May 24

May 24, 2004

Creations of a mad scientist?

Contrary to the advice of my technical advisors, and almost the entire online HPV community, I went ahead and wasted a ton of valuable time building a linear drive. Rather than peddle two cranks around in a big circle, a linear drive has your legs pumping back and fourth. The main advantage to a linear drive is possibly a reduction in the fairing area at the nose. This is because the legs go back and forth rather than around in a circle - thus, the fairing can be smaller at the front where you want it to cut into the wind. Other than the possible aerodynamic advantage, it's pretty hard to compete with the standard, tried and true, roundy-roundy chain and derailleurs 98% efficiency.

But I tried anyhow.

What I ended up with is a single length of chain that is pulled by a lever connected to a crank on a large radius pivot. When the peddle is pushed forward, it pulls the chain across the gear on the wheel and turns the wheel. Then a spring pulls the chain back across the freehub with the return of the peddles. Currently, I have both the right and left peddle connected together at the bottom bracket, but the idea was that I could re-create the entire drive on the left hand side with a double BMX free-hub. That way both legs would be independently driving the wheel. The rider could alternate leg strokes, or choose to pump them in unison if desired.

It's a fairly simple system and has many possible advantages over the roundy-roundy system:

1. Infinite gearing. To change gears on this linear drive, you simply slide the chain up or down the crank thereby changing the length of the lever. Your legs always move the same distance, but the amount of work on each stroke changes in relation to the lever length.
2. Potentially lower frontal area because your legs are down close to the ground. In my prototype, to keep things simple, I raised the pivot so high that this would not be an advantage, but there would probably be ways of accomplishing a long lever length without having to raise the pivot point so high.
3. More even power transfer through the length of a stroke. A standard chain wheel and crank has a sweet spot at about 90 degrees where something like 70% of your power goes. The linear drive is like a leg press machine - your power is evenly distributed along the entire stroke.
4. Possible increase in efficiency due to perfect chain alignment and fewer pulleys.

But, after a test ride and a long while just standing beside it staring at it I decided that to get it working the way it needs to would be a lot of work, and the gains at this point would probably not be worth the effort. First of all, my lever length is still not long enough for a 'high gear'. Secondly, the whole idea is to reduce the area at the front of the fairing which means I need to figure out some other way of making a long pivot without moving a bottom bracket way up into the air like I have (which defeats the whole purpose). Thirdly, to get both legs to alternate pumping, I need to duplicate the whole mess on the left hand side. And I'm not sure if there is such a thing as a hub with TWO freehubs. And lastly, if I do figure out a way to accomplish that nice long lever without raising the pivot way up, then I might not have any means to adjust the gearing from a high gear to a low gear. And a final lastly, I'm not sure those pawls in the freehub were meant to be used on every single stroke - they would probably wear out after a few hundred miles.

In the end, I figured that the old roundy-roundy cranks and chain ring works pretty good, and I can just buy really good quality parts rather than worrying about making them properly.


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