A friction drive is a simple, mechanical transmission in which the face of a large, primary drive wheel is mounted perpendicular to the face of a smaller, secondary wheel or disk. The drive wheel, typically connected directly to the engine, turns the secondary wheel that is connected to the drive shaft. If the secondary wheel can be moved linearly across the face of the primary wheel, its velocity can be increased proportionally as it approaches the center of the primary wheel.
As opposed to a standard transmission with sprockets or gears, a friction drive has an infinite number of potential gear ratios while using very few transmission components. Since the transmission reverses direction as the secondary wheel passes the center of the primary wheel, it has as many potential reverse speeds as forward speeds. Vehicle speed can also be increased with no corresponding increase in engine speed, making it one of the smoothest and easiest-to-operate transmissions ever invented.
While the friction drive enjoyed a brief popularity among automobile pioneers, its lack of power and limited applications made it unpopular with mainstream designers. Due to slippage of the secondary wheel, a friction drive transmission is notoriously inefficient, especially as torque increases. Although it requires no clutch or oil bath, the secondary wheel endures substantial wear and has to be resurfaced often. In heavy or high-speed applications, the friction between the two wheels can generate substantial heat, requiring a cooling mechanism. Their practicality, therefore, was limited to fairly lightweight vehicles and smaller machinery.
Today, a variant on the friction drive is the continuously variable transmission (CVT). While they all lack set gearing, CVTs take many forms, including variable diameter pulley, toroidal, and hydrostatic transmissions. While the technology is complex, CVTs are friction drives at heart and are used to power snowmobiles, grain combines, and many Formula 500 racing cars.
In the most basic sense, a friction drive may also refer to a primary drive wheel, which runs parallel to and powers a secondary wheel. This simple but ubiquitous drive mechanism is seen on scooters and some motorcycles, and a number of friction drive kits are available commercially for converting bicycles into mopeds. The kit is composed of a knurled drive wheel hooked to a small gas motor which provides power to either wheel of the bike.
Leonardo Da Vinci commonly is credited with establishing the principles behind the friction drive, but he never got it patented. The patent was awarded in 1904 to American John William Lambert, who introduced the Lambert Automobile. The following year, fellow American Byron J. Carter used a friction drive in the production of the Cartercar, the “Car of a Thousand Speeds.