First Ride: Trek Thru Shaft shock, a new take on old tech

To be honest, when Trek told me, somewhat cryptically, that there were shock updates coming this summer, I was a bit baffled. The regressive valve RE:Aktiv shock, combined with the rest of Trek’s suspension tech, makes for a stellar trail-bike suspension package. I didn’t see a lot of room (or need) for improvement.

And also to be honest, even after looking at the press release materials, talking to Trek marketing people, and riding the new shock, I was still a little in the dark about just what was going on inside the damper. Fortunately, I’ve been digging deep into suspension technology and pulled out my copy of the Race Tech’s Motorcycle Suspension Bible and got in even deeper.

Thru-shafts are nothing new, and Trek is up front about that. But Trek claims this is the first time a thru-shaft is functionally superior to the dominant shock technology on the market.

Current shocks use an internal floating piston (IFP)to compensate for changing oil volume due to uneven displacement and heat. Since a standard shock only has a shaft on one side, the volume behind the piston versus in front of the piston is different, which is compensated via the IFP. But since the IFP has some type of spring behind it (usually either a nitrogen charge or a coil spring) there are moments where the internal oil pressures fluctuate, which can lead to lag in the damping effects and an inconsistent feel.

In other words, when the shock first compresses, instead of forcing oil through the damping circuits, it has to compress the IFP, so the damping can’t do its job, resulting in lag in damping performance. Throughout the travel, the IFP movement can create “stick and slip” moments that frustrate attempts at consistent damping.

Mounting the piston to a shaft equalizes the oil volume on either side of the piston, eliminating the need for an IFP to compensate for changing oil volumes.

What you’ll notice is the shaft needs to go somewhere at full extension, and in this case, you can see it needs to poke out of the bottom of the shock.

The IFP also does the job of dealing with heat-related oil expansion, so a thru-shaft shock needs another system. Trek’s FAQs about this shock says: “Due to the use of a flow control check valve, thermal compensation on RE:aktiv with Thru Shaft takes place during moments when the shock is static. This eliminates the need for a dynamic IFP function and an associated IFP gas charge, which is necessary for traditional dynamic IFP shock function.” I’m not totally clear on how this works, but I am imagining it is similar to a brake master cylinder, where brake fluid can expand into the reservoir when the brake lever is released. I’ll be chatting with Trek’s suspension guru, Jose Gonzales, to get a more in-depth description about the hows and whys of Thru Shaft soon.

Currently, Thru Shaft shocks will be OE only, no aftermarket options, although they will fit in current frames. The Fuel EX uses a Fox damper, the Slash and Remedy have a RockShox shock with a piggyback reservoir.

Maybe Cam McCaul can explain it to you:

Currently, you’ll find Thru Shaft on:

Fuel EX 9.9 29
Fuel EX C Frameset
Remedy 9.8 27.5
Remedy 27.5 C frameset
Slash 9.8
Slash 9.7
Slash C frameset

I would expect to see this tech trickle down in the future, but right now, it is for those with deep pockets or deep dedication to the sport.

Ride Impressions

Trek got me on a Fuel EX a few weeks ago, and I’ve gotten a few good rides in on the new shock. Unfortunately, I haven’t managed to track down a non-Thru Shaft shock to do some real back-to-back testing, but that should happen soon. I’m guessing that much like the RE:aktiv shock, the performance differences are best felt when both old and new tech are ridden on the same trails.

As I expected, this seems to be a refinement of an already excellent shock and bike. The middle platform position seems even more useable than before, plush when needed, but resisting unwanted movement from pedaling and weight shifts.

Our Take

With the growing competition between kinematics vs shock tech, Trek sits firmly in the shock tech camp, and the Fuel EX is an excellent example of the breed. We’ll have a full report at later date, and hopefully an interview with more info on the F1-derived shock technology that Trek continues to adapt, with much success, to mountain biking.


Some FAQ from Trek:

RE:aktiv with Thru Shaft FAQ

Is this technology proprietary to Trek?

Trek has an exclusivity agreement with both RockShox and FOX Racing Shox for two years. There are no patents on Thru Shaft, so after two years, other brands and FOX or RockShox could implement parts of the design. However, Penske still owns the patent for the RE:aktiv valve, so only Trek will benefit from RE:aktiv’s regressive damping.

