RockShock’s designers were uncertain as to how long the inverted RS1 fork was in development. Some say three years, others say five. Product manager Jed Douglas said that they have made a number of attempts and each time, the results were divided between two camps: good, but heavy; or lightweight and flexy. Turns out that RockShox’s original goal of maintaining torsional stiffness between the inverted sliders while keeping the weight of the fork competitive with its conventional brethren – AND doing so using standardized components – was far more elusive than the engineering team imagined it would be.
| Five years in the making, the RockShox RS-1 became a reality when its creators abandoned convention and adopted a whatever-it-takes design criteria to solve the problems that haunted previous attempts to produce a successful inverted XC fork.|
The adage: “lightweight, strong and economical – pick two;” could have been rewritten by RockShox as: “lightweight, torsionally stiff and standardized.” The eventual success of the RS-1 project was made possible by advances in carbon manufacturing, and by the decision to engage the SRAM family to develop the inverted fork and the front wheel as an integrated system dubbed, "Predictive Steering." The key player in this role is a special hub that employs a monstrous, 27-millimeter tubular axle which is clamped into place by a standard, 15-millimeter Maxle Ultimate through-axle. RockShox calls this a “Torque Tube hub” and the end result is an inverted cross-country racing fork that is more rigid than its conventional counterparts, and it weighs only 1666 grams (3.6 pounds). By comparison, that is only 55 grams heavier than a SID World Cup fork. Just in case you wanted to know, the RS-1 only fits 29-inch wheels, it can be configured for 80, 100 or 120-millimeters of travel, and it will cost $1865 USD. (Silk handkerchiefs are included with every RS-1 to dry your tears.)
• Purpose: XC racing/XC-trail
• Chassis: Inverted type, one-piece carbon upper, 32mm aluminum stanchions
• Steerer: Integrated tapered carbon
• Spring: Solo Air, internally adjustable spring rate
• Travel: 80 / 100 / 120mm
• Damper: Accelerator, cartridge type
• Lockout: XLoc hydraulic remote (Sprint or Full Sprint)
• External adjustments: Rebound and spring pressure
• Offset options: 46 and 51mm
• Wheel options: 29” only, with proprietary Predictive Steering hub.
• Wheel choices: SRAM Rise XX, Rise 60, Roam 50, DT Swiss - TBD (Predictive Steering hub available separately)
• Axle: Predictive Steering, 27mm Torque Tube type only
• Claimed weight: 1,666g (3.67lbs)
• MSRP: $1,865 USD, € 1,658
• Contact: RockShox
Meet the new RS-1Predictive Steering:
One look says it all: RockShox did not follow the rules when it created the RS-1. Its tapered steerer tube and crown are a single piece of carbon which is then bonded to carbon legs that house just enough aluminum with which to thread its new remote-lockout Accelerator cartridge damper and Solo air-spring in place. Below its oversized carbon chassis, Fast-Black coated, 32-millimeter stanchion tubes slide effortlessly on bushings and seals which are constantly lubricated by the oil bath above them. Slide the 15-millimeter Maxle Ultimate through-axle from the RS-1’s large, forged-aluminum dropouts and you will see the most important link in the fork’s chain of success: its massive, 27-millimeter “Torque Tube” axle, and the matching slots on the dropouts’ inside faces. The Torque Tube spans the entire distance through the hub and between the dropout faces with no pressed on caps, so when the Maxle Ultimate is tightened, its serrated circumference can grip the dropouts and become a, “structural member of the fork,” says RockShox.
The fork’s intended role is to win World Cup XC races and impress XC trail riders, but it is built to exceed the strength and stiffness of competitive conventional types. We probably will never see it, but the RS-1’s post-mount caliper bosses are certified for rotors up to 200 millimeters, which underscores the robustness of its chassis. As a side effect of its designers’ quest for rigidity, the real estate opportunities created by the RS-1’s massive carbon uppers are a graphic designer’s dream, but RockShox showed their sensitive side by offering a glossy, red-white-and black billboard version, as well as a subdued matte black colorway for the less inclined. Regardless of which graphic is on the fork, however, RS-1 owners will not be able to hide from curious onlookers – so read up and be prepared for a barrage of trailhead questions.
