Fox Racing Shox has all but ditched coil springs in favor of a new air-spring system that it developed over a two-year period, first in the original 40 chassis and later, inside the new, lighter-weight RAD chassis that made headlines when it debuted at the 2012 Leogang World Cup. Is it good? Fox reported that in its early development, some of its female racers switched back to coils until Fox could tailor its negative spring system to lighter weight riders, but since then, none of its riders have gone back – and the heresy continued as Fox announced that only its ‘entry-level’ OEM 40 will retain the coil spring, and that with the addition of new lowers, the Air 40 will be ready for this season’s onslaught of 650B DH bikes. The Air 40 is presently in production, it weighs 5.98 pounds and with the steerer tube cut for a direct-mount stem, it is 521-grams (1.15 pounds) lighter than its predecessor. Its official name is ‘Fox Factory 40 Float RC2’ and it’s not going to be cheap. Fox pegs the MSRP at $1700 USD. They will be in stores this summer.
Greg Minnaar testing the RAD porototype 40 Float RC2 at
Leogang, 2012. Tomas Dietze photo
Fox 40 Float RC2 Highlights:
• Travel: 203-millimeters (8 inches)
• Adjustable air spring system with titanium coil-type negative spring
• Air-volume adjustment feature to control end-stroke spring rate
• RC2 damping system
• Lighter weight: 482 grams less than 2012 40 (521g w/direct-mount stem)
• All new chassis: Taper-butted stanchions, new lower casting, new crown design
• Air-bleed buttons on sliders to equalize internal fork pressure
• Kashima coated stanchions and RC2 damper shaft
• Low-Friction SKF seals throughout
• Post-type caliper mounts
• Wheel options: 26 or 27.5-inch wheels (different sliders)
• Stated weight: 5.8 pounds
• MSRP: $1700 USD
• Available: Summer 2013
After three days of testing, we wouldn't want to return to coil springs either. Fox invited Pinkbike to a pre-season suspension testing session at La Fenasosa Manor near Alacante, Spain, where we were briefed about the inner workings of the fork, before sessioning the private bike park’s chunky downhills for a back-to-back comparison of the previous coil-sprung 40 and the new air-sprung model. Guests were treated like sponsored pros, with Fox tuners waiting at the race truck to chart suspension performance and to make necessary changes between runs. With a half day in the bank on 2012 suspension, Fox switched out the bikes to the 2014 fork and shock and we continued. The comparisons were not subtle. The good news? Fox says that its air system will retrofit to existing 40 forks.
Why Bother With an Air Spring?
Fox states that better adjustability and lighter weight were the motivating factors to develop an air-sprung 40 and like all good things from Fox, it didn't happen overnight. Two air-assist systems were developed previous to the air-sprung 40. Most know about the hybrid air/coil system that Fox co-developed with Trek, but few know about the exclusively Fox PABLO system – an acronym for ‘Pneumatic Assist Bottom Load Optimizer.’ Basically, both versions use compressed air to give a coil-sprung fork a progressive spring rate near full compression, and both aptly demonstrated the value of a micro-adjustable spring rate for downhill.
| Aaron Gwin at Plattekill during 2012 testing. Gwin, along with the Athertons, were instrumental throughout the development phase of Fox's Air-40 program and reportedly, Aaron has been racing them since day one. Matt Delorme photo|
PABLO was raced by Fox’s Elite RAD (Racing Applications Development) riders throughout 2011, when Aaron Gwin reportedly won five World Cup races and the overall title on the PABLO fork. The success of the air-assist led Fox to move directly to the development of an air-sprung system. By 2012, all RAD racers were on air-sprung 40s, including the current World Champion who admitted that when the Santa Cruz Syndicate switched to Fox they began with the air-sprung fork – Greg Minnaar mentioned that he has not tested with the coil version. The air spring reduced a good measure of the 40’s weight, which encouraged the design team at Fox to redesign the ten-year-old chassis to carve even more weight from the fork. More about that later.
Pressurizing the fork's air spring pushes on the white seal head (upper left)
and completely compresses the blue titanium negative spring.
