TruTune Flow MTB Air Fork Upgrade

TruTune TruTune Flow MTB Air Fork Upgrade

Product Description

Activated carbon plug and play insert that installs like a token, but acts to increase the effective air volume of your fork, resulting in a more linear and less speed sensitive response. Our first edition inserts are assembled in our workshop, and tested and validated with our dynos and our team of test riders. 


  • Better grip, comfort and small bump compliance without compromising sag 
  • Linear, coil-like feel but with the adjustability and weight advantage of air
  • Unlocks more travel - make full use of your suspension
  • Makes the fork more consistent and predictable by dramatically reducing speed sensitivity


Short variants have more ramp up, and are more suitable in the following cases:

  • Heavy rider (over 90 kg with gear)
  • Ebikes
  • Run 3 or more tokens as your preferred setup
  • Run high pressure (100 psi or more)
  • Aggressive style / frequent bottom out  
  • Typically ride large jumps where extra bottom out resistance is needed 


The activated carbon contained in our inserts acts to increase the effective positive chamber air volume in your fork. This happens due to its high porosity and internal surface area, that traps and releases air molecules, mitigating changes in pressure (more information on that process can be found on our Technology page). The effect is similar to what happens when you remove tokens: the spring rate is more linear, with less ramp up. This may be hard to believe, because it is a solid material after all!

The activated carbon in our inserts exchanges heat with the air, which mitigates the temperature changes in the air and dramatically reduces its speed sensitivity, resulting in a more consistent and predictable feeling.

When using our inserts, the charge pressure is usually increased, to achieve similar pressures at the end of stroke compared to the original setup. This results in the right amount of support, but with improved tracking, bump absorption and comfort provided by the more linear spring rate.


TruTune inserts come in specified equivalent number of tokens. Do not exceed the manufacturer specified maximum number of tokens / spacers for your fork model. You can check your Fox fork here and your RockShox fork here.

Fox 36 / 38

Standard: 5 (equivalent token length), Short: 4 (equivalent token length)

  • Fox 36 FLOAT NA2 2018+ (ORANGE spacers)
  • Fox 38 FLOAT (YELLOW spacers)

Not compatible with Fox 36 Rhythm FLOAT NA 2 (PURPLE tokens) - our Fox 34 model should be used.

RockShox 35/38mm

Standard: 4 (equivalent token length), Short: 3 (equivalent token length)

  • 35mm and 38mm Rockshox DebonAir(+) (ZEB, Pike, Lyrik, Yari)
  • For SID 120mm choose the Short variant.

Equivalent to GREY / RED bottomless tokens. Not compatible with Dual Position Air springs.

Heavy rider (90+kg) or on E-bike? Choose our Short model where available.


WARNING: modifying any aspect of your suspension without the appropriate training and experience can result in serious injury or even death. Installation should be carried out by a competent technician. We cannot be held responsible for the results of incorrect installation of our product and assembly of your suspension.

The following are guidelines for installing Trutune plug and play inserts for mountain bike front air suspensions. Please refer to the fork manufacturer manual for up to date instructions on assembly / disassembly for your specific model.

  1. Firstly, secure the bike in a suitable stand to ensure the bike is safe to work on. If the forks are separate from your bike, use a suitable vice or the clamp from a bike stand with soft jaws to avoid damage to the forks or bike.
  2. Unscrew the dirt cover to expose the valve and put the cover in a safe place.
  3. Using a small tool (Allen key) remove ALL the pressure by gently pressing the valve.
  4. Using an appropriate socket or cassette tool (depending on year/model) remove the top cap, gently applying pressure to ensure the tool does not slip.
  5. Once the top cap is removed, remove ALL plastic volume spacer tokens. OEM tokens can be used in conjunction with a TRUTUNE insert, however it is important that the number of tokens plus the token length of the TRUTUNE insert does not exceed the manufacturer specified maximum number of tokens for the fork.
  6. Securely attach the insert to the top cap (for RockShox forks, this will generally be a screw thread, for Fox forks a snap fit).
  7. Apply a small amount of grease to the cap o-ring and then Install the top cap with the TRUTUNE insert into the air chamber and tighten to the manufacturer recommended torque using a torque wrench
  8. Using your air pump, pressurise the positive air chamber to your desired pressure, or alternatively consult the manufacturer pressure charts for your fork model and weight. Allow 30 seconds for the pressure to settle, as it will drop slightly after charging, and top up to the right value, ensuring to pump the fork a few times to equilibrate both chambers. With a TRUTUNE insert installed, we recommend to increase pressure by 5% above preference / manufacturer recommendation as a starting point. 
  9. With the aid of the floating o-ring on the fork (if available), ensure the ride sag position is within the manufacturer specification (or personal preference). If not, increase or decrease pressure as appropriate.  
  10. Finally, replace the dirt cover onto the top cap valve carefully to finger tight.

