Of all of the possible measurements of movement, and therefore stiffness, that are possible with a bicycle, Cervelo has defined through research the particular stiffness measurements we believe are most critical to bicycle performance. How did we do this? Through a long process of real-life testing at our Project California facility, we were able to “record reality” on a bike. Based on these tests, we determined not only the correct forces to use to measure stiffness, but also the three types of stiffness that have the greatest impact on bicycle use:
This is the frame stiffness characteristic that most affects how a bicycle handles. It is commonly called torsional or headtube stiffness. However, differences in how these parameters are measured within the cycling industry have significant effects on the results. Put simply, not all steering stiffness tests are equal.
Steering stiffness is roughly defined as how much the bicycle 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 (considered a system when in motion). Even while riding smoothly on a straight road, there is continuous feedback between the rider and front wheel. We are always making minute adjustments to keep the bike going on the path we want it to follow.
In the case of cornering, this dynamic condition 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 seatpost. For our stiffness testing purposes in the lab, we mimic these boundary conditions by either supporting or applying known forces at these points on the frame. We even mimic the forces at the contact points of the tires, which, it turns out, is especially important to make our lab test match reality. Why does this matter? When we set stiffness goals for the frame that are verified in the lab, we want to be sure this translates into performance that can be felt by the rider. If the tests don’t match performance in the real world, then all our work is for naught.
Also keep in mind that the bicycle has many parts, and the fork stiffness, wheel stiffness, tire, hub, axle, bar, stem, seatpost and saddle all directly affect the steering feel of a bicycle. Frame steering stiffness is only one link in the chain, but it is the most important part — the backbone, if you will.
What is the right amount of steering stiffness? That depends on the intended use! Certainly it is possible to have too little stiffness, which loosely translates to “wet noodle” descriptions of the bike. We also believe it is possible to have too much steering stiffness. At some point, a frame can become so stiff (in steering) that it is not possible for a person to notice the difference. Also, making a frame overly stiff in steering has been shown to negatively affect the rider’s perception of comfort at the bars (as more vibrations are transmitted to the hands). So having the right amount of steering stiffness is key.
Testing: Our steering stiffness testing sets our approach apart. 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 headtube. A torsional load is then applied to the headtube 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 (as depicted below), 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.
This 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 into deforming the frame. This type of stiffness is most easily felt by riders on the road, and is commonly checked in the parking lot pedal push test.
How much pedaling stiffness is needed depends on many factors, including rider power output, how the bike is used, and frame (and rider) size. Therefore it is always a tradeoff to determine how much pedaling stiffness to build into a given frame. Track frames generally require more pedaling stiffness than endurance or triathlon bikes, but all benefit from higher pedaling stiffness.
Can pedalling stiffness be too high? Yes: As was the case with steering stiffness, it is possible to increase pedalling stiffness to a level beyond which the rider will notice the difference. From that point on, any added stiffness only adds material, which means weight.
As with steering stiffness, the overall pedaling stiffness of a bike also includes the stiffness of the pedals, cranks, shoes, bottom bracket, chain, wheel, hub, axle, fork, stem bar and other parts. The frame is the largest component of this overall stiffness, but other elements such as the crank and wheels also contribute significantly.
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
This stiffness value expresses how much the base of the seatpost will move when a rider sits on the saddle. It is the inverse of the “vertical compliance” that is often referenced in magazines or in discussions about bicycle comfort. In fact, Tour Magazin uses this test to assign a frame a “comfort” score. More compliance, or less stiffness, at the rider’s contact points (the handlebars, saddle and pedals) will reduce the peak force transmission from road inputs to the rider.
This stiffness is related to how comfortable a frame is to ride, but many other factors affect frame comfort, including vibration damping, weight, sound transmission and materials. (Read more about bicycle comfort in our Engineering Fundamental on Ride Quality.)
We measure this type of stiffness without including the effects of the saddle or seatpost, both of which contribute significantly to the vertical saddle stiffness. By doing this we isolate the performance of the frame only in the measurement. At a complete bike level, the tires and wheels are the most important part contributors to vertical stiffness, with seatpost, 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, so this stiffness, while easy to “feel” as a rider at bike level, is not always easy to compare on different bikes or between individual frames.
As with steering stiffness, there is an ideal range for vertical saddle stiffness that depends on many factors: rider weight and power output, road surfaces and so on. 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 steel analog saddle & seatpost effectively removes these components from contributing to the measurement.
For all of the aforementioned stiffness parameters, we need to carefully select the values needed for any given frame design based on its intended use, size, cost and so on. Our selection process is based on an engineering-driven approach to performance — define and measure, build, then test and refine — all of which results in the biggest difference of all: One you can feel.