Cyclists have been waiting a long time for this one. Based on a 1490s
sketch by Leonardo da Vinci, The Ride’s rear hub mimics an infinite
number of gears, rather than the mere 21 offered by the usual
chain-yanking transmission.

you can always find the perfect gear ratio, whether starting from a
stop or speeding down a hill.
Twist a dial on the handlebar, and ball bearings in the bike’s NuVinci
transmission tilt between two rotating metal discs. (Your pedaling
turns one disc; the other transfers power to the rear wheel.) As the
balls tilt, they touch the discs at varying angles. This changes how
fast the wheel spins relative to your pedaling—slowly for low gear
ratios, where pedaling is easy but the wheel doesn’t turn much, and
quickly for high ratios. The balls can roll to almost any angle, giving
you precise control over the bike’s torque (and your exertion).

This latest take on da Vinci’s continuous transmission has potential
uses beyond bikes. Within four years, expect to see the NuVinci in
cars, tractors, even wind turbines
—the possibilities are nearly as limitless as the gear ratios. 3,000;

Via Popular Science