A friction wheel (also called a drive tire or kicker wheel) is a motorized wheel that alters or sustains the speed of a train through friction between the surface of the wheel and the ride vehicle.
Friction wheels can be orientated horizontally or vertically. When orientated horizontally, friction wheels are used in pairs and typically grip a fin on the underside of the train, like on friction brakes. However, some roller coasters use horizontal friction wheels that contact either side of the train. When orientated vertically, friction wheels contact the underside of the train.
Friction wheels are often used to move a stationary train at low speed. Within stations and brake runs, friction wheels are used to push the train. Bolliger & Mabillard roller coasters feature horizontal friction wheels before the lift hill to synchronise the speed of the train with that of the chain, allowing it to enter the lift hill smoothly.
In wet conditions friction between the wheels and the train is reduced which could cause the train to slip.
Friction wheels are often used on smaller roller coasters to power the lift hill. Chain lift hills are almost always straight whereas friction wheels allow the lift hill to be curved easily.
Typically each wheel or each pair of wheels is powered by its own motor. However, Schwarzkopf roller coasters used a single motor which transferred motive power to each wheel via cardan shafts.
Friction wheels can be used to launch a train. The wheels can be powered by either hydraulic or electric motors. Schwarzkopf's Bullet roller coaster introduced in the early 1980s is thought to be the first to use friction wheels to launch a train. The technology was used again on the Incredible Hulk at Universal's Islands of Adventure, which opened in 1999. Both take advantage of the technology, with Bullet launching the train in two directions and round a curve, and Incredible Hulk launching the train up a steep incline.
Intamin first used friction wheels to launch a train on Galaxy Express 999 which opened in 2005 and subsequently used the technology to power its Family Launch Coaster line.
Japanese roller coaster manufacturer Meisho Amusement Machines has used horizontal friction wheels to grip the sides of the train and slow it down. Friction wheels were used on Bullet to slow down as well as launch the train.
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