Trek released their original RE:aktiv suspension units early summer 2014. Working in conjunction with Penske Racing shocks, a high-end suspension manufacturer focused on the racing worlds of Indy and NASCAR, they initially brought the technology to the market using Fox as their supplier of this exclusive technology. For 2018/19 Trek has exclusive use of a new technology compressed down from motor racing and into mountain bike suspension. RE:aktiv adds the 'Thru Shaft' label, which is a replacement for longstanding internal floating pistons found on nearly all shocks.
Trek admits that this technology isn't brand new; AMP, White Brothers, Manitou, and RockShox all experimented with this technology back in the 1990's, but manufacturing techniques at the time meant that a reliable product was tough to create. Fast forwards twenty years and things have moved on, and Trek now believes they have a durable product with improved performance. Why does the performance improve? An IFP creates some friction and lag in all shocks; even as seals and coating have improved, there is always a static force to overcome. The Thru Shaft works around this issue by allowing oil to be displaced freely and eliminates the IFP and nitrogen/air charges.
More proprietary technology? Fox and RockShox will be supplying Trek exclusively with shocks for the next two years, but the technology is open source, so we may see this appear on other brands in the future. Penske still retains the patent on the regressive RE:aktiv valve, so this will continue to be unique to Trek for the foreseeable future.
Trek Explains Thru Shaft:
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.
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. 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.
The RockShox Thru Shaft models use an external reservoir for heat induced fluid expansion, the Fox shocks package this inside the shock.
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.
At full extension, the Thru Shaft is out of sight inside the shock.
At the shock cycles through its travel, the Thru Shaft can be seen between the lower mounts.
For 2018, Thru Shaft is limited to higher end Trek models including the Slash 9.8 / 9.7, Remedy 9.8 and Fuel EX 9.9. It will also be supplied on Slash, Remedy, and Fuel EX carbon framesets. Expect to see the tech trickle down to cheaper models in the future, but buying aftermarket shocks to upgrade your old bike will not be an option yet.