Together, SRAM and RockShox coined “Predictive Steering” as a blanket term for all of the components that were used to keep its 32-millimeter inverted stanchion tubes locked in place and torsionally immovable. Predictive steering’s key players are the RS-1’s novel dropouts, the Torque-Tube hub and a 15-millimeter Maxle Ultimate through-axle. The hub rolls on a pair of narrow, large-diameter sealed ball bearings on the Torque Tube - similar to those you would find inside a quality, 1-1/8-inch headset. The bearings and the hub are retained on the 110-millimeter-long, 27-millimeter wide tubular axle by collars that slide over each end and are retained by small set-screws. Like a conventional 15 QR hub, the big torque tube axle locates into U-shaped slots in the dropouts – very big slots. The Maxle Ultimate through axle assembly in this case, functions only as a very large and powerful quick release skewer to clamp the hell out of the interface between the dropouts and the hub’s 27-millimeter axle, and it also ensures that the wheel will not fall out if the user forgets to tighten the Maxle.
No hub, no RS-1:
| The Torque Tube axle (left) keeps the RS-1's 32-millimeter stanchion tubes sliding parallel inside the carbon uppers. The 27-millimeter diameter serrated axle requires a dedicated SRAM 'Predictive Steering hub' (center), which is securely clamped to the RS-1's oversized dropouts by a Maxle Ultimate, 15-millimeter through-axle (right).|
Neither RockShox nor parent company SRAM have plans to license Predictive Steering’s Torque Tube hub design and the fork will not function without it. At present, only SRAM and DT Swiss are making wheels to fit the RS-1 fork, although SRAM will offer the hub alone in a non-branded configuration to be used by racers riding RS-1 forks, who may have conflicting rim or wheel contracts. At present, SRAM offers the Predictive Steering hub in three 29-inch wheelsets
: Rise XX, Rise 60, and Roam 50. 29ers only:
The lion’s share of elite-level XC racers and XC trailbikes are 29ers, so it makes sense that RockShox targets the RS-1 exclusively for big-wheel bikes. One benefit of the inverted RockShox design for a 29er is its increased torsional stiffness at the same weight. More obvious is at the fork crown, where the lack of a reinforcing arch gives RS-1 racers a huge advantage in sticky mud. Almost invisible, yet significant, is the extra-wide spoke-flange spacing made possible by the 110-millimeter-width Predictive Steering hub design. Wider spoke angles add necessary lateral strength to the larger-diameter 29er wheel. Two offset options:
Dropout offsets are available in both 46 and 51-millimeter options, so bike designers can optimize the steering action for different head angles and travel requirements. The 46-millimeter offset indicates that the way is already paved for a 27.5-specific RS-1 and there were hushed conversations at the launch alluding that a mid-size-wheel version is slated. Don’t hold your breath. The RS-1 is at present, a very complicated fork to produce in large numbers – to the point where RockShox officials plan a soft release of the fork, beginning this June as their factories come to terms with its intricacies.
| What's inside the carbon uppers: The Accelerator damping cartridge (left) has a spring-loaded floating piston to isolate its fluid from air and to prevent cavitation. A look up the RS-1's carbon uppers reveals the lower bushings and seal-head. The fork's air piston, with its top-out and bottom-out bumpers is on the right. Both items are threaded into the carbon uppers using a long, tubular socket wrench - the only special tool the fork requires for service.|
RS-1 forks use RockShox’s hydraulically actuated XLoc handlebar-remote lockout system. The button opens the damper when it is depressed and, with a second push, the button extends to lock the fork out. At first, the action may seem counter-intuitive, but it works well. The remote hose enters the sloping carbon crown through small port, which looks much nicer than the plumbing that sprouts from most XC racing fork crowns. When not employed, a rubber cap snaps over the port for a clean look. RockShox offers the Xloc in the fork-only “Sprint” version, or with the wider “Full Sprint” double-hose remote that switches off the shock and fork simultaneously.Adjustable spring rate:
Everything is inverted in the RS-1, including its Solo Air spring that is pressurized from below. Like the Pike, RockShox designed the RS-1’s air spring to use its snap-in “Bottomless Tokens” to adjust the spring rate. Tokens reduce the volume of the air chamber and thus increase the spring rate near maximum compression. They can be easily added or removed in about ten minutes by releasing the air pressure, unscrewing the air-cap and then threading the Tokens into the underside of the air-cap, or into each other. Up to three may be used, and as their name infers, Bottomless Tokens are used primarily to ramp up the end-stroke spring rate to prevent harsh bottom-outs.
