Two Bikes in One: Cannondale Jekyll
[album id=39 template=extend]
All-mountain bikes are always a compromise. They don’t climb as well as cross-country or trail bikes and they don’t descend as well as downhill or freeride bikes.
That hasn’t changed, since Cannondale has chosen to not classify the 2011 Jekyll as an all-mountain bike. However, the Jekyll arguably climbs as well as a trail bike and descends as well as a freeride bike; it truly is a no-compromise rig that works as well as a purpose-built bike in both of its two applications.
And to avoid it being lumped in with bikes that don’t do either thing well, Cannondale coined the term “Overmountain” to describe the category that the new Jekyll arguably uniquely inhabits along with its sister, the women’s specific Scarlet, and its big brother, the bigger-travel Claymore.
I rode a Jekyll for months when living in Italy nine years ago, and there is no comparison between that Jekyll and this Jekyll. The old one had no Mr. Hyde and didn’t have the nice personality of Dr. Jekyll on steep climbs. I rode this new Jekyll all over the Round Valley trail system in Park City, Utah, as well as on rail trails and an asphalt climb, and it is truly amazing in its versatility; like nothing I’ve ever ridden before.
At the heart of the 2011 Jekyll is the revolutionary Fox DYAD RT2 pull shock developed by Cannondale’s whiz-kid bike designer Peter Denk in collaboration with Fox Racing Shocks. This three-barrel shock is literally two shocks in one with completely separate and characteristically sweet Fox damping: a cross-country-specific air shock featuring firm damping and pedal platform, and a downhill shock with the spring curve of a coil spring and buttery downhill damping.
This shock makes the Jekyll effectively two bikes in one; a lever on the handlebar shutting off one damping-and-spring circuit and opening another allows the rider to toggle between a 90mm-travel trail bike and a 150mm-travel freeride bike.
And the bike’s name is finally appropriate; rather than drinking a potion, you in essence install a cross-country shock on the fly to create Dr. Jekyll — a lightweight who climbs mountains. When you want to succumb to your darker impulses, you install a much bigger shock to create Mr. Hyde — a bad-boy who bombs down them.
Flipping the handlebar lever into “Elevate” mode lifts the bottom bracket height, stiffens the damping, reduces the sag by 40 percent (to around 20 percent of travel), sets the travel at 90mm, provides a progressive, low-volume air spring, steepens the seat and head angles, and gives the bike a riding position and active bump compliance suitable for climbing rough terrain easily. At 25.4 pounds for the top-end Ultimate model, it doesn’t exact a weight penalty, and I was able to catch and pass riders on 29er hardtails on a paved climb, something that would have been impossible on a long-travel bike gooshing along with a dropped-down, tilted-back riding position good for gravity-assisted speed but making uphill pedaling inefficient.
Flipping the handlebar lever into “Flow” mode drops the bike down into a super-plush setup with over 35 percent sag, lower bottom bracket, high-volume air spring with a low-slope spring curve, soft damping, 150mm of travel, shallow angles and a riding position and attitude that beckons you to let it rip going down. With these characteristics and the “center stiffness” described below, it can be flipped adeptly from one angle of attack to the opposite one through linked corners, easily drifting in control with both wheels sliding over braking bumps without losing traction, and then railing out of them.
A key feature of the Fox DYAD RT2 shock is stationary damping pistons (or damping valves) in each damping cylinder, so the oil moves through the piston (valve), rather than the piston moving through the oil, something Denk patented back in 1998. The three graphics of the shock are worth 1,000 words each, and each one of the first two (“Short Travel” and “Full Travel”) shows two different views of the shock taken at 90 degrees from each other. Unlike prior shocks intended for dual purpose there is a separate damping circuit (with a separate rebound adjuster) for each purpose.
You can see that the shock body with the two eyelets on it houses only the negative spring, which in this case lengthens the shock, since it is a pull shock, and its oil chamber is connected through a thin cylinder housing a spool-shaped valve to two large damping cylinders. (The pull shock, which gets longer as the bike compresses, reduces the force on the bottom bracket by pulling on it rather than pushing downward on it as most shocks do when the bike hits a bump.) The last of the three shock illustrations compares side by side only what is happening in the two damping/positive spring cylinders in each of the two modes (Editor’s Note: There is a typo in the title of the image; it should be entitled “Flow” [not Full] Mode vs. Elevate Mode).
Movement of the shock shaft pumps oil through the spool valve and into the damping cylinder(s). The spool valve is raised and lowered by means of a cable attached to the handlebar-mounted lever, and big openings around the large spool allow unrestricted oil flow, so damping is controlled only by the shim stacks on the damping pistons (I really ought to call them valves, not pistons, since they don’t move), not by the shock body restricting flow due to insufficiently large transfer holes. In the Elevate mode, only the large damping cylinder and the single air spring is utilized. In the Flow mode, both damping cylinders and both air springs come into action. The shock requires very high air pressures (300-400 psi), and Cannondale provides a special shock pump for that purpose. A single Schrader valve allows pumping both cylinders simultaneously to the same pressure, and a patented valve inside separates the two air chambers once the pump is removed.