Secrets Inside the New 40
Two key elements that Fox engineers developed for the new 40’s air spring make its action equal to and in most cases, superior to a coil-sprung fork. The first is an advanced negative spring system that employs a long, titanium coil-spring. Air-type negative springs release abruptly in the first 15-percent of the fork’s stroke, but the linear rate of the titanium spring creates a seamless, soft-release of the air spring at 50-percent of the fork stroke. Negative springs will be sold in four levels of stiffness, but the medium, green-colored spring that will ship with the Air 40 is reported to be perfect for almost everyone. Most of Fox’s elite racers are on the green spring.
|Want the feel of the original 40? Simply set the spacers to raise the volume-adjust piston as high as it goes and it will emulate the linear compression rate of a coil spring.|
To cause the fork spring to be more progressive, unscrew the retaining bolt
at the end of the shaft (lower right)
and lower the piston by rearranging some
of the eight spacers on the shaft.
The second half of the Fox 40 air spring system is a simple volume-adjustment piston assembly that sits beneath the left-side top cap. By switching spacers to change the location of the piston on the shaft, nine volume options are possible. Want the feel of the original 40? Simply set the spacers to raise the volume-adjust piston as high as it goes and it will emulate the linear compression rate of a coil spring. A more progressive spring rate is accomplished by lowering the volume-adjust piston. The progressive option allows the 40 to be sensitive over the chatter, which greatly improves traction, yet still firm enough at full compression to eliminate bottoming during maximum events. Switching out the negative spring requires dropping the lowers from the fork, but adjusting the air volume stack can be done in ten minutes by removing the left top cap.
Seal friction is always a detriment to air-spring systems, and Fox has paid particular attention to eliminate it. The 40 chassis has room for a large-volume air chamber, which reduces the pressure necessary to suspend the rider and puts less squeeze force on the seals. Only 70 psi is required to suspend a 200-pound rider and the pressure range between the heaviest and lightest riders is 45 to 80 psi. Fox uses the slippery Kashima coating on the stanchion tubes as well as the shaft of the RC2 damper cartridge, and
Suspension Product Manager Bill Brown said that all of the seals are specifically engineered for the 40 by SKF, in collaboration with Fox to minimize friction. Damping Improvements
Damping forces are still controlled by the RC2 damping cartridge. Because the air spring system can better resist bottoming, the hydraulic bottom-out function has been eliminated from the new damper which, beyond its new seals and slippery Kashima-coated shaft, remains essentially unchanged from the 2012 40. All of the damping adjustment dials are their familiar places. Compression damping has been adjusted to be lighter because the new spring system takes away much of the damper’s workload in the second half of the fork’s stroke.Chassis Redesign
Fox had ten years to find ways to improve its flagship DH fork, but beyond the slenderized magnesium lowers and a couple of air-relief buttons, the Air 40 appears at first glance to be nearly the same as its predecessor. However subtle, every component of the new chassis is essentially a new part. Fox says it took something from almost every component of the fork to achieve its remarkable 512-gram weight reduction. Many changes in the chassis, however, were instigated to create some flex in the 40, which has been criticized for its rigid ride over chatter sections.
| The entire 40 chassis has been redesigned, but remarkably, the internals of the air-sprung 2014 40 are retrofittable to the original. Thomas Dietze photo|Crowns:
|However subtle the changes may be, every component of the new chassis is essentially a new part.|
In its quest to reduce weight, the cavities under the upper and lower crowns have been redesigned to eliminate X-webbing and the stanchion clamps have been reconfigured with thinner sections near the tubes to prevent stress risers from forming. The clamp-bolts have been moved to the front and the clamp-angles have been reconfigured to improve grip while using less material. Fox anticipated that some racers will use a splash guard, so it added threaded bosses on the underside of the lower crown.
| The transition areas where the crowns meet the stanchion tubes have been made thinner to relieve stress at the clamps. Underneath, the webbed structure has been removed in favor of a smooth transition to the outer walls of the crowns.|
Taper-butted stanchion tubes: Air-pistons sliding up and down inside the left stanchion tube required an ID as smooth as the tube’s OD. The Kashima-coated tubes are tapered internally to be thicker near the lower fork crown. The tubes are tapered thinner both above and below the lower crown, which encourages the stanchions to flex a little. This gives the new 40 a welcome degree of comfort over the original. Both the right and left stanchions are the same, and the internals of the original will retrofit into the new chassis.
| Fox carved some magnesium from the back side of the arch and then used most of it to reinforce the front of the 40 Float sliders with a wrap-around ridge. Pressure relief buttons equalize unwanted pressure under the fork seals.|
Arch-Brace: Fox took some material from inside the brace arch by hollowing it where it intersects the seal heads on the sliders and then added some metal in the front of the arch across the face where it would do more good. Like the RAD prototype sliders, the area around the seal heads has been visibly pared down. Threaded bosses are provided for arch-mount splash guards.
