How Does a Cycling Shoe Sole Actually Flex and Bend?

How does the sole of a cycling shoe bend when you’re pedaling?

Stiff carbon fiber soles that provide high stiffness and low weight are a must-have for high performance cycling shoes. But how do these high-tech soles actually bend and deform when pedaling? Where is it important for the soles to be stiff, and where is it less important?

How carbon fiber soles perform and the benefits they provide are poorly understood by most people. Discussing the effects of sole stiffness on efficient power transmission is a surprisingly complicated topic beyond the scope of this article. Instead we will focus on how the sole actually bends when you’re pushing hard on the pedals.

Below is an image of a computer model of a cycling shoe sole showing some simulation results for 150 pounds of force (68kg or 667 N) while engaged with a Shimano SPD-SL cleat and pedal. The cleat and pedal are not shown, but the stress being suffered by the carbon fiber material is depicted; you can pick out the area where the cleat makes contact with the sole.

Stress depicted on a cycling shoe sole

The cleat, supported and reinforced by its contact with the pedal, provides additional resistance to bending. Therefore the force the rider is putting into the sole causes it to bend around the edges of the cleat. Stress is a measure of the internal forces within a material that cause it to deform and eventually break if enough stress is applied.

You can clearly see the highest area of stress along the edge of the cleat. As you might guess, the area of the highest stress is also the area where the material is being bent and deformed the most. In other words, these are the areas where sole stiffness is the most important.

Another measure we can depict is called strain, which is a measure of how much a material has changed in shape compared to its original state. Below we show the same sole, with the same load, but we greatly exaggerate the deformation to make it easy to visualize how it is deforming. The view is from slightly behind, also to help us see how the sole is bending. The colored areas show us the amount of strain in the carbon fiber. In this case, the areas of high strain and high stress are the same.

Strain in a carbon fiber cycling shoe sole with exaggerated deformation

The force of the rider causes the sole to deform into a semi-bowl shape around the pedal cleat. So what is this going to feel like? While the edge of the cleat is where the highest stress and strain are happening, this is not where you are going to feel the bending. That’s because these areas remain almost stationary with respect to the pedal. But the outer edges of the sole move the most with respect to the pedal and the rider may perceive this as the outer edges of the sole flexing, even though the flexing is happening closer to the middle.

The image below shows the regions that have moved the furthest from their original positions in red. Again the view is from slightly behind the shoe and the deformation is exaggerated.

Displacement of a carbon fiber cycling shoe with exaggerated deformation

Here you can see that the parts of the shoe that move a lot from their original position are areas that were under relatively lower stress. This movement is coming from the strain that is happening at the edge of the cleat. How does that work?

Consider a long, thin piece of wood clamped to a flat surface on one end while the other end protrudes far out, unsupported. When you add a heavy weight to the unsupported end, that end bends far down, giving it the most displacement, but the part of the wood that is bending the most is the part that is right at the edge of the flat surface it is clamped to.

Cantilever example

In the above example, imagine the edge of the ladder step as the edge of our cleat. That’s where the bending is happening, but the displacement is most way out at the end of the stick, like the outer edge of the sole.

So what do we conclude from this? First: Stiffness is most important in areas bordering whatever attachment system you use for your pedals. This will differ between Shimano, Wahoo, or the new Ekoi PW8 system, but it will always be centered around the areas that the cleats and/or pedals are in contact with the sole. Stiffness at the heel of the shoe is not important. The VeloVetta Monarch shoe has an aerodynamically shaped component that wraps over the sole not very far behind the cleat and a lot of people ask me how far back the carbon goes. The answer is about 2 more inches – well beyond the point that the carbon is under stress from the pedaling. The second thing we can conclude is that where you feel the flex in your sole may not actually be where it is flexing. Next time you are evaluating the soles of shoes that you are considering adding to your collection, keep these dynamics in mind!