|Features||Shielding Seat Stays
When the S5 was first unveiled to the public just days before the 2011 Tour de France, it didn’t take long to demonstrate its superior performance. Quickly boasting seven days in yellow in the Tour de France including two stage wins, the S5 embodies the perfect synthesis of speed and stiffness.
With an overall stiffness increase thanks to the BBright™ platform, this is not only a super-fast bicycle, but one that’s agile and responsive at top speeds, while smooth and comfortable on the toughest road surfaces.
Built For Bottles
Frames are but one component of an entire system. That system includes wheels, bars, all components and the most important part, the rider. Since the rider has needs such as hydration, we consider bottles as part of the aerodynamic equation.
We have designed the S5 to be the fastest road frame with or without bottles. The S5 fits two water bottles in the usual positions: one on the seat tube, one on the down tube. In the standard 2-bottle arrangement, the bottles fit together tightly with each other and the frame, reducing the gaps where turbulence can form resulting in lower aerodynamic drag.
In addition, an extra bottle boss on the down tube creates a new lower position for a single bottle. Using the low position on the down tube saves 14 grams of aero drag, a reduction of about 1.4 Watts, compared to a bottle in the usual position. The lowest drag bottle for the S5 is an Arundel Chrono on the seat tube, which added no measurable drag in our wind tunnel testing. An Arundel Chrono on the down tube added only a few grams. In essence, the S5 is optimized for more than one option.
In general, a good aero bike uses airfoil tube sections. This may seem obvious, but many “aero styled” bikes have arbitrary, “made-up” tube sections that can actually increase drag. In Cervélo’s early days, “aero sections” meant NACA (National Advisory Committee for Aeronautics) airfoils.
We still use some NACA foils where they make sense, but in our long quest to help Cervélo athletes go faster, we’ve spent enough time studying bicycle aerodynamics to not only design a family of our own aerodynamic sections (our TrueAeroTM library of airfoils) that improve on NACA for bicycle performance, now we’ve arrived at the next generation of aero design. We no longer design just a tube section (two-dimensional) – we now design entire bike frame skin surfaces (three-dimensional), taking into account the different air flow characteristic in each aero zone
We’ve always engineered our aero bikes using the lowest drag tube shapes we know of. But the lowest drag bike is more than low drag tube shapes. During the years we spent developing the P4, we also considered the aero interaction of the various tubes with other parts of the bike. The result is Cervélo’s AeroZone engineering principle.
The downtube AeroZone includes nearby components, such as the front brake, fork and wheel. Dropping the down tube “hides” the down tube in dirty air, reducing the high pressure normally found on the down tube’s leading edge, and eliminates the low pressure air that normally pulls rearward on the fork crown.
Asymmetric, stiffer, lighter: BBright™ is the only Bottom Bracket standard that uses an oversize 30mm axle and allows for oversized frame tubes (up to 16% wider than standard bottom brackets). It offers the optimal combination of stiffness and weight for the overall system (crankset and frame together).
In the past, frames and cranks were designed to work with existing bottom bracket standards. We recognized that these existing standards limited system performance. By designing the crank and frame together, as a system, BBright yields a significant increase in system performance. BBright was designed with the widest range of crank options in mind while allowing us to offer the stiffest frame possible.
First, we decided what not to change: the derailleurs, chain line, and pedals (Q-factor, or stance width) all remain in the same place for best drivetrain function and familiar ergonomics. We looked next at what parameters would optimize performance of the whole system (frame + crank). For the crank, a 30mm axle is stiffer, lighter and directly compatible with easily available Press Fit 30 bottom brackets. For the frame, BBright’s 79mm bottom bracket shell permits oversized frame tubes that save weight and increase stiffness, addressing a major limitation of old 68mm bottom bracket standards.
How did we get to 79mm? On the left side, compared to external bearing bottom brackets with 68mmm shells, the bearing in the same position, but we replaced the metal bearing cup with frame material. This allowed us to increase the left side chainstay width by nearly double. On the right side, compared to BB30’s 68mm shell, we kept the bearing in the same position, leaving room for power meters and compatibility with existing BBB30 crank molds. Also, the tubes can’t get any wider on the right: they’re already limited by chain and chain ring clearance, so making the shell wider doesn’t improve performance.
The net result is a stiffer, lighter crank and a stiffer, lighter frame, a natural result of our system engineering approach. BBright truly combines system optimization and simplicity.
Cervélo’s ComfortPly™ technology is a technique gained from the intense composite analysis skills learned through our experience with Project California. To develop ComfortPly™, we’ve used the same composite analysis software tools not only to optimize strength, stiffness and weight, but also to fine-tune frame properties in different directions. During layup development, once strength and stiffness targets are achieved, ComfortPly™ technology actually removes unneeded fibres to reduce weight and vibration and improve ride quality.
Since Project California began, our engineers have used ComfortPly™ technology when designing the layup of all new frames. Whether it is fresher legs on the run when you come off the P5 during a triathlon, or the ability to push a little further on your R- or S-series bike, every rider benefits from our hands-on engineering knowledge.
Smartwall intelligently redistributes material around the tube cross section to increase lateral and torsional stiffness while adding the least weight. Smartwall was inspired by Cervélo’s aero bikes, whose shapes are driven primarily by low drag considerations. Reducing the frontal area of the frame is a part of an aerodynamically efficient design, but narrow tubes have inherent challenges when trying to construct a stiff and light frame. Smartwall addresses this elegantly.
Most unwanted frame flex is out of plane, putting one side in compression and the other side in tension. Thus, material farthest from the center plane has the greatest effect on improving stiffness. Smartwall adds material only at the outside walls, farthest from the center plane, to maximize lateral stiffness and minimize weight.
Smartwall clearly makes sense for all frames. In fact, we use Smartwall to set new standards in frame stiffness, strength and weight throughout the R series. And we still use Smartwall to create stiff and light aero frames.
Extended Seat Tube Cutout
By shielding the spinning wheel from the wind, the cutout yields one of the largest single reductions in aerodynamic drag. The concept is inherently intuitive, however, executing the design requires true aerodynamic expertise.
In order to most effectively reduce drag, the cut out must have the right curvature and width to take full advantage of contemporary aerodynamic wheel technology. The result is that the wheel is completely shielded while maintaining its own aerodynamic advantages. We incorporate this knowledge on every Cervélo model with a rear wheel cutout.
On some Cervélos, the cutout is extended upward to be even more effective; covering the top of the wheel is particularly important, as it moves forward at twice the riding speed. Yet even on Cervélos that don’t have this extended rear wheel coverage, the seat tube width and shape is engineered to be an aerodynamic match with the tire and wheel along the cutout’s entire height, even changing the seat tube’s cross section where necessary.