20 May 2009

Ergonomic Revolutions

Six meters per pedal-turn. This is the gearing-specification which I would use to design a single gear bike. Building this today is a piece of cake that only requires basic skills of multiplication and division. Let your wheel be of the twenty inch diameter class, which yields (depending on the tire) about 1.6m in circumference. Add a chainwheel with 52 teeth and a 14 teeth sprocket and there you go: 5.96m of forward motion per one pedal turn! (If you think small is beautiful, go with a 42/11 combination at 6.13m of forward motion per pedal turn.)
And all this is made possible by John Starley's 1885 invention of the bicycle chain! Before the chain drive was invented you would need an almost two meter high wheel to mount a pair of pedals on, so that you move your six meters per revolution. But sitting on such a wheel you wouldn't reach the pedals anymore since they are over one meter away from the seat!
Most Hi-Bi's of Starley's time only had about 1.5m high wheels which was very dangerous for the riders sitting on top of them, but only delivered 4.7m per turn. Starley's "Safety Bicycle" with a chain drive was thus not only safer, but also more comfortable to ride! And, since the bike's engine is a human, being more ergonomic, also made it faster.

Fast forward 124 years. I am riding a twenty-inch bike with 8 internal gears. With a 42/14 tooth combination my neutral gear (which happens to be the fifth) takes me forward 4.85m on each turn of the pedals. If I would ever like to do a ride without shifting at all, my sixth gear gives me just about the perfect six meters of development (that's what we call the forward motion per one full pedal turn).

Interestingly, when I ride in the city, I am only using my gears number four to seven. That's four gears who have an overall transmission ratio of 167%. The surprise here is that this is just about the same ratio introduced by the three-speed gear hubs invented almost exactly one hundred years ago. Unlike modern bicycle buyers who just want the highest number of gears and the most sporty (or sexy) looking bike they can get, the inventors of one hundred years ago apparently thought very well about what was needed for best performance and just built that: Three gears, evenly spaced, 186% from lowest to highest. And those gear hubs were so well-built that many of them are still running Today. Especially in Toronto, you can see a lot of good, old bikes with those hubs on them.

Today, there are only a few people like my friend Kate, who actually know what they need and buy a brand new bike with just those three internal gears. Simple, reliable, ergonomic.

The revolution has happened a long time ago. Anybody joining in?


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