It’s funny how, at every step in the design process, issues of different magnitudes look like they’ll break the whole project. For example:
On the proof-of-concept alpha prototypes, we had problems with friction in the tension system. Combinations of high friction coefficients and too many lacing points meant that adjusting the shoe into the aggressive state almost always left slack somewhere in the tension system. While climbing, this slack would redistribute and the whole shoe would relax. So we minimized the lacing points and spent a great deal of time making bearings to provide a low-friction surface for the tension system to run through.
On the beta prototypes, the friction in the system has been decreased enough that tightening and loosening the shoe is quick and the tension is distributed evenly. This is great progress. However, only once the system had been optimized for low friction, did it become clear that there may be an issue with how the shoe is locked into a certain state. We’ve been using ladder locks on webbing to secure the shoe state, and the first time I climbed in the beta prototypes, the locks slipped, allowing the shoe to relax. Same failure as with the alpha prototypes, but a different cause.
After inspecting the ladder locks in different states on the shoe, it became clear that because of the curvature of the shoe side, tightening the tension system put the ladder lock on the heel, resulting in a self-unlocking position.
I shortened the travel of the ladder lock so that it remained on the side of the shoe and this appeared to resolve the slippage issue. However, after testing these prototypes on longer duration climbs, it became apparent that there was still a very slow slippage rate. Estimating the force carried by the tension system for a person of my proportions and weight yielded a loading scenario of about 2 kN. The company which manufactured the ladder locks and webbing we’re using rates them for 440 lbs before they slip, which converts to about 1.96 kN.
I am a small person, so my characteristic load should be lower than most of the population. Therefore, load forces are almost always going to exceed the rated load for these ladder locks.
But this does not mean total failure! Note that the load forces should exceed the rating for these ladder locks. In fact, this is an excellent development, since now I have a metric to spec the locking mechanism for this tension system. And if there are no ladder locks which are small enough with high enough ratings, then I can always deliberately reintroduce friction into other parts of the system.