GIRONA, Spain (VN) — Having missed the breakaway of aero road bikes when it went up the road five or so years ago, Cannondale worked hard on this category to bridge back up and try to go off the front. About its new disc-brake aero road bike, it says, “this race-bred machine is the lowest-drag, most efficient, all-around fastest UCI-legal road bike on the market today.” I had the pleasure of riding this speedy bike on beautiful, traffic-free, narrow, rolling roads northeast of Girona, Spain.
The six-part foundation of the SystemSix consists of the frame, fork, seatpost, bar, stem, and wheels all working together as a system. Individual components are denoted by the “Knot” model name, in this case referring to vehicle airspeed.
Six main components
It is becoming well understood that bigger tires can deliver higher speed due to lower rolling resistance and greater comfort, and Cannondale worked fatter rubber into its aerodynamic designs for this bike. Starting from the ground up, the 64mm rim depth and 32mm outer width of Cannondale’s new KNØT64 wheels are optimized for a 26mm tire (in other words, a tire labeled 700 X 23C, which would measure 26mm wide when mounted on the wide rim). And mounting a 700 X 25C tire, which becomes a 28mm on these wheels, adds precious little to their drag due to their rim shape and width. In order to produce this wheel, Cannondale has licensed HED Design’s patent covering design technology of wider rims; it allows the designer to get better aerodynamic performance at chosen angles to the wind (i.e., yaw angles), based on the angle relative to straight ahead of a line tangent to both the tire and the rim sidewalls.
Cannondale claims that KNØT64 wheels are faster aerodynamically overall with either 23C or 25C tires on them than comparable-depth wheels with either tire size from Mavic, Zipp, ENVE, Hed, and Shimano, but are slightly slower than Roval 64 DBs with 23C tires.
The fork of the SystemSix has a very smooth crown devoid of a rim brake atop it. Viewed from the front, the thin blades seemingly pour out of the head tube and down to the hub, with the left blade being internally beefed up for the disc brake.
The other components at the all-critical front of the SystemSix, namely the handlebar and stem, could have been molded into a single, sleek, air-cheating piece. But this would have afforded no adjustability for stem length, bar width, or bar angle. It also would not solve the problem of full internal wiring without a lot of futzing around to get the wires through the bar and stem, nor would it hide the wires once they leave the stem.
Cannondale has solved these issues with an extremely aerodynamic stem and bar system that are not molded as a single piece, but rather are separate pieces. Fancy that. Thing is, if you really want an aero front end you can’t have a clamp around the handlebar. Enter the KNØT SystemBar, a thin-profile carbon bar with a convex underside with four bolt holes backed up by an aluminum plate inside. It sits in the curved cradle of a broad, aluminum stem whose four bolt holes are slotted, allowing eight degrees of pitch adjustment before securing the bar in place with four bolts from the bottom.
The innovative internal hose and wire routing was designed by mechanical engineer Denis Kuerner and industrial designer Ian Surra in the Cycling Sports Group (Cannondale’s parent company) development center in Freiburg, Germany. It starts at the levers, passes through the bar, out an opening in its underside, and into the wide-open mouth of the two-piece, scissoring plastic cover that screws onto the underside of the stem. Headset spacers that meld seamlessly with the underside of the stem and its cover have a channel at their front for the wires and hoses to pass through. They are also able to swing open so that spacers can be added or removed without having to pull out any wires or hoses. Those wires and hoses then continue down into the forward projection of the head tube ahead of the headset bearings. From there, they can go either into the down tube to the derailleurs and rear brake or, in the case of the front disc brake, into the top of the fork crown.
While internal wire and hose routing is always more of a pain for a mechanic to set up than external routing, this at least saves mechanics much of the agony common with aero bikes face when a rider’s stem length or stem height changes. Being able to swing open both the under-cover of the stem and the spacers will make life much easier for mechanics. And the seamless way the shapes of the spacers and the stem cover flow together is both refined and aerodynamic.
Truncated airfoil shapes on the down tube, fork legs, seat tube, seatpost, and handlebar flats stay within UCI constraints while being designed to keep the airflow attached as long as possible at a wide range of yaw angles. While the UCI is dropping the 3:1 aspect ratio for major frame tubes that restricted how deep of a wing shape they could possess, the truncations of the head tube, down tube, and seat tube are because of the UCI’s “8cm box” that requires all frame members to fit within seven rectangular boxes 8cm wide.
