We then dug deeper into user experience. Since most ellipticals are rear driven (the linkage that spins is in the back) we looked at different four bar linkage combinations. There is a great four bar simulator here: http://www.mekanizmalar.com/fourbar01.html
We realized that since we were committed to front driven ellipse that the foot angle would be opposite what the foot is naturally inclined to angle at each position in motion. We created a platform sitting on a tension spring to act as foot angle adjustment throughout motion. Think of this like how the pedal spins on the bicycle crank arm to allow your foot to assume the most natural angle.
After looking at research from an Australian journal Sports Technology:
Use of an asymmetric ovate footpath to overcome the
mechanical constraints imposed by non-impact cross-
training devices in high-intensity exercise
William H.P. Thomasa
& Steven Cranitcha
We modeled what we wanted our elliptical path to be to best simulate running. We made an analysis looking at ranges for body size, stride length etc for both men and women and calculated a ellipse that accommodated at least 80%. We are still deciding how much adjustability should be built into the product. A minimum criteria of at least 18 inch stride length and ideally more than 20 inches was established. The design also shifted the foot platform location farther back on the linkage instead of at the crank arm creating a more elliptical foot path.
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