To ensure perfromance is maintained over time, when kept outside the fork, store Trutune inserts in cool and dry conditions, and avoid exposing to water, oil or other contaminants. Your bike may be stored upside down with a Trutune insert provided the quantity of lubricant does not exceed the amount specified by the fork manufacturer. When servicing the fork, do not exceed the manufacturer recommended quantity of lubricant and grease as this may affect the performance of the insert. 

Setup Guide

WARNING: modifying any aspect of your suspension without the appropriate training and experience can result in serious injury or even death. Installation should be carried out by a competent technician. We cannot be held responsible for the results of incorrect installation of our product and assembly of your suspension.

These guidelines are based on our test program so far, and will be updated as we learn more from rider feedback. Our recommendation depends on your current setup, riding style and weight.For all riders, the starting point is to remove all tokens / spacers you currently have and using a Trutune insert on its own. When charging the fork, allow 30 seconds for the pressure to settle after the chambers have equalised.


After installing the insert, start with 2-5% more pressure than your usual setup. If you feel a lack of support or are bottoming out the suspension too easily, increase air pressure in small steps (3-5 psi) until you find the right balance of plushness and support. It may be necessary to increase the rebound speed on your damper (if the setting is available) to help with hold-up.


If you struggle with using all the travel of your fork, and / or tend to run low pressure, and / or have a stiff fork when set at your preferred sag position, we recommend charging to the same pressure (or only 1-2% higher to match sag position) as your usual setup as a starting point, which will simply make more travel available and increase your comfort and grip.


The starting pressure required may be over 10% of your usual setup. Some riders in these categories have found 15 or even 20 psi higher to be the right balance. If the increase in pressure is still not sufficient, causes unacceptable initial stiffness, and the fork is bottoming out too easily, then we recommend combining the TruTune insert with one OEM volume spacer (if the total length does not exceed the maximum number of spacers for your fork model / travel).



It may seem counterintuitive, but adsorption is how this happens. As the fork or shock is compressed, some air adheres to the surface of the carbon. The air adhered to the carbon can have much higher density, and so reduces the pressure in the rest of the gas (compared to a chamber without any activated carbon). You can find out more on our Technology page.


Having activated carbon in your fork is not the same as just removing tokens. In addition to the main physical effect - adsorption - which reduces pressure build up, there is heat exchange between the air and carbon. Usually an air spring is softer at slower speeds than fast speeds, which is not desirable, as high speed movements are associated with chatter, and low speed movements with how much support there is. Increasing the air volume requires higher pressure to get the right support, but the dynamic stiffness can become too high in the process. The heat exchange between air and carbon reduces the high speed air stiffness compared to an equivalent volume setup. In practice it means a much higher charge pressure can be used before the initial stiffness of the fork becomes unacceptable, allowing greater support and better small bump absorption than just running fewer tokens.


So far, our inserts have individually spent 100+ riding hours in forks without any noticeable changes to performance. Our earliest prototypes have been in forks for over a year and have maintained their adsorptive properties over time. We are continuing to test and monitor their durability and as we learn more about the long term lifespan we will make more information available.


The effect is influenced by moisture, as water vapour competes with air in attaching itself to the carbon surface. In the sealed fork chamber, the amount of moisture is not sufficient to influence the effect or degrade performance over time. Provided the inserts are stored in dry conditions when out of the fork the performance will not be affected. Even after exposure to high humidity, the difference in performance is small as the surface area available for the air molecules at high pressure is still very high.


