By Eric Ramirez — If you’re wondering what an air spring is or how it works, you’re probably not alone. Chances are if you’re a mountain biker, you already have an intimate relationship with an air spring. Most of our bikes are equipped with a suspension fork and the full suspension bikes are also equipped with a rear shock. Despite the brand of the bike, it’s hard not to overlook the limited number of suspension manufacturers, such as Fox, RockShox and DVO.
How we know the air spring
The bumpy nature of mountain biking is felt primarily in your pedals and handlebars. As you ride your new trail bike over any stretch of dirt and rock, notice that the harshness is toned down and sometimes just makes the worst feature go away. This behavior can be considered the oversimplified version of what suspension does.
While the wheels feel every nuance of the trail, your front and rear suspension units soften the blow by separating you from every big and small impact with a little cushion of air. This cushion is the spring. Hence, the “air spring”, for the purposes of this article.
Overall suspension feel may not provide enough clues to tell you if you are using an air spring. If you have an air valve on your rear shock or fork, it’s an air spring.
How it works?
When we think of a spring, our mind often imagines a coiled, bouncing wire called a “coil.” Air springs are different in many ways. First, a coil spring is capable of existing on its own. An air spring without a dynamic container would simply be ambient air, and not a spring at all. In most suspension forks, the air spring container is the stanchion tube. There is usually a dynamic part of this container, which looks like a piston.
This piston cannot extend beyond a certain limit of the container, but it can move in this chamber, able to compress the air inside. Although not under load, this compressed air looks like a barbecue propane tank, just much more expensive. The air continually pushes back the pressure inside the chamber, requiring some external force to compress. By pushing the “piston”, or “air spring assembly”, into the air chamber, it becomes an air spring.
This act of initiation is called a cycle of compression, also “displacement”, and is called “detachment”. Stall force refers to the amount of energy needed to start the trip. If an air spring chamber contains 68 psi of air, then it will take some external pressure from the rider’s weight or impacts to start moving: for example, a 120 pound rider is sitting on the bike. But the air spring requires a rebound to set in motion. It can have a harsh feel on the trail.
Newer suspension designs, like the Fox Evol (extra-volume) or the RockShox Debonair (same), include an enlarged air chamber on the other side of the piston crown. This is called negative air. Manufacturers have designed this chamber to absorb a given ratio of air from the main air chamber.
Returning to our 68 psi chamber, let’s assume the negative chamber contains about 38 psi rather than 0 psi. Initiating a compression cycle is much easier because there is already 38 psi pushing against the main air spring. This improves the feel of the air spring and the ride quality of the suspension.
As the compression cycle continues, atmospheric pressure increases exponentially. The air inside the chamber stays there, in a shrinking space. This compression of space and increase in pressure is exponential, which is why you hear cyclists refer to it as “ramp”.
Imagine that the pressure inside the chamber doubles each time the length is compressed by half. A rider might need 68 psi in their fork to support their riding weight and we can assume this is a 160mm race fork. The rider hits a medium bump and uses half the travel. A half reduction in chamber length corresponds to a double increase in pressure to 136 psi. At mid-stroke there is 80mm left for full compression. If the chamber length is again halved to the remaining 40mm (where 120mm is used), the air pressure has again doubled to 272 psi.
|Tube length (mm)||Pressure (psi)|
If you plot a graph of this information, a rudimentary graph would show the relationship between displacement and internal air pressure on a line that looks like a ramp.
Inflation, deflation and maintenance
As you read this, you might think of all that is involved in containing all that ram air pressure. Well, simply put, there are tons of little gaskets of different types installed on suspension units.
The air spring itself has multiple seals that wear out over time due to the constant pressure and cycling inside the air chambers. These will need to be replaced periodically. High quality lubricating oils are used to keep the shock smooth and reduce friction. These should also be rinsed and replaced periodically. When servicing suspension components, the technician should check the condition of the seals. Many technicians will insist on replacing gaskets as a precaution.
To avoid erratic air spring behavior, it’s important to run your shock while inflating, about every 20 psi or so. This allows the main air chamber to balance with the negative air spring. Never deflate the fork or shock quickly, as this prevents the air from the negative chamber from escaping; always deflates very slowly. Also avoid deflating the air springs when loaded; air springs must be fully extended when inflating or deflating.
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