How do bicycle manufacturers measure frame stiffness? Is there an industry standard for comparing it?
The closest we come to an “industry standard” is the Zedler stiffness-to-weight (STW) protocol, which will be explored later in this article. It is used by Tour Magazine to compare the stiffness value of frames. But manufacturers do not universally apply this standard.
This is part of the reason Cervélo developed its own protocol, but more importantly, we have found that Zedler doesn’t incorporate the same boundary conditions as real-life riding. Boundary conditions are the constraints and forces acting on a system. If the system is a bicycle, then the constraints and forces exist at the rider’s contact points, at the tire contact points with the road, and to a lesser extent the gravity and aerodynamic forces acting on the bike. The challenge is to mimic the boundary conditions of real life while keeping the test simple enough to conduct thoroughly and accurately while being capable of easily testing as many different frames sizes and geometries as possible.
First we need to define what aspects of stiffness are important to the rider and record what happens in reality. What forces is the bike under? Where in the frame? Through a long process of real-life testing, we were able to “record reality” as a part of Project California.
Based on these tests, we determined that there are three types of stiffness tests that matter:
1. Torsional stiffness: This is what sets our approach apart. The traditional 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 (as in Figure 2 below), we have been able to reduce frame weight by removing carbon plies that had no effect on torsional stiffness.
2. Bottom bracket stiffness: When measuring bottom bracket stiffness, common approaches measure deflection under a force applied at either a horizontal or vertical plane. In our case, we apply force at the same lean angle as you do when pushing on the pedals at peak torque, 15 degrees. The head tube is fixed to simulate out of saddle sprinting and measurements are taken in the same direction of pedal force vector to get an accurate measurement of pedaling efficiency.
3. Vertical stiffness: This is the simplest case. Apply a force straight down on the saddle and measure how far it deflects. The Zedler test protocol achieves this accurately so we stay very close to the same process.
The takeaway: When you hear claims of a frame being stiffer or the stiffest, ask yourself how it was tested. How were the forces applied? Do those measurements reflect real world riding conditions? How were those conditions defined? How can you trust a company when they say it’s so? In Cervélo’s case, we’ve shared our engineering process and the data linking our test standard reality.
The stiffness of a frame is important to the rider to provide responsive handling and efficient transfer of power. Another importance is that a stiff frame under the pedals and during cornering will feel better and give the rider more confidence by behaving predictably and react quickly to your inputs. While the “feel” of a frame is subjective, exhaustive data collection and realistic testing is drastically improving our understanding of how to make the best riding frame possible.