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.