The term unicycle is often used in robotics and control theory to mean a generalised cart or car moving in a two-dimensional world; these are also often called "unicycle-like" or "unicycle-type" vehicles. This usage is distinct from the literal sense of "one wheeled robot bicycle".
These theoretical vehicles are typically shown as having two parallel driven wheels, one mounted on each side of their centre, and (presumably) some sort of offset castor to maintain balance; although in general they could be any vehicle capable of simultaneous arbitrary rotation and translation. An alternative realization uses a single driven wheel with steering, and a pair of idler wheels to give balance and allow a steering torque to be applied.
A physically realisable unicycle, in this sense, is a nonholonomic system. This is a system in which a return to the original internal (wheel) configuration does not guarantee return to the original system (unicycle) position. In other words, the system outcome is path-dependent.
A unicycle is a vehicle that touches the ground with only one wheel. The most common variation has a frame with a saddle, and has a pedal-driven direct-drive. A two speed hub is commercially available for faster unicycling. Unicycling is practiced professionally in circuses, by street performers, in festivals, and as a hobby. Unicycles have also been used to create new sports such as unicycle hockey. In recent years, unicycles have also been used in mountain unicycling, an activity similar to mountain biking or trials.
An axle or axletree is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to the vehicle, with the wheels rotating around the axle. In the former case, bearings or bushings are provided at the mounting points where the axle is supported. In the latter case, a bearing or bushing sits inside a central hole in the wheel to allow the wheel or gear to rotate around the axle. Sometimes, especially on bicycles, the latter type axle is referred to as a spindle.
A tricycle, sometimes abbreviated to trike, is a human-powered three-wheeled vehicle.
An electric unicycle is a self-balancing personal transporter with a single wheel. The rider controls speed by leaning forwards or backwards, and steers by twisting or tilting the unit side to side. The self-balancing mechanism uses accelerometers, gyroscopes, and a magnetometer. In 2020, suspension models were introduced by three major manufacturers Begode, Kingsong and Inmotion.
Steering is a system of components, linkages, and other parts that allows a driver to control the direction of the vehicle.
A single-track vehicle is a vehicle that leaves a single ground track as it moves forward. Single-track vehicles usually have little or no lateral stability when stationary but develop it when moving forward or controlled. In the case of wheeled vehicles, the front and rear wheel usually follow slightly different paths when turning or when out of alignment.
A traction control system (TCS), also known as ASR, is typically a secondary function of the electronic stability control (ESC) on production motor vehicles, designed to prevent loss of traction of the driven road wheels. TCS is activated when throttle input and engine power and torque transfer are mismatched to the road surface conditions.
A gyrocar is a two-wheeled automobile. The difference between a bicycle or motorcycle and a gyrocar is that in a bike, dynamic balance is provided by the rider, and in some cases by the geometry and mass distribution of the bike itself, and the gyroscopic effects from the wheels. Steering a motorcycle is done by precessing the front wheel. In a gyrocar, balance was provided by one or more gyroscopes, and in one example, connected to two pendulums by a rack and pinion.
A differential wheeled robot is a mobile robot whose movement is based on two separately driven wheels placed on either side of the robot body. It can thus change its direction by varying the relative rate of rotation of its wheels and hence does not require an additional steering motion. Robots with such a drive typically have one or more castor wheels to prevent the vehicle from tilting.
A monowheel, or uniwheel, is a one-wheeled single-track vehicle similar to a unicycle.
A power steering is a mechanical device equipped on a motor vehicle that helps drivers steer the vehicle by reducing steering effort needed to turn the steering wheel, making it easier for the vehicle to turn or maneuver at lower speeds.
A drive wheel is a wheel of a motor vehicle that transmits force, transforming torque into tractive force from the tires to the road, causing the vehicle to move. The powertrain delivers enough torque to the wheel to overcome stationary forces, resulting in the vehicle moving forwards or backwards.
A beam axle, rigid axle or solid axle is a dependent suspension design in which a set of wheels is connected laterally by a single beam or shaft. Beam axles were once commonly used at the rear wheels of a vehicle, but historically they have also been used as front axles in four-wheel-drive vehicles. In most automobiles, beam axles have been replaced with front and rear independent suspensions.
Eight-wheel drive, often notated as 8WD or 8×8, is a drivetrain configuration that allows all eight wheels of an eight-wheeled vehicle to be drive wheels simultaneously. Unlike four-wheel drive drivetrains, the configuration is largely confined to heavy-duty off-road and military vehicles, such as all-terrain vehicles, armored vehicles, and prime movers. Other types of smaller 8x8 vehicles include such things as the Argocat.
A Braitenberg vehicle is a concept conceived in a thought experiment by the Italian-Austrian cyberneticist Valentino Braitenberg. The book models the animal world in a minimalistic and constructive way, from simple reactive behaviours through the simplest vehicles, to the formation of concepts, spatial behaviour, and generation of ideas.
The Uno is a novel self-balancing electric motorcycle using two wheels side by side. The Uno III adds a third wheel that allows it to transform into a tricycle.
The Honda U3-X is an experimental self-balancing one-wheeled personal transporter shown in 2009.
The following outline is provided as an overview of and topical guide to bicycles:
A pentacycle is a human-powered five-wheeled vehicle.
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