Innovation check: TruTune volume spacer made of activated carbon enlarges the air chamber
Peter Nilges
· 24.10.2023
The topics in this article:
The small, black volume spacer in my hand costs 117 euros and feels more like simple plastic than sophisticated mumbo-jumbo. The part, which is inserted into the air chamber of a suspension fork like a conventional spacer, cannot be compressed or deformed in any way. Only a thread at the upper end and a grid structure at the bottom create a slightly technical look.
And yet the British manufacturer's miracle token is intended to do the exact opposite of the familiar, colourful plastic spacers that are usually supplied with suspension forks. As a reminder: With the usual volume spacers, the air chamber of a suspension fork is reduced in size, thereby increasing the progression. This means that more force is required to fully compress the suspension fork under otherwise constant conditions. The TruTune spacer, on the other hand, is supposed to function like a kind of negative spacer. In other words, despite its existing, unchangeable volume, it enlarges the air chamber, which is also unchangeable in terms of its design. The declared aim is a more linear characteristic curve, better utilisation of the spring travel and overall improved function, which results in more comfort and control. Can all this really work?
The non-believers gather around me in the BIKE test lab. It's the moment of truth. After a visual inspection by all the lab staff, the supposed miracle part goes into the Rockshox ZEB, which is already in our suspension fork test rig. The last bets are placed. The odds look bad for the "black cigar" from England. The pneumatic cylinder compresses the ZEB with incorruptible precision and records the new spring characteristic curve with TruTune spacer. It's hard to believe, but we can see it in black and white: the characteristic curve is actually flatter than before with an empty air chamber. Magic? The test bench needs around 250 Newtons less to fully compress the fork at an identical pressure of 60 PSI in the air chamber. But how is this possible?
The answer is hidden deep inside the material of the volume spacer. This consists of activated carbon. An extremely porous and fine-grained carbon that has a gigantic inner surface area. Unimaginable: just four grams of activated carbon have an internal surface area the size of a football pitch. TruTune utilises this huge surface area to adhere air molecules to it. During compression, the air chamber is compressed and the air pressure rises. As the pressure rises, the air molecules adhere so compactly to the surface of the activated carbon that the pressure rises less than with an air chamber without activated carbon. As if the air chamber were actually larger. As soon as the suspension fork rebounds and the pressure decreases, the air molecules are released again. The process can be repeated as often as required.
Conclusion on the TruTune token from Peter Nilges, BIKE test manager:
Our innovation check confirms the function of TruTune and proves that the volume spacer has a clear influence on the characteristic curve of a suspension fork. If you are looking for more linearity and a lower spring rate without riding the fork too softly, the investment of 117 euros is well advised. However, as TruTune only influences the suspension and not the damping, the effect remains manageable in practice.
Into the fork: How to fit TruTune
Conversion to the TruTune spacer is extremely easy and can be carried out in just a few simple steps using the appropriate tool. To do this
the spacer is inserted into the air chamber of the suspension fork, just like any standard volume spacer.
So deflate, unscrew the air chamber and screw the spacer into the thread of the valve cover from below. Close the air chamber again, inflate, done. Depending on the rider's weight, TruTune recommends using an air pressure 1 to 10 per cent higher than usual to compensate for the lower progression. During pumping, you have to wait a few seconds for the air pressure in the air chamber to adjust. This is because the air is pumped through the spacer.
Spring characteristics: Less progressive
The spring characteristic curve test bench in the BIKE test laboratory confirms the function of TruTune. The spacer is available in two different versions (short and standard). While most riders need the larger standard version, TruTune recommends the short version for heavy riders over 90 kilos. The biggest difference between the characteristic curves and the corresponding forces can be seen when the suspension fork is compressed once without a spacer (orange) and once with the largest TruTune spacer (red). With an identical fork air pressure of 60 PSI, you need around 250 Newtons less with TruTune.
Dynamically better
According to TruTune, the effect is even greater at higher compression speeds (our characteristic curve test bench works slowly) (see diagram below). The
resistance of air changes with the speed of compression. During rapid compression, the air heats up, which results in a higher increase in air pressure and thus increases the spring rate. With high-speed compressions, the resistance is 40 per cent higher than with slow compressions. With TruTune, the suspension is less sensitive to speed.