“Only the seatstays and fork legs are affected by the 3:1 rule,” says aerodynamicist and Cannondale design engineer Nathan Barry, Ph.D., “and the rule also still applies to the bar and seatpost. We did not want to modify our design with the rule change [3:1 rule being dropped for major frame tubes], as there are more potential negatives with little to be gained in performance. It would add much more side area, mass, and in-plane stiffness, with only diminishing aerodynamic returns. Truncated shapes are to do with the 8cm boxes and the required widths of tubes. For example, the head tube has bearings and internal cables outside the steerer, so its starting width is ~55mm. We then dictate the shape of the tube to ensure no early separation, inclusive of yaw. These [truncated] shapes are both aerodynamically functional and structurally beneficial, as they have a high second moment of area, so out-of-plane and torsional stiffness are higher than a slender or low-aspect-ratio tube.”
Even on tricked-out aero road bikes, riders still need to drink and shift gears, which affect frame design. “The down tube is shaped to not only put a bottle in the wake, but also provide a true NACA leading edge and fit in the 8cm box,” says Cannondale’s Industrial Design Director Ian Hamilton. “The seat tube was designed from the seat post Di2 battery outward, using a true full truncated airfoil from the bottom bracket to the saddle. The trailing edge was designed to sit slightly inward of the rear tire leading edge to reattach airflow.”
The puzzle-piece fit of the back of the fork crown to curved chines coming down off of the down tube further keeps air turbulence at bay. A pin at the base of the head tube where it meets the down tube projects forward into a crosswise groove in the back of the fork crown to limit the fork’s rotation to 50 degrees. This prevents extreme fork rotation, which causes the swinging edges of headset spacers or the fork crown passing across the edges of the head tube from severing or mangling the hydraulic brake hoses or shift wires. In the case of a crash that flips the wheel around backward, past the stop, the pin is intended to tear the fork crown so that the rider will replace the fork (and of course the wires and hoses that it wiped out), while the frame will be saved.
A screw-on “SwitchPlate” atop the down tube allows the use of whatever electronic or cable-actuated shifting system its owner chooses with nary an exposed wire, save for cable-actuated systems having exposed cable housing between the underside of the stem and the plate. The Di2 SwitchPlate accepts the round Junction A that Shimano makes to fit in the handlebar end, and a blank plate is available for SRAM eTap; no plate exists for Campagnolo EPS.
Bike speed calculations
Using a constant feedback loop of computational fluid dynamics (CFD) to clarify why aerodynamic drag is occurring and wind tunnel data to quantify how much drag there is, Cannondale’s design team studied each of the above-mentioned six members of the bike and their interactions. To better assess the overall wind drag across all yaw angles of the bike, or of its wheels, frame, fork, seatpost, handlebar, and stem, Barry used “yaw-weighted drag,” rather than trying to assess which is faster overall from the confusing line graphs that one often sees showing aerodynamic drag as a function of yaw angle from -20 to +20 degrees for a number of different bikes or wheels. Instead, yaw-weighted drag is a single number for the drag of the bike or wheel based on a statistical distribution of a predicted likelihood of each yaw angle to be encountered while riding.
Using this single yaw-weighted drag number, and measuring it for a number of its competitors’ bikes, Barry claims that the SystemSix is the fastest overall. The claims of power savings due to aerodynamic efficiency over other bikes are big, perhaps big enough to take their magnitude with a grain of salt. For instance, Barry claims that Scott’s aero road bike, the Foil, would cost a rider 23 watts more power at 30mph than a SystemSix, and, furthermore, that the SystemSix saves, also at 30mph, around 6 watts over a Trek Madone, around 12 watts over a Specialized Venge, and 50 watts over a traditional lightweight bike like the SuperSix EVO.
While it takes a lot of power to pedal a road bike at 30mph on the flat (perhaps 350-400 watts), 50 watts is 12-14 percent of that, and that amount is not assessed as the drag of the bike itself, but rather as only the difference between two bikes. And, if true, it grants a rider doing a solo effort of this magnitude for the final 10km of a race a free, 30-second time reduction. Taking this a step further by figuring that drafting reduces a rider’s drag by 40 percent, this still predicts a 30-watt power savings when pedaling this bike at 30mph within the peloton. Big numbers indeed.
Barry’s analysis also predicts, in a 200-meter, 60kph sprint with similarly-sized riders both putting out 1,000 watts, the rider on the SystemSix would finish four bike lengths ahead of the rider on the SuperSix EVO. From that perspective, it makes Peter Sagan’s many sprint victories on a Specialized Tarmac (not the aero Venge) against other riders on aero road bikes especially impressive.