We use a fine filter that we have selected out of many, having subjected it to shakers, test riding and dyno testing to ensure that there was no build-up of powder over time. We also apply a coating to the carbon to minimise powder production and allow a tight pack of granules in our inserts, reducing internal movement.


The effective increase in volume is temperature dependent, with around 5-10% variation between 0 and 40C. This is a small difference In terms of total fork volume, and so does not cause a dramatic change in feeling. The air pressure in the fork is slightly more sensitive to temperature than a regular air spring - the reason for this is that the carbon stores air in its pores which can be released by heating (and vice-versa trapped by cooling). The net effect in the fork depends on the amount of material, so we have found the right balance based on our test riding. The effect is also dependent on charge pressure, with higher pressure resulting in a smaller (but still stiffness reducing) effect. The pressures at which the activated carbon is no longer effective are much higher than those found in MTB air suspensions, including shock absorbers.


When activated carbon is used in filters, substances accumulate and saturate the carbon as contaminated air or water flows through, and so over time the filters become less effective and need to be replaced. In the sealed environment of the fork, the only contaminants that can affect the behaviour are suspension oil and grease, and they do not continuously flow through the material and the degradation in performance is much lower.

Based on our lab tests, the inserts stop adding effective volume when they are half full of suspension oil (by volume), which for most of our products would be around 20ml. Typically, the total amount of oil inside the air chamber for a service is much less, around 3ml. In practice, most suspension oil makes its way to the negative chamber through the equalisation valve. There is also clearance around the sides of the insert, and away from the piston, which combined with our choice of filter help to avoid any oil or grease from coming into contact with the carbon in the first place. We do however recommend not using excessive oil or grease in order to minimise any performance changes over time.


For spare parts, if they are in stock we will send them out the same day if we receive the order before midday (except weekends). If we receive the order after midday, we will do our best to despatch them same day, but it may be the following working day. Orders placed after 3pm on Friday will be despatched on the following Monday. We normally send spare parts by Royal Mail post (see below or our 'Shipping' page for cost/delivery times).

Forks, shocks and large/heavy orders go by courier. UK deliveries are with DPD Local, for overseas customers it's DPD, FedEx or DHL.  In all cases these are fully tracked services and you'll need to sign for the delivery - please provide a suitable address with extra instructions if necessary.


You may return your order unused, within 14 days for a refund if you no longer require it, provided it is returned with its original undamaged packaging and is unused and unmarked. Please note, we do not refund postage unless items were supplied in error or are faulty.

Picking mistakes do sometime happen. If we supply the wrong part or there are parts missing from your order please let us know within 14 days of receipt so that we can correct the mistake. In the case of picking errors we will refund your postage costs on the returned item.

Faulty goods will be replaced with the same item or a suitable equivalent, as appropriate.

TF Tuned cannot be held responsible for the non-receipt of returned goods so please ensure you use a tracked and/or insured service as appropriate to the value of the goods you are returning.

Customer Reviews

"I had had been looking into this for a while, but like others I wasn’t sure if this £120 small black cylinder would be all that good. After seeing TFTuned were now stocking them I reached out and asked them, are these any good? The issue I had with my 2023 Zeb Ultimates was I couldn’t get them feeling less harsh out dropping the air pressure, but doing that, even with 3 tokens, I lost any mid stroke or bottom out support I needed. Installing the TruTune was easy and I included 1 air token as I ride an ebike and I also weigh a bit, added 10% PSI and a couple of clicks of LSC. My first ride I noticed the difference instantly, the fork felt more supportive yet more supple and on bigger days out I add a couple of clicks of HSC and that's it. Thanks again TFTuned!"
Star Rating 5
Staff response...
"NIce review Lance ... magic innit!!! Ruth @TFT"

Plug and play insert for mountain bike air forks, powered by activated carbon.Better small bump compliance, increased mid-stroke support and end stroke absorption. Reduces fatigue, improves grip and comfort, and unlocks more travel. Experience a coil-like feel without the weight penalty and with the tunability of air. Heavy rider (90+kg) or on E-bike? Choose our Short model where available.

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