TruTune MTB Air Fork Insert

TruTune MTB Air Fork Insert review – The TARDIS Token

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The TruTune MTB Air Fork Insert is not a volume spacer. It’s the opposite in fact. Volume spacers reduce the air volume in your fork’s positive air chamber. The TruTune effectively enlarges it.

  • Brand: TruTune
  • Product: MTB Air Fork Insert
  • From: TruTune
  • Price: £120.00
  • Tested: by Benji for 4 months
TruTune MTB Air Fork Insert
TruTune MTB Air Fork Insert clicks into place just like a volume spacer

Er. How does putting something inside a constrained space make it bigger? I don’t really want to spend ages describing the physics of this. I just want to tell you if and how it all works on the trail.

The single physics paragraph

Air molecules can pack tighter together when on a surface compared to when in empty space. TruTune MTB Air Fork Inserts are stuffed full of surfaces. The surface-tastic material in question is called ‘activated carbon’. A TruTune is a plastic container full of activated carbon. With a TruTune installed you can fit more air (molecules) in your fork compared to without a TruTune in there. That’s all my science bit done. Want more? Read about adsorption.

Does it work?


Okay. Review done. Next!

Not really.

Why run a TruTune?

Some riders, such as myself (73kg), have found modern fat stanchioned air forks to be increasingly progressive in spring nature. As negative air chambers have got bigger (to make them more supple at sag) this has the knock-on effect of making them also much more progressive towards end-of-stroke.

In other words, I find it hard to ever get much beyond about 3/4 of fork travel even when running healthy amounts of sag and winding off all the compression damping. In particular, its the 38mm breed of fork (RockShox Zeb, Fox 38 Float) that seem to be excessively rampy for little ol’ un-steezy me.

The TruTune MTB Air Fork Insert makes air forks more linear.

Coil. ish.

The word ‘linear’ will make a lot of people instantly think of coil springs. And yes, in a way, running a TruTune gives you some of the aspects of running a coil.

To cut a longer story short, I think the TruTune gives you the linearity, consistency and improved midstroke of a coil. But it doesn’t give you the pitter-patter super-supple traction-hoovering suppleness of coil.

But at £120 it’s a fair bit cheaper than a new fork or a coil conversion for an existing fork. And it clearly has the advantage of air’s precise pressure adjustment; you aren’t limited to a choice of a handful of coil spring weights. You can dial in the pressure to the exact PSI you want.

You can also still run volume spacers (check that the overall TruTune+spacer length can be accepted by your fork!) which, although we don’t really imagine many people will ever do, it may be useful for fine-tuning the linearity-progressivity feel of the fork.

We had the normal and short versions to try

By the by, TruTune offer the insert in two lengths: normal and short. The short size, somewhat counter-intuitively, is for heavier and/or more aggressive riders. I tried both the regular and the short. The regular was the one that suited me best.

Does it work (slight return)?

All in all, the TruTune MTB Air Fork Insert afforded me the ability to get 38mm forks to work better. No more compromise on having to choose where you can get the fork to work ie. having to choose between small bump or bottom-out or mid–stroke, or choosing between slow-speed support on steeps or high-speed big bump soaking.

I mainly tested the TruTune MTB Air Fork Insert on Fox 38 and Fox 36 forks. The effect on Fox 36 forks wasn’t that noticeable to me. Again, I don’t find 36 forks to be anywhere near as progressive as 38 forks though, so that’s to be expected.

With the TruTuned Fox 38 I could finally have my suspension cake and eat it. No more being restricted to certain areas of the bouncy buffet. Supple-enough sag? Check. Good support once into the stroke? Check. Able to use full-travel when required? Check. No harsh bottom-out? Check. Consistency of action no matter whether on slow speed or high speed terrain? Check.

Quick note about setup, I ended up putting about 10% more PSI into the fork when running the TruTune. This gave me the best balance of plushness, support and full-travel accessibility.


