TruTune is a cartridge that replaces your fork’s volume spacers to make its spring curve more linear. While it might not be for everyone, it increases front-end grip, performance and small-bump sensitivity. If you’re a lighter rider struggling to get full travel out of your fork or you’re looking for a coil feel without dropping the cash on a new fork, look no further.
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TruTune MTB Air Fork Insert - Technical details
Most suspension forks on the market use air springs and with good reason. Firstly, they’re lighter than their coil-sprung counterparts but they also provide much more adjustment, and that adjustment is very easy to access. Riders can tune their suspension to suit their weight with just a shock pump, rather than get into the insides of the fork and swap coil springs for a pricy replacement.
One of the other benefits of the air spring is that it’s naturally progressive. So the more it’s compressed, the more it resists compression. This allows for even more tuning options with the use of volume spacers or bottomless tokens but the mountain biking world has become obsessed with the progression of spring curves.
That’s where TruTune comes into the equation. It uses very clever science to do the very opposite of volume spacers - effectively increasing the size of the air chamber, making a fork’s spring rate more linear and less progressive. This is done by employing activated carbon for its ability to absorb air molecules as they’re compressed and desorb them when the pressure drops or the fork extends.
TruTune also addresses the stiffness of air and how it varies with speed. The brand says that when air is compressed, it heats up and when air is compressed quickly, as what happens so often in mountain biking, the excess heat doesn’t have enough time to dissipate, which results in a stiffer feel. We’re told that activated carbon reduces this effect, resulting in more consistent and lower stiffness at varying speeds.
It gets really techy when we start talking about pressure throughout the travel as this insert makes it possible to increase the charge pressure for more initial stiffness in the travel. It’s said that this results in lower pressure at the end of the stroke but higher pressure at the start of the stroke. TruTune says that in its field data, a higher charge pressure can achieve lower peak pressure.
Overall, the idea behind TruTune’s fork insert is that it should increase suppleness, and improve small-bump sensitivity and end-of-stroke absorption resulting in a plushier fork feel. This then allows the rider to inflate the fork to a higher-than-usual pressure to achieve good mid-stroke support without having to battle the harsh feel that occurs with high fork pressures.
The TruTune insert comes in two sizes: standard and short. The latter of the two is designed to work better for heavier or more aggressive riders or those who ride e-bikes, where the standard sizes’ linearity isn’t totally desirable. The brand says that the shorter insert offers a slightly more progressive spring rate but its small size means that it can be run with an additional volume spacer if more progression is required.
As for fork compatibility, the insert can be bought to suit all Fox forks as well as RockShox forks with a 35, or 38mm stanchion diameter that run the Debonair air spring.
TruTune MTB Air Fork Insert - Installation
Although the science behind the product is a bit on the complicated side, TruTune has made its insert incredibly easy to fit. In fact, it’s just like adding volume spacers. Release all of the fork’s pressure, open up the top cap, slide off any other spacers present and replace them with the TruTune insert.
You can add a volume spacer on top if you wish. But make sure to do this in a clean, dirt-free environment so as to not introduce muck into your air chamber.
Though setting this setup apart from the usual affair, it’s worth upping your usual pressure by 10psi or so to achieve good mid-stroke support.
If I were to really nitpick, the insert itself feels pretty plasticky which doesn't bode well in the durability department. However, once installed, you'll likely never see it again and during testing, I've never experienced any issue with it breaking.
TruTune MTB Air Fork Insert - Performance
Once installed, I pumped up my Fox 36 Performance Elite to my usual pressure which still resulted in my normal sag measurement. Just bouncing the fork in the garden showed a clear difference in the suppleness of the fork’s initial stroke. It took far less force for the seals to break from their static friction - a clear indicator of the fork’s newfound suppleness out on the trail.
When riding, the suppleness was stark but the fork certainly blew through its travel a bit too quickly and lacked mid-stroke support. Thus I bumped up the pressure by 10psi and on my second outing and my fork’s performance was transformed.
Initially, my fork did feel firmer, as per my favourite garden squish test, but when riding it was a very different story. The suppleness of the fork’s initial stroke was still very much present but the fork stood up far more in its travel, which results in better support generally, but even more so when tackling steeper tracks.
The whole of the fork’s travel is delivered a lot smoother and the fork affected my body position over the bike far less, increasing control. I managed to ride at a speed that was slightly above my comfort zone but, because the fork was well-damped and so controlled, I was more composed and able to deal with the extra speed.
Importantly, what the TruTune insert eliminates is that wall-like feeling as the fork gets deep into its travel quickly, a feeling that is present with volume spacers. Again, this boosts control over the front end during big hits.
Movement through the fork’s travel felt super consistent, too, thanks to the insert’s ability to encourage heat dissipation during faster movements of the suspension. Though, even during big hits, the fork had just enough resistance around bottom out to stop the inevitable clang of the air piston whacking the top of the air chamber. But it allows the fork to progress through its travel more freely, upping its capability to absorb all levels of trail bumps.
Then because of the increase in small bump sensitivity, the front wheel grip increased and I felt I was able to corner with much more confidence. Overall comfort is improved, too, also as a result of the fork’s freedom to move. Arm fatigue wasn’t a worry.
Though what’s really cool about this insert is that it’ll help lighter riders, or those who struggle to reach full travel, to get more out of their forks without having to resort to more drastic measures.
Over the internet I’m sure you’ll find comparisons to coil forks and rightfully so as TruTune adds the more linear performance associated with coil-sprung units but in a lightweight and easier-to-adjust package. But more importantly, it’s a hell of a lot cheaper.
Though, with all that said, it isn't for everyone. If you're particularly aggressive, riding big jumps or experiencing big hits on the regular, this product won't add many benefits.
TruTune MTB Air Fork Insert - Verdict
Comparing the value of TruTune’s insert is a tricky endeavour as there are few similar products on the market. Though one product that does a similar thing but very differently is Vorsprung’s Secus Air Spring Upgrade Kit. It’ll set you back £295.35 and compared to TruTune (priced at £120) it looks rather unsightly but claims to offer more advantages, including more tunability and it’s compatible with other top cap-mounted fork accessories, such as MRP’s Ramp Control. The biggest advantage is that it allows for more bath oil to be run in the fork, which directly results in better lubrication.
Of course, you can choose a coil conversion kit, which is another thing the Vorsprung offers in the form of the Smashpot at a fairly decent price of £295.35. A coil spring comes with advantages that TruTune doesn’t such as almost no stiction, no moving seals and again, more bath oil but you’ll be adding more weight for more money.
To bring up another Vorsprung product, the Luftkappe is said to soften a fork’s initial stroke and add more grip to the front wheel but it’s difficult to install by yourself and it actually makes the spring rate more progressive. That’ll cost £95.47.
Then with air-sprung forks having such a firm grip on the world of mountain biking, coil-sprung offerings are few and far between but they are out there. For example, Marzocchi has a version of the Bomber Z1 with a coil, but then you’re looking at spending £849.
If it’s a coil-like spring rate and consistent feeling travel you’re after but you’re not keen on spending the money on a coil conversion kit or buying a whole new fork, the TruTune MTB Air Fork Insert is an excellent place to start. It’s light, does exactly what it says on the tin and is comparatively cheap.