Accelerator damper cartridge:
| The RS-1's air spring can be tuned to be more or less progressive using Bottomless Token air-volume spacers. The air-cap (center) is located beneath the right dropout. The chart on the right shows the changes in ending-stroke spring rate created by the addition of the plastic spacers.|
Like the new Pike, RockShox prevents air from entering the damping fluid in its Accelerator cartridge, but instead of using an in-line bladder system, the RS-1 damper has a spring-loaded internal floating piston to compensate for the volume of the long damper shaft as the damper is compressed. To prevent brake dive, the compression side of the damping piston employs the two-stage DIG valve, while the rebound circuit features a two-stage Rapid Recovery valve system. Rapid Recovery is a high-speed rebound circuit that returns the wheel quickly after full impact events, while managing slower shaft speeds and mid-travel events with more damping control. The aluminum accelerator cartridge is designed to be easily removed and serviced, but it is the only item in the fork which requires a special tool – a long tubular “socket wrench” that slips around the damper tube and engages a machined interface near the top of the unit where damper threads into a metal insert which is molded into the fork crown.External Adjustments:
To make room for the Accelerator’s remote lockout function, external adjustments are limited to low-speed rebound and spring pressure – a situation that wouldn't fly with long-travel trail riders who prefer a low-speed compression dial as well - but one that has been widely accepted among World Cup XC competitors.
|RockShox earns high marks for steering precision with the RS-1. Few if any truly lightweight forks match its tracking ability over rough ground and no 32-millimeter-stanchion 29er fork can.|
RS-1 forks will be offered with 80, 100 or 120 millimeters of travel, but don’t expect to see the 80-millimeter fork in North American bike shops, it’s pretty much a Euro-specific racer-boy product. We had the opportunity to ride the RS-1 in both the 100 and 120-millimeter configurations, concentrating on faster, less technical terrain with the Trek-mounted shorter-stroke fork, and pounding the Specialized-mounted 120-millimeter RS-1 on Moab’s UPS and Porcupine Rim trails. Without a doubt, the RS-1 is a whole new animal. Smoother action:
The action of the RS-1's inverted stanchions is much smoother, with so little starting friction that the fork seems under-sprung and under-damped at first. Most of the RS-1's enviable responsiveness is due to the fact that its seals and bushings are constantly bathed in lubricating oil, but there is no escaping that the bending forces imparted on short stanchion tubes, placed near the dropout ends of the fork are considerably lower than the forces that a conventional fork is subjected to with its bushings located much closer to the crown. The RS-1’s distinctly smoother action makes one wonder how much “compression damping” is actually the function of the stanchion tubes momentarily binding on the bushings inside of a conventional fork during a major impact event.
For as lightweight as the RS-1 is, it holds
a line around corners with uncanny precision.
The same can be said for the fork's straight-
line performance in the rocks. - Adrian Marcoux photo
Suspension setup: As mentioned, the low friction of the RS-1’s inverted configuration requires more attention to air pressure settings. The stock air spring is configured without Bottomless Tokens, because pro XC racers run super-high spring pressure settings. At zero to ten-percent sag, the 100-millimeter RS-1 feels rough over the small stuff, but strangely compliant over the kind of roots and rock gardens that pepper XC courses. With a typical trail setting of 20 to 30-percent negative travel, however, the 100-millimeter fork rides like an 80-millimeter conventional-type slider, with a more sensitive feel over the little stuff and with almost nothing in reserve for significant rock hits.
Tuning the longer-stroke, 120-millimeter RS-1, however, provided more options than the fractional increase in travel would suggest. Set up for XC-type trail work, its small-bump sensitivity is second to none. For the all-mountain style terrain we subjected the RS-1 to in Moab, we had to give up a measure of that small-bump compliance in order to attain the larger-impact performance necessary to maintain pace on the square-edged hits and drops that punctuate the landscape. Lowering the spring pressure to achieve a more comfortable ride would encourage the RS-1 to blow through its travel and cause the bike to nose into bomb holes. When pushing the RS-1 beyond its comfort zone, RockShox's Rapid Recovery rebound circuit played a commanding role, allowing us to use slightly more low-speed damping to keep the suspension smooth at trail speeds, while maintaining ride-height over long, fast stretches of deeply notched sandstone. Once dialed in, the longer-stroke fork was a hard charger.
