RockShox has always been at the cutting edge of mountain bike suspension, driven by two main formats- XC and DH racing. With 29er's coming to dominate the XC race category, wheel, hub, and fork stiffness have come under scrutiny. The RS-1 ACS Fork was developed to answer the stiffness deficiency, as well as improve ride quality, overall fork function, and handling. The RS-1 ACS achieves these goals by considering the fork and hub to be an integrated unit, while adding additional width between the spoke flanges. The completely new fork and hub design result in what RockShox calls Predictive Steering, the most structurally sound relationship between the fork and hub. In order to achieve this, old ideas had to be re-considered, and new standards developed.

Inverted fork designs have been used in motocross and mountain biking prior to the RS-1. There have been disadvantages to this design, first among the being that the lowers tend to flex more in torsion. In motocross, larger hub interfaces and axle designs have overcome this problem. For bikes, the space and dropout limitations had to be addressed in a similar way. The Torque Tube was developed as a 27mm outside diameter axle that rests in a matched receiver machined inside the drop outs of the fork. The hub had to grow in both diameter and width to best accommodate this, and the result is larger, more efficient bearings, as well as a 110mm overall spacing, which widens the bearing spacing, as well as the spoke flanges, increasing the spoke angle to the rim. With the Torque Tube interface integrated directly into the fork, a Maxle Ultimate thru-axle holds locks down on to create the stiffest hub/fork mating on the market. This is called Predictive Steering, and the results are an increase in wheel stiffness due to the wider flanges, an overall stronger hub design, and essentially a 1:1 relationship of the fork to the hub. The fork and hub combine to defeat torsion from terrain input, and complete braking control.

Predictive Steering also answers inherent weakness of the fork lowers so close to the wheel axle, making the inverted design feasible. Now, that alone doesn't address full stiffness and performance. RockShox employs years of Black Box carbon technology to build the tapered steerer, fork crown, and leg uppers from one carbon lay-up, with exceptionally strong and resilient uni-directional carbon fiber molded in a single process. This results in a light and strong design that exceeds the strength of aluminum. The 32mm stanchions are aluminum, and treated with proprietary FastBlack coating, which drastically reduces friction, and protects the surface from scratches. With an inverted fork design, gravity keeps oil on the seals of the fork bushings, which sit closer to the axle. This increases sensitivity and reduces stiction so that small bump compliance is improved, and compression and rebound is much smoother, keeping the wheel traveling cleanly on the terrain, for maximum traction and control. The stanchions fit into carbon lowers, which carry the dropouts and post disc mount.

Of course RockShox also had to develop a new version of the Rapid Recovery valve rebound system for the RS-1 ACS Fork. This system accurately and smoothly rebounds the fork from large and small impacts,and has been proven highly effective in forks from the BoXXer Series through the SID World Cup forks. Rapid Recovery is powered by the Dig Valve, which provides the optimal level of control for both low and high speed compression, carefully controlling the oil flow to provide the rider with the right amount of support and impact absorption. The all-new Accelerator Damper, which combines incredible small bump performance with an ultra-efficient lockout, optimizes overall performance when less, or zero travel, is needed such as when powering up a hard pack rise, or sprinting to close a gap or for the win. The lockout system is the firmest RockShox has ever offered, and is managed with ease by the X-Loc Sprint hydraulic thumb lever. The equilibrium balanced, highly regarded, Solo Air system allows the rider to easily set and tune air spring pressure that sets-up the base functionality of the suspension system.

The RS-1 ACS Fork is truly the apex fork of RockShox's line. Will it be a game changer for XC race, or even marathon? The Predictive Steering is a massive change for fork/hub design. Still, riders have to feel the end result is is worth the cost. If you must have the purest form of front wheel handling and fork integration, the RS-1 will be on your wish list. At the very least, we expect the RS-1 and Predictive Steering to be a harbinger of widespread advances, and RockShox's willingness and capability to innovate and push boundaries is crucial for development. The rate of change has been incredible over the decade, but has mostly represented upgrades and refinements. The RS-1 ACS Fork is the potential beginning of a new era. Do you lead, or follow?