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INTRODUCING THE CERVELO S-SERIES
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Elbow to elbow, weaving through the peloton, straining to hold the wheel in front. 200 metres to go. This is the moment. Shifting gears and jumping out of the saddle, every part of you straining to pedal as hard as you can to the line.
Cervélo has always been the leader in building fast, aerodynamic bikes. We continue the tradition with the new S-Series. Designed for our World Tour racers and boasting significant improvements in aerodynamics and stiffness, these bikes are about all out speed. If you live for the thrill of the sprint and going fast, the S-Series bikes are made for you.
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Aerodynamic drag is the major factor affecting a bike and rider – it can account for up to 90% of the overall resistance that a rider must overcome. There are several types of drag relevant for us. First, pressure drag. As a body (in this case a bike and rider) passes through the air, it forces the air molecules to move out of the way in order to pass through them. These molecules push back on the body, creating pressure. The component of this pressure that faces aft (to the rear of the body) is called pressure drag.
Secondly, there is friction drag. Air, like all fluids, has viscosity (or “thickness”). The air molecules that come into contact with the body stick to its surface and stay stationary in relation to the body. As the body continues through the air, other air molecules pass by the stuck molecules as they flow around the body in layers, following parallel paths. This is the laminar boundary layer. The viscous nature of air creates a shear force, or friction drag.
At some point on nearly all bodies, laminar flow cannot be maintained and the air molecules tumble and mix instead of flowing smoothly. The transition point is where this turbulent boundary layer begins. This behaviour of flow is related to a parameter called the Reynolds number, which is determined by several physical characteristics of the flow. The laminar flow regime exists up to Reynolds numbers of around 10,000. Beyond Reynolds numbers of 10,000, the flow transitions to “turbulent” flow, as shown in the figure below.
Maximise your power
Through many hours of research and testing, Cervélo has defined the particular types of frame stiffness that have the greatest impact on a bike’s performance.
Steering stiffness is the type of frame stiffness that most affects how a bike handles. It is commonly called torsional or head tube stiffness. Steering stiffness is defined as how much the bike frame twists when it is ridden around a corner. Generally speaking, higher steering stiffness leads to more responsive handling by reducing the lag time between input from the hands and reaction in the bike and rider.
Cornering can be described in engineering terms by a set of forces (the “load case”) applied at the handlebars, the saddle, and the tires’ contact points on the road. Some of these forces are in opposite directions, essentially twisting the frame. The load path from the handlebar flows into the frame through the headset bearings, and the load path from the saddle flows into the frame through the seat post.
In the lab, we mimic the application of these load paths by supporting or applying force at these points on the frame. We even mimic the forces at the contact points of the tires. We want to be sure that what we learn in the lab translates into performance that can be felt by the rider.
Cervélo’s testing of steering stiffness sets us apart from other bike manufacturers. The traditional industry test calls for the frame to be fixed to a jig at the rear dropouts and supported in the centre of the head tube. A torsional load is then applied to the head tube and the frame is essentially twisted. While this does put the frame under torsion, it is not a realistic load case. But by simulating the cornering loads from the tires as well as from the rider’s inertia, we have been able to reduce frame weight by removing carbon plies that had no effect on steering stiffness. The end result is reduced frame weight for the same effective steering stiffness.
The right amount of steering stiffness depends on the intended use. Too little stiffness, and the result is a “wet noodle” riding experience. You can also have too much steering stiffness: there is a point where the frame is so stiff (in steering) that the rider does not notice any benefit and may find that it makes the bike feel less comfortable, as more vibrations are transmitted to the hands.
Pedaling stiffness is also known as bottom bracket stiffness. When a rider pushes down on a pedal the frame deflects laterally. Stiff frames deflect less, so more of your energy goes into turning the rear wheel, rather than deforming the frame.
How much pedaling stiffness is needed depends on many factors, including rider power output, how the bike is used, and frame (and rider) size. Track frames generally require more pedaling stiffness than endurance or triathlon bikes, but all benefit from higher pedaling stiffness.
