Trek’s new trail bikes, like the Powerfly FS we test rode in June, have had a little something else going on in the back end. The Fox and RockShox rear suspension units are using a new system, RE:aktiv Thru Shaft.

With the aim of providing as much control as possible, by eliminating the stiction at the very top of the stroke, Thru Shaft has been developed in partnership with Penske, and of course Trek’s own suspension guru Jose Gonzalez.

Don't miss part 1 and part 2 of our Trek suspension visit.

What’s essentially happening, is the suspension shaft runs all the way through the body, and the lower part of the shock runs along it inside the body. Compared to a normal shock where the piston pushes into the body, and has a much larger volume to displace. This dramatically reduces the amount of displacement, therefore almost getting rid of lag entirely.

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.

Better tracking for more control? We're in.

On the bike, the end result is better tracking of terrain – so that means more traction for cornering and braking, more control – and more speed. The shock responds faster as it maintains a constant volume of oil, which means it is especially great for fast and hard riding. While at Trek’s trail bike launch, Gonzalez fitted a new RockShox RE:aktiv Thru Shaft shock to Rene Wildhaber’s Trek Slash, and he raved about the changes, especially when pushed at speed.

Wildhaber got on with the new shock just fine.

Does thru shaft sound familiar? It’s the leading type of suspension in motorsport, and even what Manitou and RockShox used in early suspension products. But the seals at the time couldn’t take the pressure – hence all those blown cartridges.

What does Thru Shaft look like?

Trek are using the lighter weight Fox Float units on bikes like the Fuel EX, whereas the RockShox units sit on the Remedy and Slash. At first it looks like they have scaled the shocks down, as the RE:aktiv Thru Shaft units don’t have the big piggy back reservoir. They just don’t have the same oil displacement so it’s not required, but you will notice the small little external section, which is a thermal compensator. With less oil volume the smaller Fox shocks have the thermal compensator inside the main shaft.

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. For 2018, the RE:aktiv Thru Shaft shocks will only be on the Slash 9.8 and Slash 9.7; Remedy 9.8 and Remedy 9.8 Women’s; and Fuel EX 9.9 and carbon frame sets. There are no aftermarket options.

But can you notice the difference with RE:aktiv Thru-Shaft?

I didn't get the opportunity to ride the one bike back-to-back with a standard RE:aktive shock and then witn the Thru-Shaft version. But the Trek Fuel EX 9.9 I rode was by far the most capable 130mm trail bike I have ridden, with excellent suspension performance on long bike park runs, and extended proper alpine length descents. It also negotiated off-camber root sections with aplomb, something the higher than usual tyre pressures didn't help with. But the bike stayed planted. It was a great riding bike.