Can I achieve good cycling shoe stiffness in winter with a boot insert? Background: I ride daily in a very cold climate (Fairbanks, AK). My typical winter riding footwear includes a pair of trail running shoes underneath an insulated (NEOS) overboot, as this is a very warm and  light combination. However, on longer rides or endurance races, I find that I notice significant strain on my calves and Achilles tendons, much more so than similar efforts when wearing cycling shoes. I assume this is due to the lack of stiffness in my footwear. I am sure I lose power transfer efficiency in my pedal stroke for the same reason.

I am considering adding some type of stiff insert (e.g., plastic) in between the shoe and the overboot to address these problems. So here is my question, in two parts:


From an anatomical and physiological perspective, is maximum stiffness of the sole the ideal situation for a cycling shoe in terms of comfort and injury prevention, or is the shape of that stiffness-providing structure equally important (i.e., I realize cycling shoes are not flat pieces of carbon or plastic, but they conform more to the shape of the foot). 
From a  physics perspective, will a stiff footbed between the shoe and overboot provide similar power transfer to a cycling shoe, assuming there is minimal movement between the three components of the footwear system?


Note: I do not wish to switch to clipless pedals/shoes on this bike. Even with 45NRTH Wolvhammers or the like, most winter cyclists in my area still require aftermarket boot covers for long, cold rides.

Another note: I am going to experiment with inserts either way, but was interested to see what folks on here think about it.
It's not a very good idea, but since you're going to do it anyway, I'll give you some pointers on what to expect.


The contact area between your foot and the surface of the boot / pedal system needs to somehow conform to your foot, or you will have painful hot spots. In your idea, the sole of the running shoe should take care of that, and the insert can be indeed flat. The outsoles of running shoes are curved, though, so your stiffener would have to conform to that shape or you'd need a lot of room inside your overboots.
The shock absorption in running shoes absorbs a bit of energy on each pedal stroke, feels mushy and doesn't really offer any benefits for riding. A plate between running shoe and outer shoe wouldn't help here. This is why cycling shoes usually have thin anatomically shaped soles.


My suggestion would be mountaineering boots or hiking boots with rigid soles combined with large platform pedals. These boots are very stiff, don't have too much shock absorption and are designed for low temperatures.

As a side note, thin plastic plate isn't going to be very rigid. The stiffness of a plate is related to 3rd power its thickness, so in order to have stiff soles you'll need either a very thick plate or 3D structure. Because of the exponential relationship, using stronger materials doesn't help much.