Tuesday, March 2, 2010

CVT's Are Impractical For Bicycles

Jobst Brandt, a highly respected mechanical engineer with many accomplishments under his belt, wrote the following back in 2003. It was part of a series of FAQ's hosted on Sheldon Brown's website but borrowed from Rec.Bicycles.Tech. Take a few minutes to read this and let me know how your mode of thinking differs or agrees.

The reason I bring this up is because one or two folks, who as usual withhold their identities, discussed with me the inadequacy of the chain drive and derailleur in a previous post on spokeless wheels and how a simple infinitely variable chain less drive is the definite solution to its ills. Will this truly revolutionize the bicycle?
"The Continuously Variable Transmission (CVT) is the holy grail of many inventors who are not convinced that it is an impossibility. That is to say, the positive engagement, continuously variable transmission, that does not rely on friction, electrical, or hydraulic ratios but uses mechanical gearing, is not possible. By definition, continuously variable is analog while gears and chains are digital.

The CVT does not exist, and I am convinced it will not. If it were possible, railway locomotives, trucks, buses, and cars would long ago have used them. Strangely, it is in bicycling that the strongest believers of the concept reside... as if there were more money to be made in bicycles. In fact, the bicycle, with its enormously adaptable human motor, doesn't need a CVT. In addition, its low input speed and extremely high torque, make the bicycle an especially difficult gearing challenge. For this reason high performance bicycles use derailleur chain drive that is found practically nowhere else.

Non-gear CVT's, currently used elsewhere, have poorer efficiency than both planetary gears and derailleur chains. More importantly though, the low-speed high torque of bicycling would require transmissions that would weigh more than the bicycle, which makes them impractical."

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