While this advice is being directed at flat pedal riders, the advice holds true for clipless pedal riders as well. In short, you do not need a stiff soled shoe when riding flat pedals. Most riders get confused on the subject because they assume that since clipless pedals have stiff soles, they need stiff soled shoes for their flat pedals as well. The problem is that we think that clipless pedals have stiff soles strictly for performance reasons when, in fact, it is an attempt to solve the problem of how unnatural the clipless pedal interface is with the foot.
Because of this difference a stiff soled shoe is unnecessary and, one could argue, actually counterproductive from a natural movement point of view.
While having a stiff sole can help in some situations that I’ll go over later, in general there is a huge difference between the foot and pedal interface on flats vs. clipless pedals – on flat pedals your foot is able to drive into the pedal itself while with clipless pedals the sole of the shoe must provide the platform for your foot to drive into.
The reason that you must have a stiff soled shoe for clipless pedals is that the attachment point with the pedals is too small to drive your foot into (your foot is actually touching less than one square inch on the pedals) so the soles of the shoe itself become that platform your foot needs. Without a stiff sole providing some sort of platform your foot would be forced to balance on and drive into the attachment point itself, which would be very uncomfortable and inefficient. The stiff sole acts as an intermediary of sorts, allowing the foot to drive into it and then transferring that force into the attachment point with the pedal.
On flats a very different dynamic is allowed to take place. Because the actual interface with the pedal is so large (you have several square inches of actual contact space with flat pedals) your foot can use it directly for support and to drive into. Just like running, your foot is allowed to naturally interact with the surface it is touching instead of relying on an artificial means of support and energy transfer. Because of this difference a stiff soled shoe is unnecessary and, one could argue, actually counterproductive from a natural movement point of view.
Your body is designed to let the foot articulate as needed so it can interact directly with the ground and this doesn’t change when you sit on a bike. When you stiffen the sole of the shoe to act as the support for the foot you also change how the foot can articulate – the stiffer the sole the more you are “locking” the foot into place and interfering with how it would naturally articulate. You can not change how one joint moves without placing more stress on some other joints and over time that locked up foot can come back to haunt you.
Now, with all that said a stiff soled shoe can improve performance and safety in certain situations. For example, I like to ride with 5-10 Impacts when riding downhill or freeride type trails because the thicker, stiffer sole will provide more cushioning if I have to eject mid-air and come down hard on my feet. I don’t like them as much for trail riding because I find the sole too stiff for lots of pedaling and my feet feel more comfortable with a more minimal soled shoe like the 5-10 Spitfires or Freeriders.
A stiff soled shoe can also provide a more efficient power transfer into the pedals for racing situations – but at the expense of altering how your foot moves which can cause problems over the long run. While the idea that the same shoes that make you faster can also hurt you over the long run is new in cycling, it is not a new idea in sports. In fact, I originally learned of this idea that shoes that can increase performance can also cause long term overuse injuries while running track in high school.
In the track world we all knew that even though your racing spikes made you run faster, you didn’t train in them. They were for race day and high performance practices (which were rare) because the same things that those shoes did to give you a short term performance increase on race day would tear you down eventually if you used them too much in training.
In other words, just because something made us faster didn’t make it “better” and understanding how to juggle what would allow us to train injury free with what would make use faster was part of the game. The truth is that having a super stiff sole on a riding shoe is not only unnecessary, it is potentially harmful if used exclusively over the long run. Find a pedal and shoe system (flats and 5-10′s work real well) for your everyday riding that allows your foot to move freely and drive directly into the pedal interface itself and save the stiff soled shoes for specific racing/ performance applications.