Review: TruTune Suspension Inserts Unlock More Travel
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.
TruTune Details
• 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
• trutune.co.uk
• 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
• trutune.co.uk
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.
The more travel you have, the fewer volume spacers you need; with modern single crown forks reaching 190 mm travel there is more call to go beyond zero 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.
Image from TruTune.co.uk
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?
 | While it's tricky to get your head around how it works, TruTune does a very simple job: it makes air-sprung forks less progressive. In that sense, it does what it sets out to do, allowing access to more travel, with less harshness on big hits, albeit at the necessary expense of reduced support and bottom-out resistance. Obviously, that isn't something everyone needs. But because some modern long-travel forks are very progressive, and given they're not exclusively ridden at Rampage, plenty of riders could benefit from the ability to go more linear than zero volume spacers. |
Author Info:
Must Read This Week
Never stop pumping!
I need to use the right amount in the right terrain.
I've owned or tried the readily available coil forks on the market, and they do not feel good to me (notably missing was Ohlins, because I'm not made of money). So where I ended up was on a Fox 36 with the Grip 2 damper and a Smashpot coil conversion kit from Vorsprung. I've owned this fork from new, and I believe it is a 2019 or so. It doesn't have those burping ports on the back of the legs.
Side Note: I also own a late model Fox 38. While this fork ostensibly has the same damper, I much prefer this fork, even in its air version, to the coil converted 36. Supposedly the difference between the forks stock is air piston diameter and the resulting seal friction is reduced in the 38, but my coil kit on the 36 should eliminate that souce of a difference in feel. I think I'm just going to deal with the weight and ride the 38, if I condense my fleet moving forward.
It's also interesting to hear you like the 38 (air) more than your 36 coil. I have a RS Domain on my other bike and it feels pretty good/plush to me despite having a "cheap" damper. I'm also heavy rider. I wonder if those stiffer stanchions are the real key. Supposedly they reduce flex and thus reduce binding on the bushings--a problem you would still get on coil.
(As a side note, I just sent my Onyx in for a potential warranty repair. Hopefully they'll find something wrong, like hte bushings, and it'll come back feeling amazing)
That way you could achieve nothing but also spend a lot more AND add a load more weight; triple-win.
I like your thinking. You've then also got the added benefit of being unable to revert back to an air spring after realising the set-up is just too good.
at 85 kg, I've been struggling to use full travel on my 190 zebs ( with the correct sag and 0 tokens )
What a great tip. Does bleeding the lowers with those bleed valves cure it? Thx.
Would love to be able to try this!
My advice is to run your low-speed compression at least ~50% closed (& high-speed 25% if you have a knob for it) and drop your pressures 5-15 psi. Set your rebound fast, 2-4 clicks from fully open.
You'll have a lot more midstroke support, you'll still rarely bottom out, and the fork will be a lot plusher in the big hits. You might even want a token or two after trying it out. This thing will only make your fork dive even harder, it kills your midstroke support and will make hard braking suck.
"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."
So basically, they're saying AT LEAST 100 hours, but also, we don't know.
Yeah, only rated for 700 years unfortunately.
So basically, ignore my stupid comment.
There are only two kinds of riders who want this: those with a faulty Zeb/38, and those who haven't tried less fork pressure with more compression.
trutune.co.uk/products/insert
The other issue is the time it takes for air to infiltrate the small pores (which will depend on pore size and pore size distribution). Think of trying to blow (or push) air a lot of air (people) through tiny holes (a couple of fire exits).This time component will make the phenomenon they are interested in dependent on fork travel rate--i.e. their cartridge's contribution to a fork's reaction forces will act completely different at different compression speeds/travel rates. The graphs they show for Force vs Travel are likely done at very slow speeds where are can infiltrate the pores easily. They need to show how the Force vs Travel looks for the whole of rates expected for a fork. I'm guessing at for even fairly low compression rates, this pore structure device will be overwhelmed by the air trying to get in and then it will also be too slow to release the internal air on rebound (referred to as hysteresis), thus being ineffective at all but the slowest fork travel rates.
I would also like to see negative chamber adjustability like Ohlins on all forks (as I think that's a more appropriate solution than simply increasing negative volume!), but that's just me.
Bought some activated carbon in a pet shop (Aquarium section), put it in a 3D printed canister and voila! Truely works.
76kg rider + 14kg bike
Before (2 tokens):
80psi
LSC 16
HSC 2
Rebound 8
After (TruTune)
100psi
LSC 9
HSC 2
Rebound 10
For reference: My testing process was to get the sag into the ballpark and then huck myself off a 4-5ft drop repeatedly, using compression damping to stop bottoming out. Next was some more bracketing on a fast, chunky section of trail with some deep compressions. The surprise there was finding myself liking a faster rebound, despite the higher pressures.
This is a ripoff that will annihilate your mid-stroke support and make your fork dive like a goddamn submarine under braking.
When it comes to air forks, mid-stroke support is mostly an effect of damping.
But okay, enjoy the effects of negative four tokens
So, obviously, a TruTune user would increase their air spring pressure to account for that. But, air suspension does not increase support proportionally with pressure, ask any big boy with an air fork. Only coil springs increase support proportionally with spring rate.