With the TruTune installed I can finally get modern big negative air chambered fat-legged forks to perform better. It doesn’t give you everything that a coil fork does but it certainly improves things massively. If your bike has a 38mm fork on it that you can’t quite get to do the things you need it to do, a TruTune MTB AIr Fork Insert is totally worth a look.

Review Info

Brand: TruTune
Product: MTB Air Fork Insert
From: TruTune
Price: £120
Tested: by Benji for 4 months

Cotic RocketMAXer. Schwalbe Magic Mary Purple Addix front. Maxxis DHR II 3C MaxxTerra rear. Coil fan. I have been a writer for nigh on 20 years, a photographer for 25 years and a mountain biker for 30 years. I have written countless magazine and website features and route guides for the UK mountain bike press, most notably for the esteemed and highly regarded Singletrackworld. Although I am a Lancastrian, I freely admit that West Yorkshire is my favourite place to ride. Rarely a week goes by without me riding and exploring the South Pennines.

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  • TruTune MTB Air Fork Insert review – The TARDIS Token
  • rickon
    Free Member

    Let’s do this! £120 for fork tokens that are probably printed?!

    Could you just drill a load of holes in your current fork tokens?

    Full Member

    Rickon they don’t do what fork tokens do, so you could drill tokens, but you’d have to fill them with activated carbon ™

    So I ran mine at tweedlove in some really grim slop and came away very happy, amazingly didn’t crash once, even on hammertime! Brand new T9 butcher helped but forks felt really nice, tons of grip and plush without being divey

    Free Member

    Been thinking about the physics of these. I have my doubts that it is adsorption, air would have to be adsorbed and desorbed multiple times a second and I just don’t think it’s that fast a process. I couldn’t find any useful literature discussing speed of adsorption, papers tend to concentrate on total volume rather than speed. People seem to be getting some benefit, so I wonder if it is the friction of the air moving in and out of the pores in the activated carbon. This would act like additional damping but more rapid and I think more progressive than oil moving through a nozzle would provide.

    Free Member

    These things do seem quite interesting and the science is quite mind boggling. I did assume they were manufactured from “solid” activated carbon but it appears that the “token” is a 3d printed canister containing the activated carbon. Now the science might be funky, but activated carbon doesn’t actually appear that expensive, £106 for 25Kg. Unless there is something techy about the spec of the carbon, it’s does seem kind of pricey. The fact they also have a finite lifespan is a downer too.

    Full Member

    I’m struggling even with the initial premise that large negative chambers “cause” greater progressivenessivitude
    Surely that’d only be the case if the relative sizes of +ve vs -ve had changed ?

    I suppose maybe in a 180/190mm single crown fork there might have to be a trade off between the two and I also suppose that manufacturers might generalise that a typical owner of such a fork might be heavier than Benji and might be expected to ride mighty hard, all of which would mean prioritising -ve over +ve (for the mid- and end-stroke support (err, bro) but also feeling nice in the car-park)

    If so, I bet a 40mm DC air fork would piss on the Zeb – are DH forks ever air-sprung these days ?

    Free Member

    @scaredypants 99% of the uci dh teams run air forks

    Free Member

    I suppose maybe in a 180/190mm single crown fork there might have to be a trade off between the two

    Yep, I had a 190mm Zeb v1 and it was impossible to get full travel, way too progressive.

    V2 Zeb at 180mm is actually a lot better and I’d only try one of these out of curiosity – I don’t feel driven to by a performance issue IYKWIM.

    PS. “Tardis token” is very good Ben

    Full Member

    “I have my doubts that it is adsorption, air would have to be adsorbed and desorbed multiple times a second and I just don’t think it’s that fast a process.”

    I can’t see why adsorption couldn’t happen this quickly – you’re dealing with a pressurised gas and molecules attaching themselves to the surface of a solid. It’s not like you’re having to push a high viscosity fluid into a porous sponge. Despite the name, adsorption is very different thing to absorption.

    Full Member

    I’m not finding much on adsorption timing. The few bits I saw looked slow but with a faster start.

    Presumably a shokwhizz (?) would tell you if the pressure was ramping up less quickly.

    I wonder if a simple screw on piggy back chamber would work?

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