This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Jobst Brandt <firstname.lastname@example.org>
Date: Sat, 25 Jan 2003 13:49:06 -0800 (PST)
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.