Another result of this analysis is that it will take over 50 percent more power (309W vs. 200W) to pedal a SuperSix EVO down a 5-percent grade at 60.6kph than a SystemSix. On the face of it, this seems improbable, but the downhill grade is the key to understanding where this number comes from. I asked Barry about this, to which he answered, “As you pointed out, whilst the drag reduction on the bike is approximately a third, with the rider the drag reduction is closer to 10% of the total. So where does this very high number come from? On a descent at 60 km/h, the aerodynamic resistance is very high, over 900W in our example. But because of the negative gradient (a descent), the mass of the rider and the bike are contributing to acceleration down the slope (a thrust force). This decrease in potential energy offsets much of the aerodynamic drag; this is why we descend so fast without much pedal input. So in our example, the PE [potential energy] term is contributing about 700W of thrust, which is opposing the aerodynamic resistance in this case. Because mass is close to the same, the PE term is similar, but the drag difference at 60km/h is very high, about 100W. This is why you can achieve such a huge margin on a descent.” If this calculation holds true on the road, it is the kind of thing that could make a big difference for a team putting its riders on the front to chase down breakaways.
Using the power equation incorporating total drag forces on a cyclist (wind drag, gravity, acceleration, and frictional losses in bearings, drivetrain, and tires) that University of Utah professor Jim Martin came up with decades ago, Barry calculates that above 15kph (9.3mph), aerodynamic drag accounts for the majority of the resistance a cyclist faces. Cannondale claims that going uphill, a one-kilo lighter, non-aero, climbing bike like the SuperSix EVO is only faster once the grade gets above six percent, and for the faster-climbing pros, that break-even point is at a 7-percent grade. While Tour de France Hors Categorie climbs certainly have sections with grades well beyond these numbers, Cannondale says that “most revered HC climbs in the Tour only average 7-8%.” Barry’s analysis shows a top rider losing 10 seconds on l’Alpe d’Huez to a rider on the lighter SuperSix EVO, yet he will have saved so much energy over the rest of the stage versus that bike that he still might come out faster overall on a SystemSix.
Cannondale has worked hard for nearly four years on the bike, with rolling data analysis happening round the clock. When Jonathan Schottler, the engineer implementing CFD findings in the Freiburg development center directed by Hamilton would leave his desk at the end of the day, he would hand off number crunching to Damon Rinard in the company’s Wilton, CT headquarters, who in turn would pass the work on at the end of his day to Barry in Australia and sometimes to the manufacturing engineering team in Taichung, Taiwan. So when Schottler turned his computer on again the next day in Freiburg, the project would have moved forward, and there were frequent break-of-dawn conference calls for the Wilton team.
Slots in the dropouts for the front and rear 12mm, keyed, “Speed Release” through axles allow faster wheel changes since the wheel can drop out while the axle is still in the hub. And a double-lead thread makes it so that it only takes five turns to tighten it rather than 10. Spare wheels in the EF Education First-Drapac p/b Cannondale team car will have axles in them already, so the mechanic need not pull them out and stick them into the new wheels, as is standard with through axles.
Most SystemSix models come with a “free” power meter on the Cannondale HollowGram crank. While the Power2Max NG Eco power meter and Vision chainrings are included with the bike at no additional cost, the user must activate a lifetime subscription with Power2Max via an app for $490 before being able to actually collect any data from their pre-installed power meter.
Depending on how much you want to push the aerodynamics vs. hydration formula, a low down-tube mount is for a single water bottle. Attaching a second bottle on the seat tube mounts requires instead using the supplied higher mounting holes for the down-tube bottle.
Two frame price points of the SystemSix exist, one with high-modulus carbon fibers and the other with more layers of lower-modulus fibers. Seven color schemes are available. Complete bikes vary from Dura-Ace and Ultegra Di2 to cable-actuated Dura-Ace and Ultegra models, including a Dura-Ace women’s model, all with Shimano hydraulic disc brakes.
I quite enjoyed riding this bike. The whirring sound of a bunch of SystemSixes slicing through the air is really impressive when riding in a big group of them. It is a noticeably fast bike, whether or not it can produce the magnitude of power savings that its design engineers calculate.
The SystemSix has the same geometry as the SuperSix EVO ridden so successfully by the EF Education First-Drapac p/b Cannondale team, so it came as no surprise that it cornered really nicely. The bike was small for me in length, height, and handlebar width, so it felt overly twitchy when out of the saddle. With a proper fit, I imagine that sensation would go away.
Despite all of the tall tube sections and rim sections, I found it not to be uncomfortably rigid vertically. After three hours of moderate riding on relatively smooth roads, I had no sense of being beaten up by the bike.
If you get one, you’ll be faster. It remains to be seen if you will crush your friends in sprints and on downhills, flats, and moderate climbs by as much as Cannondale’s numbers predict.