Friction Drive is a kind of motor that instead of a chain uses 2 wheels on the engine to spin the main wheel. This kind of engine is often used on scooters, mainly go-peds, in place of a chain. Friction drive units provide very smooth motion as the mechanism is designed to virtually eliminate mechanical backlash and hysteresis.

In a friction-drive mechanism the friction elements are mostly like cylindrical, conically tapered or globoid wheels which are pressed together so firmly that the frictional force developed at this point or line of contact transmits power. The power that can be transmitted in this way will depend on the magnitude of the contact pressure and the coefficient of friction of the surfaces in contact.

In every friction drive a small amount of slip develops between the friction elements: i.e., at the point of contact the driven wheel always has a slightly lower circumferential velocity than the driving wheel. The drive is therefore not a fully positive one. Such forms of drive have the advantage that the effective radius can be varied quite simply by a shifting of the point of contact of the friction wheels toward or away from the axis of rotation, so that infinitely variable control over the transmission ratio is obtained.

We can say the simplest form of friction-wheel drive, cylindrically shaped wheels which roll against each other. The resilience of the (often rubber-covered) wheels, spring pressure or weights produces the necessary contact pressure and thus develops the friction through which power is transmitted. Some Examples are record-player drive, tape-recorder drive. Every powered traction wheel of a road vehicle or rail-mounted vehicle is in effect a friction drive. The contact pressure due to the weight of the vehicle enables the wheel to develop friction and thus get a grip on the road or rail surface. This combination is comparable to a rack and pinion, the wheel being the pinion and the road or rail the rack.

In Fig.1 we can see a friction drive that is similar in construction to a planetary gear and is of the type used for the tuning mechanism on some radio receivers. The balls correspond to the planet wheels; they roll on the inner shaft, which corresponds to the sun wheel. When the fine-adjustment knob mounted on the inner shaft is rotated, the cage formed by the hollow shaft will rotate at a lower speed and thus make possible precision tuning.

A friction drive present for infinitely variable speed control comprising two conically tapered friction elements and an intermediate ring is illustrated in Fig.2. When the ring is in the right-hand position, the upper shaft will rotate faster than the lower shaft, which is the driving shaft, because in this position the driving radius R is larger than the driven radius r.

When the ring is being moved to the left-hand position, the driving radius r will be smaller than the driven radius R, so that now the upper shaft will rotate more slowly than the lower shaft. Fig.3 shows a friction drive comprising a large flat wheel and a small friction wheel which can be slid to different positions on the driving shaft on which it is mounted.