Prototype 3 a success
Prototype 3 was our first implementation of the asymmetric ovate footpath. The pointed apex at the front of the teardrop used in Prototype 2 was replaced with the rounder shape from the stouter ellipse experiment. The slope of the drive phase was also made shallower. A linear track was substituted for the swing arm pivot to create a mechanically simpler reciprocating motion.
The Prototype 3 design started life as a 2D parametric simulation , using the hybrid footpath from the Prototype 2 experiments as a guide. Various parameters were altered interactively until a footpath very close to the traced footpath was achieved.
The resulting design used parts cannibalised from the previous prototype. This was a quick and dirty proof-of-concept build, so the plans were just a list of cut and shut instructions to convert what we already had into what was needed. Initially we tried using fixed foot platforms, but soon realised that pivoting pedals were required to get rid of the hiccup.
The design employed a linear track with pivoting spiders on rollers that traveled up and down the central track. These were connected to one end of a set of stirrups. At the other end, the stirrups were connected to opposing ends of the crank arms. The pedals hung below these two points on the stirrups.
Multiple pedal positions were tested to find an optimum pedal placment.
This mechanism was a vast improvement over the previous designs. It had all the desired attributes we required, the leap, the high foot position during the recovery phase and of course, it was non-impact.
However, the Q-factor (horizontal distance between pedals) was far too wide. As you can see above, the feet are splayed out from the hips. The legs also rubbed against these stirrups.
The scrappy quick and dirty Prototype 3 had been a success. It captured the key elements of running, however the stirrup design and the geometry needed considerable refinement for the mechanism to be viable training device for runners.