Sliders: Nearly invisible, but important to the 40’s performance is a taper cast into the sliders. The change in wall thickness reinforces the lower bushing area without making the magnesium sliders overly rigid. The axle clamps are completely new, formed with less material and the sometimes problematic stainless steel thread inserts have been replaced with a rectangular chip that can be easily removed should a pinch bolt snap off in the threads. The caliper mounts are now post-type that are direct-mount for eight-inch (203 mm) rotors.
| Subtle, but important, the new 40's lowers are tapered to provide a small amount of compliance. The dropouts are lightened significantly and a new rectangular threaded insert for the pinch bolts has been added for better serviceability.|
Pressure-relief valves: Downhillers who regularly bled the fork’s internal pressure by slipping a zip-tie between the fork seal and the stanchion tubes will no longer have to risk damaging the seals of the 40. Pressure relief buttons on the back of each slider are provided to equalize the fork’s internal pressure, which can be three PSI higher at the top of some uplifts due to the lower ambient pressure at altitude. Fox showed a cross section of the fork seal to illustrate how a minor increase in air pressure inside the fork can impart significant squeeze force on the stanchion tubes. Internal pressure buildup can also be caused by off-gassing of the suspension lubricants, or by an increase in temperature, so Fox recommends pressing the buttons before altering the spring or damping settings, and before the start of each DH run.
Fox Air 40 and 650B Wheels
Early season testing by a number of teams has put the stamp of approval on 27.5-inch wheels for DH. Fox insiders admitted that the low-ball time savings over a 2.5 minute course was 1.5 seconds, with many riders running significantly faster. Fox will offer a 650B-specific lower for the new 40 that has been reconfigured to fit the larger wheels and with a seven-millimeter longer offset to correct for proper trail geometry. The 26-inch fork uses a 45-millimeter offset, while the 27.5-inch fork has a 52-millimeter offset. Both share the same crowns and stanchion tubes.
2014 DHX RC4 Shock
Along with the new 40, Fox revealed its redesigned DHX RC4 shock. Changes in the current suspension configurations from nearly straight rates to a variety of rising rate linkages has all but eliminated the need for building a progressive damping curve into the shock. This was the primary motivation for Fox to revamp the DHX.
| Internal improvements to the RC4 shock, like new damping circuits in the IFP chamber and the replacement of the boost valve assembly with an adjustable air-assist system to control end-stroke spring rate are the big news for 2014.|
Fox DHX RC4 Shock Highlights:
• Damping sources balanced between the main shock piston and the reservoir circuits
• Kashima coated shaft
• High-speed and low-speed compression and rebound damping adjustment
• No more boost valve
• assist and air assist volume adjusters to tune the spring rate and progression
• Shaft reduced from 5/8 to 1/2-inch to reduce friction and improve small-bump compliance
• Increased oil volume
• Redesigned valve system
• MSRP: $600 USD
• Available: May, 2013
| Fox redistributed the damping forces of the new DHX RC4 shock more evenly between the main damping piston (lower right) and the IFP compression stack (upper left) to give the damper a more seamless feel. An adjustable air assist piston replaces the Boost Valve in the reservoir. Pressurizing the IFP reservoir now gives the coil spring more of a progressive curve, while turning in the blue Assist dial adds more or less progressiveness to the system.|
The new RC4 retains its coil spring (for the moment) and for the most part, it appears unchanged externally - but inside, the shock has been completely reworked to take advantage of damping strategies used by the Fox Factory motorsports division. Suspension engineer Brian Anderson said that they balanced the ‘distribution of damping’ between the shock’s main piston and the adjustment circuits in the IFP reservoir. Three standard tunes, each based upon performance curves collected during the 2012 race season, will be available for aftermarket shocks. Of course, custom tunes will be available from the factory for riders with special requests.
