Subject: The Continuously Variable Transmission
From: Jobst Brandt
Date: January 25, 2003
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.
[Continuously-variable transmissions have in fact been introduced by NuVinci. They are heavy compared with geared hubs, but by no means not as heavy as Brandt suggests. NuVinci hubs use an assembly of tilting spherical rollers between angled surfaces, and a special oil which resists slippage; nonetheless there is some slippage, and efficiency is somewhat lower than with geared hubs or derailer systems. The main advantage is the ability to "dial in" a pedaling cadence rather than select among sprockets or internal-hub gearing steps..-- John Allen]
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