However, pedaling stiffness can to be too high. As with steering stiffness, it is possible to increase pedaling stiffness to a level where the rider will not notice the difference. From that point on, any added stiffness only adds material, which means weight.
Testing: Common testing approaches for pedalling stiffness measure deflection under a force applied at either a horizontal or vertical plane. In our case, we apply force at a 15-degree lean angle to simulate real riding. The headtube is fixed to simulate out-of-saddle sprinting, and measurements are taken in the same direction as the pedal force vector to get an accurate measurement of pedaling efficiency. Again, the rear wheel is supported at the tire contact patch to more closely simulate real world conditions.
VERTICAL SADDLE STIFFNESS
Vertical saddle stiffness expresses how much the base of the seat post will move when a rider sits on the saddle. This stiffness is related to how comfortable a frame is to ride. We measure vertical saddle stiffness without including the effects of the saddle or seat post, both of which contribute significantly to the vertical saddle stiffness. By doing this we isolate the performance of the frame only in the measurement.
Many other factors affect a bike’s comfort – tires and wheels are the most important contributors to vertical stiffness, with seat post, saddle and frame next on the list. For this reason, it is possible for frames with the same vertical saddle stiffness to feel very different. That means it is not always easy to compare vertical stiffness on different bikes or between individual frames.
Generally, we want the frame’s vertical saddle stiffness to be as low as possible for the most comfort; however, when it gets too low, there can be unexpected bobbing or movement when pedaling that decreases efficiency and rider control. On the track, where comfort is less of a concern, a high vertical saddle stiffness can actually be beneficial.
Testing: This is the simplest load case to test. We apply a force straight down at the saddle and measure how far it deflects. Using a steel analog saddle and seat post effectively removes these components from contributing to the measurement.
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Focused on your ride experience
The most common definition of ride quality is “a comfortable ride.” This is accurate – to a point. When you dive into the engineering side of ride quality and comfort, it starts to become very complex. After conducting a long-term collaborative study with the University of Sherbrooke, we concluded that despite subjective opinions on what is “more or less comfortable,” there are no universally accepted definitions for “comfort” or “ride quality”.
With this in mind, Cervélo’s approach to ride quality focusses on the rider’s experience. We define ride quality as the feeling of the bike being appropriate for its intended use. That means that the application of ride quality will be specific – and different – for each bike. For example, the S5 aero road bike is all about speed. That’s why we lowered the front end for a more aggressive rider position with even better aerodynamics and increased stiffness to aid power transfer and handling.
Different technologies and elements are applied to our C Series endurance road bikes. For these bikes we chose a shorter, more upright geometry for the frame, selected tube shapes that prioritized vertical compliance and light weight, and then constructed the frame without using ultra-high modulus (UHM) carbon for a damped ride feel. Together, these create a bike for riders who want to ride further, faster.
Elements of comfort
As we said, comfort is very difficult to define and measure because it is so subjective. However, it is helpful to look at elements that correlate to comfort in two categories: static and dynamic comfort.
Static comfort is your perception of the bike when at rest, such as when you are sitting on it on a trainer in the shop. In this situation, factors such as fit, your contact points with the bike (feet, seat, hands), and body position are in play. Context will have a big impact on your perception of static comfort – for example whether you stretched that day or how well you slept.
Dynamic comfort focusses on the experience of the bike in motion. Dynamic comfort is very complex, but there is one measurable element that correlates well with perceived comfort: vertical compliance, which is the deflection (movement) in a vertical plane that can be measured as a response to an input. Simply put, vertical compliance represents the bike’s ability to react to a bump in the road.
When the front wheel contacts the bump, even as you sense the impact in your hands, each of the elements in the system deflect to some degree in response. The components between the bump and your body make up the system. The chart below shows the relative contribution of each component to total compliance in a road bike. These charts help us to focus our engineering efforts so that we optimize the vertical compliance of our bikes: what is optimal will depend on the bike and its intended use.