How does RE:aktiv with Thru Shaft compare to other all-mountain or enduro shocks like the Fox X2 or RockShox Super Deluxe?

RE:aktiv with Thru Shaft is the only damper that eliminates oil displacement, the dynamic internal floating piston (IFP), and gas charge; as a result of this elimination, RE:aktiv with Thru Shaft has reduced hysteresis (lag) and more balanced damper pressures, which allows it to more quickly and effectively react to changing terrain. Overall, it’s a much more responsive damper with the same air spring performance.

What does all of that mean? How does eliminating oil displacement benefit the rider?

In a traditional air shock, the damper shaft displaces oil as the shock moves through its stroke. The IFP, a gas-charged piston in the damper, compensates for this constant change in damper volume. As the damper rod displaces oil, the increased damper volume creates enough pressure to compress the gas charge and move the IFP. As the shock rebounds and pressure is reduced, the IFP will start floating back to its original position, and the cycle continues.

The rod pressure from the gas charge and the stick and slip effect of the IFP’s movement create hysteresis, or lag, which keeps the shock from working as quickly as possible.

By eliminating oil displacement, we also eliminate the need for a dynamic IFP. With no dynamic IFP, hysteresis is dramatically reduced, which creates a damper that reacts to changing terrain significantly faster than anything else available.

How does RE:aktiv with Thru Shaft eliminate oil displacement?

Rather than a single damper shaft that displaces oil as it moves deeper into the stroke, Thru Shaft uses a shaft on either side of the damper valve that moves through a single, solid column of oil. As the main shaft enters the damper, the secondary shaft exits the damper on the other side. Conversely, as the main shaft exits the damper, the secondary shaft enters the damper on the other side. This results in a constant damper volume with no displacement and more balanced internal pressure.

How does all this affect the rider?

It’s a more responsive shock. With no IFP force acting against the damper shaft, small-bump sensitivity is greatly improved. Eliminating the dynamic IFP also eliminates its friction and stick and slip effect, so not only is the shock movement easier to initiate, it also changes direction much faster. The solid column of oil and immediate pressure balance result in more support and efficiency with faster response to terrain throughout the stroke. In total, this shock amplifies the responsiveness of a standard RE:aktiv shock, and keeps your rear tire glued to the trail so you can ride with even more confidence.




With no dynamic IFP, how does the shock manage heat-induced fluid expansion?

The longer-stroke RockShox version uses an external reservoir for thermal compensation.

Since the shorter-stroke Fox version has less total oil volume, Fox was able to include a thermal compensator within the main damper shaft.

Do the added seals cause extra stiction?

Eliminating the dynamic IFP’s stick and slip effect and the IFP’s gas charge nose force on the main damper shaft greatly outweighs any potential added stiction from the additional Thru Shaft secondary shaft seals.

Which bikes feature the new shock?

Slash 9.8 and Slash 9.7; Remedy 9.8 and Remedy 9.8 Women’s; and Fuel EX 9.9.

Carbon framesets of the above platforms will also include shocks featuring RE:aktiv with Thru Shaft.

Why is RE:aktiv with Thru Shaft only available on the highest-end bike models? Will it make its way onto other models?

Initially, we are only offering RE:aktiv with Thru Shaft on the highest-end models because of development and production costs. We expect to offer it on more models in the future.

Can riders upgrade other Trek mountain bikes with this new shock?

No, we are not offering the new RE:aktiv with Thru Shaft shocks as an aftermarket upgrade at this time.

Is Thru Shaft something that we could see on DH or XC bikes?

We’re currently focused on using RE:aktiv with Thru Shaft in trail bike applications, as that is where we see the greatest performance benefits. However, as with any other new technology, we’re always exploring other potential applications.

Is there a weight limit on RE:aktiv with Thru Shaft?

Since weight limitations are based on the air spring, and not the damper, the weight limits are the same as with the standard offerings from either manufacturer.

Is the setup process the same as traditional air shocks?

Yes. Riders will still set air pressure based on desired sag, and adjust the rebound damping accordingly.

Can riders adjust the air spring volume?

Yes. As with other modern air shocks, riders can add volume spacers to both Fox and RockShox Thru Shaft shocks for a more progressive spring rate.

Is the new Thru Shaft shock user serviceable?

Yes. Recommended maintenance for Thru Shaft shocks is similar to traditional IFP shocks.