I've messaged Paul Aston for his opinion. Traditionally the only way to get appropriate midstroke support out of an air spring has been heavier damping in the midstroke, which is exactly what the best-reviewed forks on the market do.
This seems like solving one problem at the cost of creating other problems. Especially since I'm skeptical about the long-term durability of something that probably emits carbon dust, and because if I'm to believe this is "speed sensitive" then surely they should have to address what that does when the fork is rebounding as well.
Sounds like it does part of the same job as the Vorsprung Secus (more linear), but by increasing the positive rather than negative air volume?
How about an article rounding up all these cunning fork gadgets and explaining what they do @seb-stott ?
I’m a heavy guy at 100kg and find I need to run high pressure to get correct sag on a set of 36s. Zero tokens and still not getting full travel.
I’m thinking this would help me get that last bit of travel if I left psi for sag the same.
If I’m right someone needs to figure out how to do the same to an X2 shock.
This is a $120 piece of charcoal that will absolutely f*ck up your mid-stroke support. If you want a fork that dives like James Cameron then go for it I guess. If you ride steep trails at 100kg you will hate this thing.
It's frustrating to tune the rebound to full slow just so that the spring doesn't overpower the rebound damping when it goes deeper into the travel - but then it's not as supple as I would like (thinking retro-Marz here) in the initial part of the stroke
Best setup for the Yari: remove all tokens, increase pressure more than you think you should, let rebound as open as you can handle. Counter intuitive, but it worked for me. The damper will still choke at some point because it can't handle successive hits, but at least the fork rides higher in the travel and feels more active this way. The long term solution is to upgrade the damper if you can (or get a better fork).
You, me, and everybody else doesn’t get prototype racing suspension.
They didn't do this for performance. they did it to lower the cost of making the fork, and make that fork marketable for a larger range of bikes. All Pikes, Zebs, 34s, 36s, etc etc etc of any travel are the same except for the length of the air shaft and number of tokens installed (caveat: Fox has been known to not make all stanchions the same length within a chassis model, but the concept is the same.)
and then they came up with a slick marketing strategy for the method they came up with to fix the problem they created: one positive air spring size does not work for a fork that can come in both 160mm and 80mm travel. but if we call them "tokens" and tell people they're for "tuning your progression" then we can pretend this isn't a hack to fix the problem. and in their defense, it isn't even entirely untrue, but they marketed the hell out of it.
On the face of it, none of this couldn't work. but it's a compromise, especially on the longest travel setting, where it's hard to get enough volume for the positive spring. and at least in RS's case, it forced them into some damper compromises as well, especially in the first gen charger. if you liked that progression, it felt fine, which is why there were so many rave reviews at the time. but if you liked a more linear fork feel, the self-equalizing air spring has been a cancer on fork design since it was introduced.
and that's ignoring the lubrication compromises that affect all users.
No, increasing the negative spring force, which is done with pressure not volume, helps remedy this. Just like increasing the the spring force of a coil negative, like DVO's OTT Adjustment can do, will decrease the breakaway force of an air positive spring.
I do believe the Activated Carbon could store and release the air a bit slower and make an sensation of softer, make the fork ride a bit lower in it's travel, as an air damper.
Also, if it's "speed sensitive" as they claim then it is certainly active when the fork is rebounding as well. Seems like a lot of money to spend on something absolutely unproven by any racer, only a claimed "100 hours" of durability testing, and has effects on the rebound that they don't even seem to touch on.
Because of the low position, increasing the air pressure increases the harshness.
Pairs very well with negative chamber upgrades (e.g. Vorsprung Secus, Everflow Tank) to provide exceptionally linear and consistent feeling through the travel.
people really will buy anything.
its like the whole "bigger tires are faster at the same pressure"...well....yea. but..
It works, it's great, it was a game changer in the Alps.
Easy.
Someone better tell Vorsprung, because the Luftkappe, and especially the Secus, increase the negative spring volume intending to make the whole spring more linear.
Yes, shrinking the positive volume will make the positive spring on its own more progressive, but you also have account for the change in pressure on the negative side. A small negative volume decreases in pressure rapidly as travel is used, greatly increasing the _net_ spring force in a very progressive way. A larger negative won't see as large a pressure drop, meaning the _net_ spring force will stay closer to linear.
A larger negative spring a la Luftkappe, MegNeg, or Secus, making the entire curve more linear could probably be better for more riders, but, hey the price is decent on this.
*(Why didn't you hang some weights from the bars instead of bouncing** on it? Make it an actual rigorous and repeatable test? Science!)
**(YOU call it "the bounce test"? Wow, what a unique name!)
***(Since it's supposed to have more effect with more pressure)
Drink every time someone tells me adding more pressure gives more mid-stroke support, blackout ensues.
If a pro starts running this and manages to prove me wrong, sure. I simply don't think that the stock RS/Fox dampers provide enough midstroke on their own to run something that definitely reduces midstroke support. Untested and overpriced, in my opinion.
I hit up Paul Aston, and I will let you know what he thinks since you are so inclined to argue without discussing it.