TruTune Fork Insert (Fox)

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  • £120


Plug and play insert for mountain bike air forks, powered by activated carbon.
Increases the effective air volume in the fork, creating a plusher and more linear feel.
Can be used in conjunction with Fox Volume spacers. Please check the Fox website for maximum number of tokens excepted in your fork..


TruTune components are a new suspension technology developed by Carbon Air. They increase effective air volume in a mountain bike air suspension without making any changes to external dimensions. Increased volume provides many benefits to riders in the form of a plusher feel, better small bump sensitivity, and end-of-stroke absorption. The charge pressure of the suspension with our inserts can be increased substantially to achieve good support without the harshness associated with high pressure.


Activated carbon, also known as activated charcoal, is a highly porous form of carbon. The raw material is usually carbonised in an oxygen free environment, and then activated by subjecting it to steam or oxygen at high temperatures.
The magic of this material happens at the molecular level. As gas molecules hit the surface of the carbon, some are trapped by the action of Van der Waals forces. When air pressure is increased, extra air molecules are adsorbed onto the carbon which are then released (or desorbed) when pressure is dropped.
This results in a much more linear pressure curve, with a lower pressure peak, despite the air chamber having exactly the same size.
In short, it behaves as if it had more volume.
Activated carbon has another important effect on the dynamic behaviour of air: reduced speed sensitivity.
The stiffness of air varies with the speed at which it is compressed. That is because when compressing a gas, it heats up. If it's compressed quickly, the excess heat does not have enough time to escape to the surrounding environment, resulting in a greater pressure increase, and so it has higher stiffness.
When compressing it slowly, the excess heat has time to dissipate, which lowers the pressure and hence lowers stiffness.
The opposite is true in expansion, which causes the gas temperature to drop and pressure to drop further. The stiffness of air for high speed compression is up to 40% greater than its low speed stiffness. Activated carbon reduces this speed sensitivity by trading heat with the air, mitigating the changes in air temperature and resulting in a more consistent (and lower) stiffness at different speeds. TRUTUNE IN THE DATA We are always putting our TruTune designs to the test, both in the lab and in the field. Our laboratory tests demonstrate the effect through the whole travel of a fork, by measuring pressure as we compress a mountain bike air fork using our dynos.
Using a TruTune insert, the pressure at the end of travel is much lower, despite starting at the same value. Because the fork has no real additional volume, it can only mean that some extra "volume" was created by the insert.
With TruTune, it is possible to increase the charge pressure without having high initial stiffness - this results in similar pressure at the end of stroke, but higher pressure in the initial portion of travel.
While laboratory tests are useful for our development, the ultimate test is a rider's experience in the field. The technology was proved on a host of challenging trails and conditions by our network of test riders. With many different TruTune designs, our riders noticed improvements to their comfort, support and end-of-stroke absorption.
Our field data confirmed our laboratory tests - a higher charge pressure can still achieve lower peak pressures with a TruTune positive chamber insert.

 TruTune Model Fork Compatibility

Fox 34 (4 Token length)

Fox 34 Rhythm, Step Cast, FLOAT (GREEN spacers).
Fox 36 Rhythm FLOAT NA2 (PURPLE spacers).
Fox 36 E-Bike (GREEN / PURPLE spacers).
Marzocchi Bomber Z2.
Not compatible with E-bike 34 forks.
Fox 36/38 (5 Token Length) Fox 36 FLOAT NA2 2018+ and Fox 38 FLOAT (ORANGE and YELLOW spacers).
Not compatible with Fox 36 Rhythm (PURPLE tokens) or Fox 36 E-bike - choose our Fox 34 model.
Fox 36/38 Short (4 Token Length) Fox 34 Rhythm, Step Cast, FLOAT (GREEN spacers).
Fox 36 Rhythm FLOAT NA2 (PURPLE spacers).
Fox 36 E-Bike (GREEN / PURPLE spacers).
Marzocchi Bomber Z2.
Not compatible with E-bike 34 forks.
Fox 40 (4 Token length Fox 40.
Marzocchi Bomber 58.