The shock shaft diameter was reduced from 5/8 inch (16mm) to 1/2-inch (13mm) to reduce the volume of fluid that is displaced into the reservoir. What resulted was less internal pressure, which eliminated much of the shock’s seal friction and provided a seamless interaction between each phase of the damping circuits as the shock cycled through its stroke. The boost valve function has been eliminated, and in its place is an air-assist function. By adding or subtracting pressure to the IFP reservoir, and by controlling the volume of the reservoir with the adjustable piston, the user can alter the progressiveness of the spring rate near bottom-out.
The result of the lower internal pressures and balanced damping is a shock that feels smooth and consistent throughout its stroke and it seems to remain that way regardless of the shock tune. Perhaps having the Fox race techs doing the setup and subsequent tuning of the shock during testing oversimplified the process, but the end result was that the new DHX RC4 proved to be an awesome feeling shock under a wide variety of riders and suspension systems during the session.
Fox suspension superman Mark Fitzsimmons charts suspension adjustments
as we progress through the second session of testing.
Learned at Fox Camp:
• Torque the clamp-bolts on the 40's fork crown to exactly 65 inch-pounds to keep the stanchion tubes parallel.
• Heat buildup does not affect the Air 40. Fox techs say that a 20-degree (6.7 C) change in temperature results in only one-half psi change in fork-spring pressure. In testing, the riders could not perceive any changes in performance due to heat.
• The most common damping setup error in DH is that riders use excessive low-speed rebound damping to keep the bike from bouncing their landings. Mark Fitzsimmons says to back the rebound dial off to smooth normal chatter and terrain, and then to increase high-speed compression damping to correct the bounce.
• Work on your suspension setup in small increments – one click or five psi – and pay close attention to the fore/aft balance. Use the suspension to tune the ride height of your shock and fork to get the bike to settle at an angle that gives you the most control.
• Pros often ask for separate training and a racing suspension tunes because their race tune feels to stiff. Once you’ve shuttled all day and worked out a great setup, it probably will feel too harsh next week on your first run down the mountain – leave the knobs alone and you will ride into your tune as your intensity increases.
|A 20-degree (6.7 C) change in temperature results in only one-half psi change in fork-spring pressure. In testing, riders could not perceive any changes in performance due to heat.|Testing With Fox at La Fenasosa
La Fenasosa’s terrain is arid and rocky with plenty of stepped limestone to put the suspension’s square-edge performance on the line, as well as a number of smooth berm turns to rate grip and stability. Where there was soil, the tracks were peppered with gravel of various sizes. Recent rains provided more traction than is usually the case there, but most of the tracks quickly dried. Fox chose a downhill with small jumps and features so we could get consistent runs and maximize the time available for tuning opportunities and to compare suspension components. It was a quick turnaround and a pair of military troop carriers with bench seats that towed trailers for the bikes ensured that there was no waiting for shuttles.
Fox suspension engineers chart the progress of each
session with rider comments and tuning info.
Team testing for amateurs:
|The plan was that the Fox RAD engineers would run the same type of test session that they would use to dial in the suspension of its Pro athletes or key suspension customers.|
The plan was that the Fox RAD engineers would run the same type of test session that they would use to dial in the suspension of its Pro athletes or key suspension customers. Each run would be documented and any suspension adjustments would be notated. Fox even provided electronic timing for those who required it. We would ride a morning session on the stock bike to familiarize the DH course and about noon, while we ate a hearty lunch, the Fox team would switch out the fork and shock to 2014 items. Day two would be spent exclusively on the 2014 components.Bike setup:
My test bike was a Pivot Phoenix set up with a Fox 40 fork and 2012 DHX RC4 shock. The Phoenix, like all dw-link suspension designs, is quite sensitive to ride height and damping adjustments, so I took advantage of the services of the Fox race truck and had my settings tweaked with each successive run. By noon, the Pivot was riding pretty level over the bare stone of the upper section and was tracking well over the smoother dirt below. I was impressed by the fact that Fox’s tuners made small adjustments to the suspension, concentrating more on the balance of the fork and shock rather than trying to solve the issue by adjusting only one side of the bike. First ride on the Air 40 and the new DHX RC4:
Buoyed with confidence from the first session and anticipating perfection from my new Air 40, I crashed my brains out mid-course, down a relatively simple section of rocks on my first run. I had plenty of warning, as the front end had been dropping into holes and lifting the rear wheel, but I figured that the mighty 40 Float would save me. Three shuttles between the mountain and the Fox truck had me back
on point – the Pivot was balanced and the sensation of the bike and its suspension faded to background noise. Successive runs felt seamless and in control, which encouraged me to try alternative lines. By day’s end, I was considering a stiffer setup, as the extra comfort and confidence of the new tune had me riding more aggressively, using most of the fork and all of the shock travel. Others were experiencing the same effect. By late afternoon, speeds down the rocky upper section were increasing and most everyone was bottoming their rear suspension there – a rare occurrence earlier in testing.
