Trek re-aktiv thru shaft rear mountain bike shock made by rockshox

After just explaining how an IFP works in our Suspension Tech series, Trek’s gone and done away with it altogether. Their new RE:aktiv shocks, co-developed with Penske and made for them by both Rockshox and Fox, use a through-shaft design to eliminate the need for a compressible overflow chamber.

Thru shocks are used in Formula One, short track sprint cars and other motorsports. Penske (and others) make through shocks for them, so the partnership with Trek wasn’t much of a stretch. The benefits are plentiful. For starters, there’s no internal floating piston pushing back on the system, so the shock more easily moves into its travel on small bumps. There’s also fewer moving parts and a much simpler flow path, so things can move more freely inside. Those features and more add up to a shock that should be much more responsive…

Trek re-aktiv thru shaft rear mountain bike shock made by fox
As the name suggests, the shaft runs all the way through the shock, so there’s got to be clearance on the tail end.

Why does a thru shaft eliminate the need for an IFP? In a regular shock, as the damping circuit moves through the oil, most of the oil is flowing through the circuit to the other side. But some of that space is taken up by the shaft, which takes up more volume as it’s pushed into its stroke. That extra volume as to go somewhere, so the IFP is pushed against its nitrogen charged chamber to make room. With a thru shaft design, the shaft is running all the way through the shock, so there’s no displacement from the shaft itself. There’s still some need for an expansion chamber as the fluid heats up and expands, but that’s another story (we’ve inquired as to how Trek is handling this, update as we get it).

The other big goal with removing the IFP was to reduce any lag in response. Stiction at the IFP piston means it might react more slowly than the air piston, which creates imbalances in oil pressure. And those pressure imbalances mean the shock isn’t responding as smoothly as it could (hysteresis). On the bike, their goal was to make it track the terrain better than ever and be ultra responsive to the smallest of inputs.

Both Fox and Rockshox will be making shocks for Trek with this design.

2018 Trek Fuel EX full suspension mountain bike with new re-aktiv thru shaft shock
All photos c. Trek Bicycles/S.Lorence.

You’ll find the thru-shaft RE:aktiv shock on the 2018 Trek Fuel EX 9.9…

2018 Trek Remedy full suspension mountain bike with new re-aktiv thru shaft shock

…2018 Trek Remedy 9.8 and 9.8 Women’s…

2018 Trek Slash full suspension mountain bike with new re-aktiv thru shaft shock

…and Trek Slash 9.7 and 9.8. It’ll also be available on framesets for these bikes.


  1. Lefty fork dampers have always been thru shaft. Hell, Judy dampers were thru shaft. I really hope they have some volume compensation. Some bike suspension companies have tried to bypass it, didn’t go so well.

      • Noleen made thru-shaft dampers for K2’s xc race bike. They later changed to a Fox air spring pull shock on that model.

        A downside to the thru shaft design is that the volume compensator on more sophisticated dampers often controls the flow of oil to/from the reservoir, allowing more damping circuits. That’s gone with thru-shaft.

  2. The midvalve piston, seeing greater oil flow compared to a basevalve setup should certainly help open up some tuning possibilities. Im cautiously excited here.

    • Yes. You can see in the pics the extended lower part of the shock, to accommodate where the shaft sticks out during compression. “Metric” already made shocks longer for a given stroke, though. Couple that with trunnion mounting, I think the impact will be minimal.

  3. I’d like to see a sealed boot on the shaft extension.
    Anyway…proof will be in the overall ride since this isn’t readily adaptable to other frames. I haven’t ridden one, but reviews indicate Trek is dialing their FS bikes well.

  4. what’s old is new again…

    On the one hand, thru-shafts seem like they would offer a lot of the benefits of a twin-tube recirculating damper in a simpler, cheaper and [if my old Amp was any indication], way easier to service package.

    On the other, there was a reason this tech has been tried and abandoned by the bike industry a couple times already, isn’t there?

  5. Does anywhere know which engineering schools have degrees in Armchair Engineering? I’m looking to get one. Copying and pasting Wikipedia pages on BR is getting old.

    • If only Wikipedia were informative on suspension design. It’s not. Sounds more like they’re copy/ pasting random jargon from bike reviews.

  6. Talk about a hyperbolic article title. Thru-shaft designs are older than I am. The application requirements and constraints determine what design you go with. There’s nothing revolutionary about this design principle.

  7. Trek gets an ” exclusive ” on this design for two years. After that Fox or RockShox can sell the design to anyone.

  8. So, isn’t that piggyback on the first shock pic still an IFP?
    And how does the inline shock have a simpler flow path than one with an IFP? Seems like flow path is the same.
    And sure there are fewer moving parts (the IFP is a moving part) but, there aren’t fewer seals, as the IFP seal is replaced with the thru-shaft’s seal.
    So perhaps you have less chance of aeration from nitrogen getting past the IFP, but is cavitation a bigger problem?
    And compensation for oil volume changes due to temperature seems impossible in th inline shock. Very curious to find out how that is handled.
    But the main question should be…do you really want to buy a bike with a proprietary shock? Likely custome eye to eye, stroke, and mounting style. In 5 years what will your upgrade options be if this doesn’t catch on?

    • A shock of the same length can have it’s travel shortened to match with travel spacers, so plenty of shocks will fit right in.
      Floating pistons are backed by high pressure, so higher friction there than on the through seal. A top out or bottom out bumper can easily be made to change volume for temperature expansion.
      In 5 years time the shock will still be operating just fine with regular servicing which is much quicker and easy to do for the home bike mechanic due to lack of high pressure nitrogen.

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