Denk, who, along with his partner in his Freiburg, Germany, office designed the entire Scott road and mountain bike lines for a dozen years, came up with a number of critical innovations that allow the Jekyll rider to fully enjoy the benefits of the revolutionary rear shock.
Primary among these is “center stiffness,” a concept that doesn’t show up on standard lab tests but anyone who has pushed a full-suspension bike hard through back-and-forth corners can appreciate. Bikes that lab tests show to be extremely stiff laterally often still feel floppy when switching from one direction of turn to another until they settle into the turn and hook up. Denk figured out that this was due to lag in the bearings in the pivots, and a lab test won’t show it because the bike is pre-loaded, thus taking out the lag, before the side loads are applied to measure its lateral stiffness.
The lag is caused by individual balls in a cartridge bearing being in full contact with only one race of the bearing when a force is pushing on the bike from one direction. When a side force is applied from the other side, the ball loses contact with that race and briefly is floating free before it contacts the other bearing race; there’s the lag.
Denk eliminates the lag by putting paired bearings in every pivot in the bike, even the ones at the dropouts. He also uses large, 15mm-diameter through-axles at each pivot (12mm at the dropouts), with the pair of bearings widely spaced out at the axle ends. Furthermore, you know that grabbing a wide handlebar only gives you control and a rigid, closed structure of your arms and shoulders with the handlebar if the grips are attached well; if the grips slip, you cannot control the bar or the bike well. So every pivot link has a pinch bolt that clamps onto the end of the axle passing through it, and the pivot links, whether carbon on the high-end models or aluminum on the lower ones, are large and hollow for yet more stiffness.
The paired bearings, long, stiff, clamped through-axle pivots and stiff links result in noticeable stiffness at the center of movement without lag when switching loads from side to side. Building on that super-stiff platform, Denk then tuned the stiffness of the frame itself to impart good rider feel, like an extension of the body, when it is laid over into a turn. If a bike is too stiff, it feels unresponsive, like pushing an I-beam around, something that makers of Moto GP motorcycles figured out long ago, building in a tuned flex to provide good road feel. This took many iterations of carbon prototypes and lots of test riding to tweak to perfection, and Denk himself laid up many prototypes himself, cutting out the carbon pieces by hand with scissors.
Denk also built in a progressive linkage into the bike; to see what it does, look at the image with the figure showing two graphs: the spring curves of the shock in the two modes vs. the spring curves of the bike. Note how the progressive linkage makes the early part of the shock’s spring curve linear and of lower slope, indicating compliance over small bumps. And note how it makes the ends of the curves steeper, indicating a high ramp-up in spring stiffness at the end of the travel to prevent bottoming out. The bike intentionally has no lockout; on rough climbs if your center of mass is going up and down as you hit bumps because the suspension is not active, your center of mass experiencing more elevation gain (requiring more work) than the hill actually has, whereas if the bike absorbs the bumps, the center of mass moves along parallel to the slope of the hill, resulting in less actual climbing for you.
Carbon Jekyll frames are made of “BallisTec” carbon, which is made in Japan for military applications requiring extreme toughness, and in an effort to keep it out of military usage in China, the Japanese government requires every fiber of it to be accounted for. It is held together with a hot-melt resin developed for carbon-fiber baseball bats. The fiber has double the elongation (how far it can be stretched before breaking) of high-modulus (HM) carbon, and combined with that resin, it can take tremendous impacts from rocks without cracking the way carbon mountain bike frames are otherwise prone to.
The down tube, head tube and top tube are molded as a monocoque, and tube-to-tube fiber-wrapped-joint construction is used to attach its mitered ends to the seat tube and bottom bracket. Aluminum Jekyll frames use hydroformed tubes with smoothed-off two-pass welds to be nearly indistinguishable in appearance from the higher-end carbon models. All models use the same shock.
The bike holds a full-sized water bottle, something trail riders can appreciate but which will not fit on most freeride bikes. Riders wanting to use chain-retention systems or Hammerschmidt cranks will appreciate the ISCG03 mounts on the bottom bracket shell. The down tube and uninterrupted seat tube protect the shock front and rear from grime thrown off of the tires. It has Cannondale’s super-stiff sandwich-style derailleur hanger that clamps on both sides of the dropout.
Finally, it has a Y-shaped top tube (dubbed “Clashnikov” due to its resemblance to a certain gun) to stiffen the seatpost area. And high-end models come with RockShox hydraulic handlebar-activated adjustable-height seatpost.
For a rider wanting to ride any kind of terrain and do it well, whether high-altitude climbing, steep descents or rough ups and downs, this is the bike. It would also be great for something like the TransAlp Challenge. Same goes for a rider competing in a range of different mountain bike events only wanting to purchase and travel with a single bike.
Technical writer Lennard Zinn is a frame builder (www.zinncycles.com), a former U.S. national team rider and author of numerous books on bikes and bike maintenance including the pair of successful maintenance guides “Zinn and the Art of Mountain Bike Maintenance” – now available also on DVD, and “Zinn and the Art of Road Bike Maintenance,” as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”
Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn.
Follow Lennard on Twitter.
Follow Lennard on Twitter at www.twitter.com/lennardzinn