Trek RE:aktiv Thru Shock Suspension

Trek RE:aktiv Thru Shock Suspension

Founded in 1975, Trek has a history of invention, innovation, and collaboration with stellar creative geniuses like Gary Fisher, Gary Klein, Keith Bontrager, and Greg LeMond to name a few. They’ve built bike frames from steel, aluminum, and developed their own proprietary process for manufacturing carbon fiber frames in the USA. (And you thought Wisconsin was only good for making cheese!) In the process of manufacturing bicycles, Trek has grown, in Waterloo, WI, from a company that occupied a small red barn into the largest bicycle manufacturer in the United States that occupies over 183,000 sq. ft. In addition, they build bicycles in Germany, Holland, and China. They employ over 1,000 workers at two locations in Wisconsin and over 800 world-wide. They are a privately held company and don’t release general production statistics but various sources estimate the number of USA-produced Trek bikes at approximately 10,000 units out of more than 1.5 million around the globe annually.


Trek’s engineers drew knowledge garnered from the aerospace industry thru a mold tooling company named Radius Engineering, which ultimately led them to the creation of their proprietary process for building carbon fiber frames, in 1992, which they named OCLV for Optimum Compaction Low Void. Today, 25 years later, Trek still uses the name OCLV for the carbon fiber in its frames.

Birth of the RE:aktiv:
Recently, Trek’s engineers drew inspiration from Penske Racing Shocks where shocks are made for Team Penske’s race cars as well as competitor’s vehicles for varied racing/driving formats like Indy, Formula One, NASCAR, motorcycles, dragsters, ATV’s, snowmobiles, and military vehicles. It began in 2015 with Trek’s introduction of the Fuel EX 27.5 mountain bike that featured the product built from their collaboration with Penske Racing Shocks and Fox Racing Shox, a new shock named RE:aktiv.

RE:aktiv was different from other mountain bike shocks in that it utilized a regressive damping system, which Trek claimed would improve climbing efficiency, offer greater speed, increased traction, enhanced cornering, plush responsiveness to large bumps, while offering a firm platform on flats and corners without the lag time encountered in progressive and regressive damping systems that were incorporated by all other manufacturers of mountain bike shocks.


Shock damping systems produce several basic shapes of damping curves that are referred to as progressive, digressive, and linear. A curve is simply a plotted line that is drawn along X and Y axes at various points in order to represent some type of relationship as defined by those axes. Progressive damping systems produce profile curves that stiffen progressively as the velocity (speed of travel in a specific direction) of the suspension increases. For a rider, this means that at slower suspension velocities, the system will be softer, more plush as the rider travels over rocks or other obstacles, while surrendering some control in cornering. However, modification in oil viscosity (Use a thicker oil) in conjunction with closed, or towards the closed range of rebound or compression settings on the shock, will provide enhanced cornering control with some sacrifice on suspension plushness over rocks at lower velocities. In general, and in order to create a more progressive damping curve, the use of lighter oil viscosity, adjusting the compression and rebound settings to reduce valve port size, and crossover shims will help reach that goal.

Oil viscosity affects the compression and rebound characteristics of forks and rear suspension systems. Lighter weight oils increase reaction speeds, while heavier viscosity oils decrease reaction speeds. Rider weight can influence viscosity choice with lighter riders opting for light weight oil and heavier riders selecting higher viscosity oils.

Shims are metal washers that can vary in thickness and diameter that are stacked atop one another against the surface of the damping piston to provide damping force when oil is forced out of a piston’s damping port. The smallest diameter shims are the stiffest, while the larger diameters are less stiff. How shims are selected and stacked seriously impacts the performance of damping because they can control oil flow thru the piston. Increased oil flow provides increased damping effects and vice versa.

Valve ports are manufactured in different sizes along with shim stacks, oil viscosity and pressure in order to control the flow rate of oil into the piston. Port size can be controlled via compression and rebound adjustments on many shocks. Additionally, check valves can be introduced to achieve directional oil flow into the piston with minimal to no oil backlash through the port.


Digressive damping systems produce profile curves that reflect higher damping forces at lower suspension velocities along with decreased damping forces at increased velocities. The higher damping rates at reduced suspension velocities mean that the suspension will quickly stabilize upon exiting a rocky section of a trail and will provided enhanced steering while cornering and lower damping rates at greater suspension velocities that relieve overall stress on the suspension system, which translates into smoother flow over rocks or ruts on the trail. However, the suspension system is now stiffer over smaller rocks/ruts, which can produce a harsher ride, until large enough rocks/ruts are hit and subsequently move the suspension into a plusher, more responsive feel at increased velocities. You can adjust digressive damping by adding or removing shim stack, changing to a higher viscosity oil, use larger valve ports, and by adjusting rebound and compression settings on the shock.

Linear damping systems provide a linear increase in damping force as the suspension velocity increases, while keeping a constant relationship between the suspension’s un-sprung mass momentum and the resultant damping forces. What this means to riders is that if you hit a rock that’s three times the size of the previous one, your impact should be three times larger. A linear relationship relative to suspension will provide a consistent experience across the full stroke of the system. However, other factors are important in developing a linear damping curve/response, and include things like oil viscosity, valve geometry, stack stiffness/taper, compression/rebound indent settings, and crossover shims.

