Bike Shop: Trek Thru Shaft – Revolution, Evolution or Neither?

What the heck is Thru shaft? Does it do what Trek claims?

Sometimes it’s not about having the solution. It’s about knowing where to find it. Ask anyone with a smartphone. Or ask Trek’s suspension guru Jose Gonzalez. He’s found some remarkable solutions, of all places, inside Formula One race cars.

In 2014, Trek introduced RE:aktiv damping, or as it’s known in the F1 world, the Penske valve. The seemingly simple circuit opens immediately under high-speed forces and stays firm under slow-speed. And it works. It works really well. There’s no unpredictability and no spike at the initial breakaway point. It’s why I look forward to riding any Trek that comes my way.

It’s also why I was skeptical when I heard Trek had made yet another leap forward. How many more leaps are there? How can you further refine RE:aktiv valving on a conventional shock? Answer: you make it unconventional.

As the shock compresses, the volume of the oil chamber stays constant because as the damper shaft enters, an extended damper shaft leaves, eliminating the need for an IFP.

Also borrowed from high-end auto racing suspension, Thru Shaft technology does just what its name suggests. Traditionally, the damper shaft simply pushes and pulls the damper valve assembly through the oil chamber. With Thru Shaft, a second, seemingly superfluous shaft is attached to the other side of the the valve assembly. As the shock is compressed, that extension gets pushed out thru the bottom of the shock and is pulled back in when the shock rebounds.

Benefits of Trek Thru Shaft

If you aren’t yet wondering why this makes any difference, you are a better engineer than I. But if you are wondering, let’s discuss traditional shocks first. Where the oil chamber dead-ends, there is usually an internal floating piston, or IFP. Behind the IFP is a high-pressure chamber, normally filled with nitrogen. As the damper shaft enters the oil chamber while pushing the damper valve, it is constantly displacing oil as more and more of it is introduced to the chamber. An IFP compensates for that displacement. But in a Thru Shaft system, the superfluous shaft is exiting the system at the same rate as the damper shaft is entering it, keeping the oil volume constant.

Thru Shaft’s ability to eliminate lag in damping pairs perfectly with the instant reaction of Trek’s RE:aktiv valve.

Now, if you aren’t wondering why that makes a difference, you again are a better engineer than I. The reason is hysteresis. Yes, that’s a real word. It’s even in the Scrabble dictionary. And it has a real effect on how your suspension performs. When the shock moves, the IFP moves, which creates a lag in the damping. Without that lag, you would get more instant, more pure damping performance.

The piggy-back on the RockShox Thru Shaft accommodates for thermal expansion and cools the oil cycling through the shock. The lighter-duty Fox version tucks a similar mechanism inside the damper shaft.

There’s also the extra stiction that occurs when compression and rebound forces slide the IFP up and down the oil cartridge. Under such high pressure, there’s a lot of force on that o-ring. There’s much less force on the seal through which the Thru Shaft exits the shock, which gives you the net drop in stiction. Time will tell, but it isn’t expected to come with a drop in reliability. Thru Shaft shocks suggest you service them at the same intervals as you would a traditional shock. The one issue that Thru Shaft doesn’t solve is that of thermal expansion. The pocket of air beneath the IFP allows the oil volume to fluctuate as its temperature changes. Trek solves this in one of two ways, depending on which bike we’re talking about.

It still stings that there is no 29″ Remedy, but the extra sensitivity of the Thru Shaft shock takes a bit of the edge off.

For the moment, Thru Shaft is only being offered on a few flagship models. It’s expected to trickle down in coming years, but for now, Trek is looking to recoup some R&D costs by drawing riders to higher-priced, higher-margin bikes. Namely, the 2018 Fuel EX 9.9, Remedy 9.8, and Slash 9.8 and 9.7 complete bikes, as well as on each corresponding frameset. The shorter-travel Fuel EX applies Thru Shaft to a Fox shock, while Trek went with a RockShox for the Remedy and Slash. The RockShox version features what looks like a tiny piggy-back, which doesn’t actually house any damping hardware. It’s actually just a way for some of the oil to cycle out of the main chamber and radiate some heat that would otherwise expand the fluid. The Fox shock on the lighter-duty Fuel Ex features a similar mechanism, but hides it in the damper shaft itself.

We’re still waiting to try the newly Thru Shaft-equipped Slash. But we can probably guess what a little extra smoothness could do for one of the most buttery bikes we’ve ridden this year.

Ride Impressions

The benefits of Thru Shaft go hand-in-hand with the benefits of RE:aktiv. I’m already a believer in that concept, and it gets turned up an extra notch with Thru Shaft. A big benefit of RE:aktiv is that it shines most brightly in the shock’s firmer compression damping settings. The Penske valve allows the platform to get out of the way on high speed impacts, and the instant transfer of energy made possible with Thru Shaft made that effect even more noticeable.

We tested Trek’s new Thru Shaft bikes in the Verbier bike park in southern Switzerland. It is so choice. If you have the means, I highly recommend it.

I assumed there would be a benefit to eliminating the stiction that came with an IFP. But I also knew it would be subtle. While pumping around berms and over rollers, my feet were listening intently for the fup-fup of rubber on aluminum. I took a run on the original shock for comparison, but it still proved elusive.

So I brought a bike and a couple shocks home from Switzerland, and I passed it around the office. Bike Mag photo editor and all-around shredder, Anthony Smith and I each spent some time on both the traditional RE:aktiv shock and the new Thru Shaft version. I wanted to bring in a second opinion on how tangible the benefits of Thru Shaft really are. So we bellied up to the bench in the Bike Shop, and had a talk about it in this video.

trekbikes.com

Related:
A Detailed Look at RE:aktiv

Review: Trek Remedy 9.9 RSL

2017 Bible of Bike Tests: Trek Fuel EX 29