Prototype 2 epic fail
By now, we had spent a considerable amount of time and effort studying every aspect of the running gait. We used motion captures of trained athletes running on treadmills and traced their movement to find the motion paths of there feet.
Armed with these insights, and our analysis of Prototype 1, we felt we had a pretty clear understanding of the type of footpath our device needed to create. As you can see in the motion analysis above, it’s something akin to a sideways teardrop.
Prototype 2 employed a rather complicated reciprocating swing arm mechanism to recreate his footpath. It had the elevated foot position during the recovery phase taken from the analysis above, and the 12.5 degree slope for propulsion derived from our Prototype 1 analysis.
The net result was a mechanism that converted movement around this sideways teardrop shaped footpath into circular motion of the crank arms using the slope for propulsion. We were so confident with this design that we decided to fabricate two machines so we would have more available for testing.
Given the level of precision required to fabricate this design, we decided to get the RHS sections laser cut. This took a little longer and cost a bit more, but it saved a great deal of manual time and effort. Once again we used a local firm, Bents Engineering to machining the bearing housings, and linkages.
Unfortunately, the welded frames suffered from the usual warping issues, which were difficult to correct given the number of moving parts involved that needed to be realigned. In the end, one frame turned out perfectly, but the other had a slight knock in it.
After such a complex build, it was with great excitement and anticipation that we tested Prototype 2 for the first time. We were expecting that the elevated foot position during the foot recovery phase would be an improvement over the previous elliptical design. However, this design still relied on the slope of the footpath and the force of gravity on the load bearing foot for propulsion. This ultimately made the overall sensation of running on this device unsatisfactory. It simply felt like striding with a high foot recovery position. Even worse, the fixed foot platforms created an unnatural hiccup at initial contact. It was very distressing when we soon realised that on the face of it this approach felt worse than the ellipse. A key element that was still missing was the leap sensation, that feeling of having both feet off the ground.
This was a rather discouraging outcome after all the effort involved in the design and fabrication of this mechanically complicated and costly prototype.
Essentially, we had made the mistake of believing that the drive phase in the gate cycle should be solely responsible for propulsion – after all it is called drive for a reason. However, after some rather heated debate and analysis, we realised that in order for the double-float phase to occur and generate a leap sensation, the mechanism really needed some significant opposing force at toe-off to launch the rider into the air. As such, it was counterproductive to rely on the slope of the footpath and the force of gravity for propulsion if we wanted to achieve a leap sensation.
It was then that we had our ‘eureka moment’. Propulsion needed come from actively leaping and landing not passively from the force of gravity sliding the feet down a slope. Essentially we needed a footpath with minimal or zero slope that forced the rider to leap and land. A mechanism that converted the impact energy from landing into propulsion.
So we re-tasked Prototype 1 to quickly test this hypothesis. The planetary arms were cut significantly shorter to create a short stout ellipse, which was tested using a very shallow slope setting. Riding this modified elliptical mechanism we experienced the leap sensation for the first time. However the stride length was so short, it felt more like stationary marching than running. So while the overall experience was still unsatisfactory, it provided real clues as about the dynamics of running. Essentially running is as much about swing vs stance phase timing, as it is about the footpath.
We traced this new ellipse from the re-tasked Prototype 1 onto a piece of cardboard (TOP LEFT, ABOVE). Then we traced the back half of the Prototype 2 footpath and combined it with the front half of this new ellipse (BOTTOM RIGHT, ABOVE). We speculated that if we could create a new mechanism to produce this hybrid footpath, it would most likely provide both the leap sensation and the high foot recovery position similar to running. So we decided to cannibalise one of the Prototype 2 bikes to quickly test this hypothesis.