TruTune is a plug-in suspension modification that takes the place of a volume spacer in your fork but has the opposite effect. When I first heard about it, I thought it was a joke, but after talking to the engineers at Carbon Air, the company behind TruTune, I realised the physics was sound. Now I've had the chance to ride it and, at the risk of giving away the ending of this review too soon, it does what it promises.
• Makes air-sprung forks less progressive (the opposite of volume spacers)
• Fits Fox 32, 34, 36, 38 & RockShox 32, 35, 38 mm forks.
• Price: £120 GBP
• Money-back guarantee
What is it?
In a sentence, TruTune doe the opposite of a volume spacer. I wrote a whole article
about how exactly it does this, but the short version is that it's made of activated carbon, which is like a really fine porous sponge with a crazy high surface area. Air molecules stick to the surfaces of its tiny crevices and pack together more tightly than in empty space. As the air pressure increases, they pack together even tighter, so essentially, each cubic centimeter of activated carbon acts like two cubic centimeters of empty space. This effectively increases the volume of the positive air chamber, reducing the compression ratio.
The result is that your air spring is more linear than if it had no volume spacers in it at all. Previously, the tuning options for volume spacers have been 0, 1, 2, 3 etc., but now that range of options has been extended to include negative numbers. It's complicated to say how far into the negative it goes, but the TruTune insert I have is equivalent to removing 3-6 volume spacers, depending on the suspension speed. TruTune also make a "short" version which is roughly equivalent to -2 to -4 volume spacers.
Who's it for?
Obviously, this isn't going to be useful for everyone - if you need volume spacers to avoid bottoming out then you have no need for this. But increasingly, fork manufacturers have been setting aside more of the available space in the air-side leg for the negative chamber (below the piston) in order to make forks softer at the start of the travel and firmer in the middle. The fact that air springs are usually much stiffer at the start of the travel than in the middle is arguably their biggest downside compared to coil, and increasing the negative spring volume helps remedy this.
But increasing the negative volume (which usually necessitates decreasing the positive volume as there's only so much room inside the fork) makes the fork more progressive. And so, in recent years forks have generally got more progressive. At the same time, long-travel single crown forks (with 170-190 mm travel) have become more common, and increasing the travel makes it much harder to use all of it.
So now more than ever, there are forks that are simply too progressive for some riders. TruTune claims to address this.
Does it work?
Installing the insert is no different to fitting a volume spacer - it just screws onto the bottom of the top cap. To check if it works, I set a 190 mm RockShox Zeb with no volume spacers to 70 psi. By bouncing on the fork as hard as I could on flat ground (I call this the "bounce test"), I was able to use 133 mm of travel. The amount of travel I can access in this test is surprisingly consistent. I repeated the test with TruTune installed and got to 139 mm. This may not sound like a very big difference, but the deeper you go into the travel, the more pronounced the effect becomes - (that's the point).
According to this graph from TruTune's website, the insert in a RockShox Pike reduces the ramp-up of force towards the end of the travel significantly but doesn't make that much difference in the middle of the travel (roughly the opposite of adding two volume spacers). By the way, the reason this graph shows a loop rather than a single line for each volume spacer setting is that the force was measured on compression and rebound, and rebound forces are always lower due to friction and heat loss (see below).
On the trail, I was consistently getting more travel out of the fork, and it was less harsh (more like a downhill coil fork) on big hits. Of course, there was also less support when deep into the travel, but I was able to use a little more air pressure and more compression damping without excessive harshness, and with the 190 mm RockShox Zeb on a Pole Voima
eMTB, the progression of the front matched the rear better - without the insert, there was a noticeable ramp of force at about two-thirds of the travel in the fork, which wasn't matched at the rear. In that context, adding the TruTune insert improved the ride noticeably, making the bike feel more balanced and even more forgiving on the rough descents where it was most at home. Although the fork was noticeably softer, I wasn't bottoming out unduly, and I didn't need to compromise on sensitivity to avoid bottom-outs.
Nerd Section: reducing speed sensitivity
TruTune claim that their product doesn't only reduce progression in the fork, but also reduces the effect of compression speed on suspension stiffness. I explained this concept in more detail in the first look article
, but essentially, when air is compressed quickly it gets hot (this is why your pump gets hot when you inflate your tires quickly, or how a diesel engine ignites its fuel without a spark); and when air gets hotter in a fixed volume, the pressure increases.
Normally, the heat that's generated in your air spring as it compresses has time to escape into the walls of the fork, but if the fork compresses quickly, the heat can't escape in time, so the pressure becomes higher than normal for a given point in the travel. That means the spring provides more force when it's compressed quickly versus slowly. This extra force can be as much as 40%.
Whether this is a bad thing is up for debate, but it certainly can contribute to air springs lacking support during slow-speed compressions such as braking and cornering while feeling harsh and being unable to use all the travel on fast, high-frequency hits like slamming into a curb.
Trutune say that because their insert has a high surface area and a lot of mass compared to the air in the spring, it absorbs the heat generated during fast compressions, and releases it during rebound, reducing the extra force required to compress the fork quickly.
In the chart above they plot the measured air pressure (which is a proxy for spring force) against travel as the fork is cycled through its travel first slowly then faster. This creates a loop in the curve because the pressure is lower during rebound than compression, but as the speeds increase, the force required to compress the fork increases dramatically with nothing in the fork, but more modestly with TruTune installed - hence the black lines keeping closer together than the red lines.
To simplify things, Trutune tells me that their standard insert is equivalent to removing three volume spacers at very low speeds, and up to six at high speeds. TruTune also offer a "short" option which is roughly equivalent to removing two volume spacers at low speed and four at high speed.
If you think that sounds like you're going to be bottoming out all the time, well yes that is a possibility if you're using close to full travel already. But it's worth pointing out that the situations which cause the fork to bottom out (such as the G-out at the end of a rock roll) aren't necessarily the ones that cause the fastest compression speeds (eg. hitting a curb-sized rock at pace), So the effect on bottoming-out is not the same as removing six volume spacers permanently. Plus, on those fast hits, you're still getting more force than on slower hits, just not as much additional
force as you'd get in a standard air spring. Remember that with a coil spring, the stiffness doesn't increase at all with compression speed or with travel, yet people who fit coil conversions aren't necessarily bottoming out all the time.
The graph above is from TruTune and I don't have the equipment to verify it, but on the trail, you can certainly notice there's a more forgiving feel on big hits. I can't say how much of that is due to reduced speed sensitivity and how much is simply due to the lower compression ratio.
What's the bottom line?