Debunk variable speed transmission invention?

Question

A friend describes his invention for a variable speed tranmission for bicycles, which involves a novel design for a sprocket wheel. This sprocket wheel has a flexible system of spring-like spokes so that it can have an eccentric axle, i.e. the axle is off center with the sprocket wheel. The theory is that the "effective lever arm length" is continuously adjustable. What's the best way to explain why this idea won't work?

Answer 1

Continuously variable transmissions almost always rely on static friction instead of intermeshing teeth since, obviously, you can't change the number of teeth on a gear or sprocket. (although people have tried......) Modern CVT's either use friction rollers or adjustable pulleys.

Having an eccentric axle wouldn't work, because at the end of the day, the chain still has to travel the *same distance* around the shaft, per rotation, in order to transmit work. Hence the location of the axle doesn't matter, only the size and/or the number of teeth in the sprocket.

Interestingly, Ellsworth Bicycles out of California make a bicycle with a CVT, if you are willing to shell out $3,000..
http://www.gizmag.com/go/6132/
http://www.ellsworthride.com/

Answer 2

Actually, variable speed transmission works. It's just that it's a really complicated thing to actually produce. However, a lot of hybrid cars use it nowadays (like the toyota prius). The problem with your friend's idea is that the number of spokes is fixed, meaning that the gear ratio is practically the same no matter what. It would be better if the number of spokes could be adjusted. The article below gives a clear picture of how CVT works.

Answer 3

The problem with this system is that the sprocket will always have the same number of teeth, so if you change its geometry, the chain won't fit the teeth.

That's not to say it's a bad idea, but you might need a belt drive or some other solution. Gear ratios are determined by number of teeth, not necessarily by the diameter of the sprocket.

Answer 4

I'd love to see a diagram, but as I understand it, 2 things can happen because of the eccentric axle:
1) The saddle will just sink to the floor once you sit on it, if the springs are not stiff enough.
2) You will get a really bumpy ride if the springs are stiff enough.

Answer 5

Maybe the easiest way to explain it to him is that even if he had a continuously variable transmission on a bike, nobody would buy it. Those things are notoriously unreliable, heavy and inefficient. Which is exactly what the average biker wants: to be slowed down by a gimmick.

Answer 6

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Answer 7

If it's a fixed number of spokes around the circumfrence, then nothing is variable. If he expect it to be variable, the chain would have to jump discontinuously between different spokes.