40 Float RC2 - First Impressions
|Cliche as the word is in the context of mountain bike journalism, if I had to capture the performance of the air-sprung 40 into a single word, it would be 'seamless.' |
Cliché as the word is in the context of mountain bike journalism, if I had to capture the performance of the air-sprung 40 into a single word, it would be ‘seamless.’ Most of the time when I am on a big bike, I am aware to some degree of what the fork and shock are doing. Sure, with a good setup, there are segments of a DH run where I am not conscious of the suspension, but the fact that suspension setup is a compromise, there are always sections that fall outside the capabilities of the fork and shock that upset the balance of the bike, however slight, and they call attention to the offending component. The air-sprung 40 has a much wider range of performance. It sucks up the bigger hits without feeling harsh, and yet its small-bump compliance is beautiful.
As wonderful as the original 40 performs, in a side-by-side comparison with the Air 40, it can’t make that claim. When Fox got my coil-sprung 40 dialed, it skipped over the chunder in the upper rocky section with ease – which is exactly what I wanted from the fork - but I could sense the compromise in the smoother, lower sections, where the coil-fork rode a bit high in its travel and skipped over the gravelly turns. By contrast, the Air 40 rolled the upper section quite easily without much skipping and stuck to the trail like glue on the smoother segments of the course. In the end, after we got the Phoenix matched up to the 2014 suspension, I used most of the available travel and, instead of concentrating on the bike, I was sizing up the few features on the course where I had to pick a good line. In short, I was riding the course, not the bike. That is my definition of ‘seamless.’
The 40 Float RC2 needs no stamp of approval. It’s already racked up a World Championship and a closet full of World Cup victories. Downhillers who want to argue the merits of a coil-sprung downhill fork should start looking for alternative transportation, because the big black bus with a bushy tail painted on it – the one takes pros to World Cup podiums – has been air-spring-only for two years running and it just left the station. Humor aside, if the 40 Float fork can beat or match its performance, and all points of the compass support that notion, then there are no arguments left that can support the use of a coil-sprung fork within the context of downhill racing. Fox’s 40 Float RC2 fork takes over a pound off of the bike and offers adjustability that far exceeds the measured increments that coil-springs afford. Consider that the original Fox 40 represents ten years of racing development and that its air-sprung successor – barely two years old – can out-perform it. It’s only going to get better with time. Expect an air-sprung shock soon. DHX RC4 Shock - First Impressions
Adding the 2014 DHX RC4 shock to the equation is especially important because the performance of the front suspension largely depends upon that of the rear. In the sensitivity department, Fox’s modifications to the 2012 DHX shock paid off. Because the new RC4 shock moves more freely, it feels like the spring rate is softer, so there is a tendency to start turning in the low speed compression to ensure that the tail end will not wallow – but that is not necessary, as the shock’s high-speed rebound and compression damping are speed sensitive. Santa Cruz Syndicate rider Josh Bryceland, who was also testing that week, mentioned that '...the new suspension doesn’t feel like it’s going to be right in the car park, but on the course, it is great.' Fox revamped the damping curves for the new DHX shock to compensate for rising-rate linkage trend, so that may also be a factor in the shock’s improved feel. Add the quicker response of the smaller shock shaft and the revised damping circuits to the new shock and the result is a wider performance range than last season’s RC4 out on the course, which makes it a good match for the new fork