Carbon Air Aims to Make Air Suspension More Like Coil - Pond Beaver 2021

When a press release entered my inbox for a product made from coconut shells and sawdust, which slots into your air spring but somehow has the effect of increasing its volume, I assumed it was a late April Fool's submission. But Carbon Air is the real deal. They've been making products for the air suspension in Audi's A6 and A7 cars since 2017. Now, they're developing their product for mountain bikes.

What is it?

First off it's worth refreshing how conventional volume spacers work and their limitations. Adding spacers increases progressiveness, making it harder to bottom-out for a given amount of sag. But in some applications, it can be hard to use all the travel even with all the volume spacers removed. This is particularly true of long-travel single-crown forks, because the longer the travel, the closer the piston gets towards the top-cap at bottom-out, so the higher the compression ratio. Air spring upgrades which increase the size of the negative chamber - such as RockShox's MegNeg or Vorsprung's Luftkappe, make it harder still to bottom-out because they soften the beginning-stroke far more than the end-stroke, and require more pressure to achieve the same sag.


Put simply, Carbon Air's product does the opposite of a volume spacer. It's made of activated carbon, which is produced by exposing carbon-rich material like sawdust, coconut husks or coal to steam and chemicals, and is used in many industrial applications like hydrogen gas storage (hydrogen likes to leak out of steel tanks) and air filtration. You can think of it like a super-fine foam, with lots of tiny pores within pores in a fractal structure, giving it a surface area of well over a thousand square meters per gram. (For American readers, that's about twenty football fields per quarter-pound).

This enormous surface area allows it to adsorb (yes, adsorb with a "d") more gas molecules than would normally occupy that volume at a given pressure, because they "stick" (adsorb) to the material's surface through something called van der Waals forces. And the higher the pressure, the more air the material adsorbs. According to Carbon Air, this allows one cubic centimeter of their material to take up as much air as two cubic centimeters of empty space. So, counter-intuitively, the more volume of an air spring you fill with activated carbon, the bigger apparent volume it has.





What does it do?

So Carbon Air can make air suspension less progressive. This could be useful for those who struggle to use all the travel while maintaining appropriate sag. We could stop there, but there's much more to it than that.

While air springs are progressive in the last part of the travel (and this is a good thing, within limits), they are digressive in the first part of the travel. In other words, they have a stiffer spring rate (that's the change in spring force per unit travel) at the start of the travel than the middle. In fact, air springs are often more than three times stiffer off the top than in the middle (older air springs could be over ten times stiffer). It's as if you had a coil spring suited to a 150Kg rider at the start of the travel, then swapped to a spring for a 50Kg rider once you got to the middle. This is what causes air suspension to lack suppleness and traction at the beginning of the travel, while also lacking mid-stroke support.


One solution is to increase the volume of the negative air chamber, which reduces the initial spring rate relative to the mid-travel. But larger negative chambers make it harder to use full travel for a given sag. To ensure most riders can use all the travel with a reasonable amount of sag, mainstream manufacturers want to keep the ratio of positive to negative volume constant. In an ideal world, the positive and negative chambers would be so large the spring curve would be basically linear (but could still be made progressive with volume spacers if desired). The main problem is space. In a single-crown, long-travel fork there basically isn't enough room to do this. The same goes for an air shock which must fit into multiple frames and around water bottles. Apparently, space is a primary constraint in car suspension too, which is why Audi worked with Carbon Air.

This is where activated carbon could be very useful. Whether used in the positive chamber to reduce the bottom-out force when combined with a large negative chamber (e.g. MegNeg or Luftkappe), or in both chambers of a regular air spring.


There's one more potential advantage to Carbon Air's product, but to explain that we'll have to cover a little physics. In an air spring, the stiffness depends on speed. This is because when a gas is compressed it generates heat, and when it expands it takes in heat - this is why your hand pump gets hot when compressing air and your CO2 canister gets cold as the gas expands. This heat generated during compression causes an additional increase in pressure over and above what you'd expect if the gas was at a constant temperature. This causes an increase in the force required to compress the gas quickly (adiabatically) compared to slowly (isothermally), where the heat has time to escape.

This heat dissipates very quickly (with a half-life of about 0.1s, according to Steve Mathews of Vorsprung suspension), so it's most significant in very fast suspension movements. But essentially, air springs are softer in slow compression like cornering and braking, when compared to very high-speed compression like a hard landing. Whether that's a bad thing is debatable, as it can be offset with reduced high-speed damping, but this is another reason why air suspension can lack support under slow movements like braking and cornering, but can be reluctant to use all of its travel on a big impact.

According to Carbon Air, their product absorbs the heat generated during compression and releases it during rebound, keeping the temperature of the gas very consistent. The high surface area and high thermal mass (compared to the gram or so of air in an air spring) of the activated carbon makes it a great heat sink. So, according to its manufacturers, it virtually eliminates the speed-sensitivity of air springs, making them perform even more like a coil while still preserving the lightness and tune-ability of air.

Tobias Ackroyd, Product Development Engineer Carbon Air, also mentioned the possibility of using their product in conjunction with a volume spacer if you wanted to maintain the same progressiveness but still offer the benefit of reduced speed-sensitivity.

I did wonder if the material takes time to adsorb air during compression, which could introduce another speed-sensitive effect as the insert would act like a volume spacer until the air had time to "soak in" to the material. I asked John Coakley, CTO of Carbon Air, "Can it adsorb air fast enough to keep up with the fastest suspension movements, which are somewhere in the region of 10Hz [ten cycles per second]?" Apparently, activated carbon can store and release air so fast it's effectively instantaneous as far as suspension is concerned: "It's used in loudspeakers where you can see adsorption and desorbtion [the reverse of adsorbtion] in these materials up to 400Hz," John told me, "so for this application, you're not going to see any speed-sensitive effects."

The future

Carbon Air is discussing with suspension manufacturers the possibility of integrating their product into future air spring designs. For obvious reasons, they were not keen to share any more details on that. As for whether it will be available aftermarket to effectively increase the positive and/or negative volume of existing air springs, that may depend on the result of those discussions.

Personally, I'm a a big fan of negative volume upgrades like the RockShox MegNeg and Vorsprung Luftkappe, but in some applications (frames with progressive leverage curves and long travel forks, respectively) they can make the spring too progressive. An aftermarket option which makes it possible to make use of these upgrades without excessive end-stroke force could be a step forward for air suspension.

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