and hydration pack, to mimic your static weight on the trail. Also, set any on-the-fly adjustments to their neutral positions.
There should be an O-ring on the shock body by the dust seal. This O-ring is your sag indicator. Slide the indicator up against the dust seal and sit on the bike carefully without bouncing. Dismount with care and check the distance between the indicator and the dust seal. The distance should measure roughly 15 to 25 per cent of your total travel depending on your riding style and the manufacturer’s specifications. If it is more, increase the positive pressure, and if it’s less, reduce it. Add air pressure in the positive air-pressure valve with a shock pump. Remember to check the sag regularly to make sure you haven’t lost air pressure through use.
Next, if required, adjust your negative air pressure using the shock pump at the negative air-pressure valve. This pressure should be roughly the same as the positive pressure, give or take 10 psi for personal preference.
To adjust the rebound damping, and to get a feel for how it works, try setting it to the fastest position and jump on the bike. It will probably feel like a pogo stick bouncing you right back up. Next, change to the slowest setting and do the same test; the suspension will slowly return to normal. From the slow setting, adjust incrementally until the shock returns with a quick, but controlled speed.
Now, you’re set to hit the trails. Check the various air pressures regularly. A small pressure loss is normal, but anything more signals that it’s time for maintenance. And remember, a clean bike is a happy bike.
The main function of a pneumatic shock is to absorb impact, which it does via compressing air. (Other types of shocks use metal springs and fluid.) Because air gets denser or more resistant as it’s compressed, pneumatic suspension is progressive. Compression damping manages the speed at which the shock progresses through its stroke. To make sure the suspension performs well, the initial air pressure has to be set to your body weight and riding style. All manufacturers will have specific guidelines you can follow for optimal performance.
The negative air pressure in a shock is used to help with the initial compression. Because the air cylinder is always under pressure, there is a constant force keeping the shock fully extended. To get the shock moving from a resting state, it takes more pressure to initiate movement than it does to maintain that movement once started. Negative pressure helps lesson the amount of energy required to start that movement by pre-loading the air spring slightly. Without this feature your suspension will feel harsh and will not absorb smaller impacts. An improper setup of the negative pressure can result in a loss of usable travel.
Sag is the most important suspension adjustment. It is the amount the suspension is compressed when you sit on the bike when not moving. Generally, a proper sag allows the suspension to sit about 20 per cent into its available travel. For example, if you have 100 mm of suspension travel, you will sit 20 mm into that travel. You’ll have 80 mm of usable positive damping and 20 mm of negative travel, which is important to allow your wheel to maintain contact with the ground over dips and undulations.
Rebound damping controls how quickly the shock returns to its neutral position after being compressed. Without this adjustment, the rider would be bucked all over the place losing control and efficiency. The damping is achieved by forcing oil through a valve, and the adjustment simply opens or closes the amount of space that oil has to pass through. The smaller the space means the slower the oil flows and the slower the shock is able to move through its travel. This feature must be adjusted any time the primary air pressure is changed for optimal performance.
The Proper Pump
Yes, a pneumatic shock needs air like a tire. But, no, you can’t use a tire pump for your shocks. A shock pump, such as this one by Filzer, delivers the low volume, high-pressure bursts of air your suspension needs. Also, it has its own gauge. (Never use a separate air gauge.) If you put in too much air, the pump lets you release the gas in 2 to 3 psi busts. ($30, mec.ca)