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Air suspension volume spacers

30/12/22 18:06 Filed in: Gear
Despite the drawbacks, air suspension is what the majority of mountain bikes use. Air is light. Air can be freely adjusted with a shock pump. These two benefits alone are responsible for why air is King.
The alternative is a coil spring. Coils are very smooth, but also heavy and to change spring rate you must change the coil.
There is another difference that is neither positive nor negative in its own right, but generally means that the bike frame has to be designed around it - that is the linear nature of a coil spring versus the progressive nature of an air spring. A 100 lb/in spring requires 100 lb of weight on it to compress it one inch. A second 100 pounds of weight will compress it a second inch. This continues until all the coils are bottomed out on each other and the spring is maximally compressed. An air spring works very differently. Compress an air spring through half of its travel and you have approximately increased the spring rate by 100% (approximately because the air chamber does not reduce to zero volume at bottom out). Compress it by another half and this happens again. Every millimetre of movement in the coil spring requires the same input as every other millimetre of movement but every millimetre of movement in the air spring is unique in the amount of input required to achieve that movement.
Downhill bikes that typically use a coil spring require the rear suspension to build in a progressive linkage for the rear end to prevent bottom out on big hits. Cross country bikes that typically use an air spring may even have a falling rate in the rear linkage to prevent the spring from ramping up too much. Some coil sprung shocks have fancy hydraulic anti-bottom-out features as well.
The design in air springs has been towards make them feel more linear; more coil-like. Increased air volume can go some way towards that. But the amount of progression any rider requires to get both a good (comfortable) feeling at their sag point (ie, just riding along) and also to not bottom out too easily varies widely between a beginner and a professional (who might well be using the same shock).
Enter the air volume spacer.
If the air chamber is quite large producing a compression ratio in the chamber of around 4:1, then beginners will be well served. They will see most of their travel used under their moderate demands on the equipment. Put that same system under a competitive professional, and the suspension will bottom out often and hard enough to cause damage. They require more ramp-up at the end. The volume spacer that goes in a RockShox fork is a plastic disc that threads onto the underside of the air chamber cap. Simply remove all the air, unthread the cap, add or subtract a token (as RockShox calls them - the Fox system is nearly identical except they snap together rather than thread together) and reassemble/reinflate. By removing some volume from the air chamber (the plastic takes up some volume in the part of the chamber that is still open at full compression - clearly it can't occupy any of the volume used by the air piston in its travel) and beginning with the same air pressure as before, the end pressure increases. Typically the difference is only from around 1/2 travel to the end.
At sag (just riding along) the fork or shock behave the same regardless of volume spacers. But in a decent hit, the air pressure increases faster after half travel and increases the resistance of the suspension to bottoming out.
 
I had an old Fox 32 XC fork that was designed to be like a modern fork with several spacers in it. The compression ratio was in excess of 5:1. I never achieved full travel. Over two trials (the first to try an intermediate reduction) I shortened the air piston rod (which connects the sliders movement outside the fork to the air piston movement inside the air chamber) enough to take the compression ratio down to 4.3:1. At this lower compression ratio I could occasionally bottom out the fork without that happening too often (or requiring so much air pressure that it didn't sag adequately).
Now my hardtail has a Fox 34 fork with 120 mm of travel. It came with 2 spacers installed and two more in the packaging. At the other end of the travel scale, my new Slash has a Zeb fork with 170 mm of travel. It had one spacer installed and two more in the packaging. To get full travel in both of these forks, I had to run slightly more than normal sag and zero spacers. I seem to be light on the fork across all bikes. Not that I'm super fast - I acknowledge I'm only moderately quick. But I'm a big guy and weigh a lot and I do like to ride steeper trails (where more weight ends up on the front wheel). Yet I don't require any spacers.
 
I've written mostly about forks, and also about suspension linkages (rear end only except for the unusual few bikes out there) - but fork and shock can be freely interchanged in this theory. They both have air chambers and they both can use spacers inside them to take up some volume and lend more bottoming out resistance to the unit. The main difference is that a fork has a 1:1 relationship between wheel travel and air chamber piston travel. They are directly connected to each other. But the shock is on a linkage that compresses the shock 1/2 to 1/3 as much as the wheel moves. A 200 mm travel DH fork has 200 mm of movement between the sliders and the stanchions. A 200 mm travel rear end on a DH bike probably uses a 75 mm travel shock (almost 3:1). While this puts more stress on the shock, it is otherwise the same.
 
Prior to suspension companies making all these volume spacers available and designed into so many higher end products, the go-to solution was a blob of grease placed in an out of the way corner of the air chamber. So it has always been possible to increase the compression ratio. And my physical modification of the old Fox fork demonstrates the decrease was possible too - if rather more involved.
 
There are also several unconventional solutions on the market. Formula suspension uses closed cell foam volume spacers. As the pressure in the chamber increases, the volume of the foam spacer decreases. This limits the ramp-up at the end to less than you'd otherwise see. Good for controlling the mid-range without overwhelming the final range of travel. There are also designed-in and aftermarket solutions with negative air chambers. If the air chamber acts upon a piston with air pressure behind it, like the close cell foam, once the chamber pressure exceeds the opposing air pressure, it will move the piston and lessen the compression ratio. Several European producers of forks use this sort of system in their own forks and the MRP Ramp Control cartridge replaces your stock air cap and spacer stack with an on-the-trail adjustable volume spacer.
 
The take away message from this is that regardless of how your air suspension behaves it can be modified quite simply to behave differently. If you can't imagine undertaking this challenge yourself, there are shops and people who specialise in helping with just such problems.

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