The takeaway from the RS-1 tuning experience was that in stock trim, both the 100 and 120-millimeter forks were best suited for pro XC racer types - riders who prefer minimal negative travel and higher spring pressures, and who are willing to put up with a bit of a beating over chatter in exchange for big-hit protection. That said, we believe that the addition of one or two Bottomless Tokens would give the 120-millimeter RS-1 the progressive spring rate it needs to soften its initial travel and become a killer XC-trailbike fork. RockShox’s ride-along suspension engineers agreed, and were using similar setups on their personal RS-1 rides at Moab
| Climbing smoother pitches out of the saddle were the rare moments on Moab's trail network where the RS-1's XLoc remote lockout proved to be a useful tool. |
When a suspension fork moves as easily as the RS-1 does, too firm of a shock tune can drive the fork low in its travel. Once we got the fork set where we liked it, we found that adjusting the shock’s spring pressure to achieve a balanced ride height fore and aft was the last and very important step in the tuning process. On the subject of Balance, the RS-1 is a very honest fork, meaning that its suspension action is stable and predictable, with no damping fade or spring-rate to increase after a tough descent. Once you find the sweet spot between the shock and fork settings, you can forget about suspension and concentrate on your lines.Pedaling/climbing:
When sawing at the handlebar during an out-of-the-saddle effort, the RS-1 feels rock solid in the sense that is doesn't feel like anything is moving underneath you in the lateral direction. Locked out, the RS-1 sprints like a road bike, which is how it should be. Don’t bother locking out the fork, however, unless you either have a dire need to stand on the pedals and attack, or there is a stretch of smooth ground or paved road that you need to get rid of in a hurry. The lockout option feels like RockShox teleports an oak dowel into the Accelerator cartridge when you release the XLoc button – and riding a rigid fork as stiff as the RS-1 on rough ground is not an experience that you will want to repeat. Steering stiffness:
RockShox earns high marks for steering precision with the RS-1. Few if any truly lightweight forks match its tracking ability over rough ground and no 32-millimeter-stanchion 29er fork can. When descending steps and drops that subjected it to maximum travel events, the bike's front end managed to track well and we could make precise steering corrections on demand.Issues:
Mountain bike customers are traditionally wary of big-ticket purchases that require expensive one-off accessories. Mention the word, "proprietary" and it conjures up expensive carbon frames with shocks that won’t fit anything else, cranks without replacement sprockets, steerers that only fit one headset option, and the list goes on. The bottom line, at least for a truly lightweight inverted fork, is that the interface between the RS-1 fork, hub and axle had to be modified in order to make it a competitive product. We expect other suspension makers will present non-standard solutions as they move forward with similar inverted fork projects as well. Whether or not you subscribe to a fork purchase that requires a proprietary front hub and axle, the fact remains that without the courage to break from existing standards, the folks at RockShox would have been forced to leave the prototype RS-1 to gather dust in the bin labeled: "almost products." Final Thoughts
| The flash value alone, of an inverted carbon fiber XC racing fork from the boys and girls at RockShox, will generate sufficient force to drive it into the marketplace. The visibility that the RS-1 will bring to the brand when it debuts at World Cup XC events is yet another story bursting to be told. Beyond the hype, though, the the RS-1 also happens to be a damn good fork - one that seems to have handily overcome the historical negatives that have been recited ad-nauseum in media reviews of its predecessors, Those should be its number one selling points. For cross-country racers, the RS-1 delivers the exact blend of firm pedaling, big-hit insurance and instant remote lockout action that they need to win, along with the additional bonus of a substantial measure of security and stiffness in the steering department. For trail riders, the 120-millimeter-stroke model is the only practical option, as the RS-1's torsionally rigid carbon chassis and effortless suspension action begs to be ridden harder than the pencil-neck 100 or 80-millimeter options can possibly handle. In stock form, the 120-millimeter RS-1 is undergunned in the plush department to compete with some elite-level trailbike forks in the same travel category, but all the pieces are in place. The RS-1's Accelerator damper has seeds for speed, and with the addition of a couple of volume spacers to correct the spring rate, we are confident that the suspension action of RockShox's new inverted fork will match its standout steering precision and chassis stability. - RC|