How TruTune drives
The promises on the TruTune website don't sound bad. Improved grip, less arm pump, better travel utilisation, greater sensitivity and better absorption capacity. In practice, we can confirm that a suspension fork with TruTune allows the travel to be utilised slightly better and that the fork feels very forgiving even on fast hits. However, the huge "wow" effect that you would expect when studying the spring characteristics does not materialise, which is why not all test riders were completely enthusiastic.
The explanation: TruTune can only influence the suspension behaviour of a suspension fork. However, the damping side remains unaffected. However, as the damping always regulates a large part of the resistance in addition to the spring during fast compression movements, the influence of TruTune is capped. However, for riders who have problems utilising the suspension travel even with a soft setup and are looking for maximum comfort, TruTune is well worth considering.
This is how it works:
The activated carbon volume spacer utilises the effect of adsorption. Adsorption occurs when atoms or molecules of gas adhere to a surface. Under pressure, the air molecules attach themselves to the huge inner surface of the Volumespacer, resulting in a higher density. When the pressure drops, the air molecules are released again.
Activated carbon is used in many areas
Activated carbon is available in various forms, e.g. as granules, powder or in pelletised form. It can even be processed into fabrics. The production of air conditioning systems alone requires 5000 tonnes of activated carbon every year. Typical applications also include drinking water filters or use in respiratory masks. The possibilities are correspondingly diverse. Carbon Air, the manufacturer of TruTune spacers, has been producing activated carbon for the air suspension (image 1) of the Audi A6 and A7 since 2017, for example. Wherever the installation space filled with air is limited, the use of activated carbon makes sense.
Interview with John Coakley, Chief Technology Officer at Carbon Air
The patent of a loudspeaker manufacturer gave us this idea.
BIKE: How did you come up with the idea of using activated carbon in the suspension of mountain bikes?
John Coakley: We are a spin-off from the acoustics research laboratory at the University of Salford. A patent from a loudspeaker manufacturer claimed that activated carbon would make the cabinet of a loudspeaker appear larger because the air would reverberate better. So a PhD student set out to prove that this was nonsense - but it turned out to be true! I was working there as a technology transfer officer and a professor described to me how loudspeakers work and casually said that they are basically like an air spring. I thought to myself: why not try that with air springs?
TruTune is currently only available for three fork manufacturers, but you are also working on a version for dampers. Is the effect even stronger at higher air pressures?
The technology actually works better at lower pressures, but it holds up to well over 30 bar, so shock absorber pressures are not a problem. One argument in favour of using it in shock absorbers is that there is so little space at the end of the stroke that a strong progression is unavoidable. Current dampers always have larger negative chambers, which require slightly more pressure. This makes the end progression even stronger and it can be difficult to utilise the full travel. TruTune solves the problem.
You are also working on tokens for negative air chambers. What are your future plans for TruTune?
We have had prototypes for negative air chambers in use for some time. There are still a few design challenges we have to work with as the spacers and end stops used are often different, even within different versions of the same fork. We believe we have a solution. And there's a lot more to do - but we can't say too much about that.
What about the shelf life of your product? Does the effect wear off at some point?
We have not noticed any wear or loss of function in well over 100 hours of operation. It is important to store the inserts in a dry place when they are not installed in the fork. Theoretically, an insert saturated with water can be dried by placing it on a radiator overnight. However, if it is saturated with oil, this is irreversible. But the small amount of oil in a fork poses no danger with the filter we use.
Is TruTune already being used in racing? For example in the Enduro or DH World Cup?
A number of racers and racing teams are testing TruTune and the initial feedback is very positive. Some have reported that they are setting personal best times without feeling like they are going fast, such is the grip and feeling of control.
The air spring is only one side of a suspension fork. Your diagrams show the behaviour of the air spring without taking the damping into account. How big is the influence of the damping in situations where the fork utilises the full suspension travel?
Our test bench tests are carried out without dampers, that is correct. But in all scenarios, the effective damping is increased when the spring stiffness is reduced. On the whole, the damper works better with TruTune towards the end of the stroke.
