Wheels

A wheel is a circular object that, together with an axle, allows low friction in motion by rolling. Common examples are found in transport applications. More generally the term is also used for other circular objects that rotate or turn, such as a ship's wheel and flywheel.

Mechanics and function
The wheel (with axle) is considered one of the simple machines and lies near the practical starting point of advanced human technology (advanced, that is, in comparison with even earlier mechanical innovations such as stone/bone knives and axes, tension-sprung projectiles, scoops and shovels, etc.,.). Since a wheel is a rigid object, it will only be non-rotating when all the torques on it are balanced. Since forces produce larger torques when they are closer to the axis, a wheel can be used to transform between large and small forces applied by friction with belts or other wheels. The combination of the wheel with the wedge produced the toothed gear, itself fundamental to the advent of industrial class complex machines. Other variations on wheels produce the pulley and the windlass.

When wheels are used in conjunction with axles, either the wheel turns on the axle or the axle turns in a vehicle (as in a cart) or a housing (as in a mill). The mechanics are the same in either case.

The low resistance to motion (compared to dragging) is explained as follows (refer to friction):
 * the normal force at the sliding interface is the same.
 * the sliding distance is reduced for a given distance of travel.
 * the coefficient of friction at the interface is usually lower.

Bearings are used to reduce friction at the interface.

Example:
 * If dragging a 100 kg. object for 10 m. along a surface with &mu; = 0.5, the normal force is 981 newtons and the work done (required energy) is (Work = Force x Distance) 981 &times; 0.5 &times; 10 = 4905 joules.
 * Now give the object 4 wheels. The normal force between the 4 wheels and axles is the same (in total) 981 N, assume &mu; = 0.1, and say the wheel diameter is 1000 mm and axle diameter is 50 mm. So while the object still moves 10 m the sliding frictional surfaces only slide over each other a distance of 0.5 m. The work done is 981 x 0.1 x 0.5 = 49 joules.

Additional energy is lost at the wheel to road interface. This is termed rolling resistance which is predominantly a deformation loss.

Wheeled vehicles
Vehicles can be classified according to number of wheels:


 * 1) Unicycle, monocycle
 * 2) Bicycle
 * 3) Tricycle
 * 4) Quadricycle