In brief summation, and relative to progressive and digressive damping systems, with progressive systems, the greater the force of impact, the greater the damping effect, while in digressive systems, damping rapidly responds to increased rock/rut sizes, and then levels off.

Trek’s original RE:aktiv suspension system utilized regressive damping, which was patented by Penske Racing Shocks (PRS). In their patent, PRS described regressive damping by stating the following, “One embodiment of the present invention pertains to a damper having regressive characteristics. During extension of the damper at low velocity, the force required to extend the damper progressively increases as the extensive velocity of the damper increases. During operation at moderate extensive velocities, the force required to extend the damper regressively decreases as the velocity increases. At still higher extensive velocities, the damping force progressively increases with increased extensive velocity.” This translates to better handling characteristics regardless of speed of impact therein eliminating the problems inherent in either solely progressive or digressive damping systems such as loss in low speed cornering with progressive systems, or the stiffness experienced with digressive damping. In theory, Trek’s RE:aktiv system will provide better traction and handling along any type of terrain, while also providing superior damping, irrespective of trail profiles.

The refinement of the RE:aktiv for 2017:
In 2017, Trek announced that they were introducing an upgraded version of their original RE:aktiv shock, by introducing Thru Shaft technology, which describes a shaft that is attached to both sides of the damper valve so that when the damper valve is pushed thru the oil-filled damper body, the shaft becomes displaced on the other side of the valve. This eliminates oil displacement by keeping the oil volume constant within the damper body. In a traditional air suspension system, a nitrogen gas-charged reservoir lies behind the internal floating piston, which in turn separates the gas-filled chamber from the oil filled chamber. When pressure exerted via the shock’s shaft compresses into the shock’s body, movement of the floating piston from displacement of volume occurs. As the nitrogen is compressed, pressure increases, which prevents the floating piston from exhibiting excessive movement. When the shaft’s pressure is removed, the gas moves the floating piston back into its original position, ready for the next input (bump or rut). This movement in conjunction with the spring returns the shock outwards to its original, pre-activation position.


Another important feature of the RE:aktiv Thru Shaft Shock is a simplified construction versus its competitors’ because it not only eliminates oil volume displacement, but has discarded the need for the internal floating piston and the gas-charged reservoir all together. This streamlines the system, which results in decreased hysteresis, which is an ancient Greek word that means “lagging behind.” In bicycle suspension systems, hysteresis refers to the lagging of the effect produced from some input, the cause, that relates to pressure, flow, torque, and displacement in that system. By minimizing hysteresis, product designers can create shocks with predictable responses when subjected to small bumps and big jolts while riding along rocky terrain, in bike parks, or down mountains. By eliminating the gas reservoir and floating piston, stiction produced by the floating piston as it travels along the damper body is eliminated. Stiction, a derivative of static friction, relates to the tendency of an object to stick and slip as a result of high static friction. In suspension system’s valves, this indicates a valve’s inclination to “stick” in the absence of applied forces, and to “slip” when forces are applied. Many things cause stiction including corrosion, oil viscosity, deposition of material, contamination, chemical reactions, and degradation of valve seals. In reducing stiction in the damper body along the shaft’s wall, the shock is quicker to respond to small impacts due to decreased resistance to movement (stiction) and to change direction more readily. In addition, friction produces heat that can impede the smooth functioning of a suspension system. In Trek’s Thru Shock, dissipation of heat occurs when the shock is at rest and the heat can readily dissipate in the surrounding oil, which fills the damper’s chamber. Special material coatings along the shaft wall and on moving components also reduces thermal buildup by decreasing friction.

This isn’t new technology, having been used by other shock manufacturers in the 1990’s but Trek argues that earlier systems weren’t reliable, yet with modern improvements in manufacturing, it’s possible to now produce reliable suspension systems using this technology. Given that from an engineering perspective, the shock is more streamlined, and if properly designed and executed, this shock should provide excellent reliability with a minimum of setup or continuous adjustments. Theoretically, this shock should provide superior responsiveness regardless of terrain. In essence, a suspension shock is designed to allow for tires to optimally retain traction on any terrain, and to provide feedback to riders so they can exercise efficient control over their mountain bikes. Trek’s improved RE:aktiv Thru Shock fills these requirements, which is attested to by its Penske-Fox-Trek pedigree. No mutt intended!

Rumor indicates that Fox and Rockshox have a 2-year exclusive marketing agreement with Trek for Trek’s sole usage. However, after that timeframe has elapsed, more manufacturers may jump on the bandwagon and produce regressive-type suspension systems. Ultimately, mass rider experience, input, and feedback will determine the success or failure of this newly reinvented system. After all the physics, beautiful charts, and marketing hype have faded, rider experience will determine the final verdict, which will be based solely on handling performance, reliability, adjustment ease, and durability. I wouldn’t bet against Penske, Trek, or Fox, if I was a gambling man.


August 01, 2017

Rear Shock › Suspension › Trek ›

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