Look up wheel in Wiktionary, the free dictionary.

Three wheels on an antique tricycle

A wheel is a circular component that is intended to rotate on an axle. The wheel is one of the main components of the wheel and axle which is one of the six simple machines. Wheels, in conjunction with axles, allow heavy objects to be moved easily facilitating movement or transportation while supporting a load, or performing labor in machines. Wheels are also used for other purposes, such as a ship's wheel, steering wheel and flywheel.

Common examples are found in transport applications. A wheel greatly reduces friction by facilitating motion by rolling together with the use of axles. In order for wheels to rotate, a moment needs to be applied to the wheel about its axis, either by way of gravity, or by the application of another external force or torque.

Etymology[link]

The English word wheel comes from the Old English word hweol, hweogol, from Proto-Germanic *hwehwlan, *hwegwlan, from Proto-Indo-European *k^wek^wlo-,^[1] an extended form of the root *k^wel- "to revolve, move around". Cognates within Indo-European include Greek κύκλος kýklos, "wheel", Sanskrit chakra, Old Church Slavonic kolo, all meaning "circle" or "wheel",^[2]

The Latin word rota is from the Proto-Indo-European *rotā-, the extended o-grade form of the root *ret- meaning "to roll, revolve".^[3]

History[link]

A depiction of an onager-drawn cart on the Sumerian "battle standard of Ur" (circa 2500 BC)

A figurine featuring the New World's independently invented wheel

Evidence of wheeled vehicles appears from the mid-4th millennium BC, near-simultaneously in Mesopotamia, the Northern Caucasus (Maykop culture) and Central Europe, so that the question of which culture originally invented the wheeled vehicle remains unresolved and under debate.

The earliest well-dated depiction of a wheeled vehicle (here a wagon—four wheels, two axles), is on the Bronocice pot, a ca. 3500–3350 BC clay pot excavated in a Funnelbeaker culture settlement in southern Poland.^[4]

The wheeled vehicle spread from the area of its first occurrence (Mesopotamia, Caucasus, Balkans, Central Europe) across Eurasia, reaching the Indus Valley by the 3rd millennium BC. During the 2nd millennium BC, the spoke-wheeled chariot spread at an increased pace, reaching both China and Scandinavia by 1200 BC. In China, the wheel was certainly present with the adoption of the chariot in ca. 1200 BC,^[5] although Barbieri-Low^[6] argues for earlier Chinese wheeled vehicles, circa 2000 BC.

Although they did not develop the wheel proper, the Olmec and certain other western hemisphere cultures seem to have approached it, as wheel-like worked stones have been found on objects identified as children's toys dating to about 1500 BC.^[7] It is thought that the primary obstacle to large-scale development of the wheel in the Western hemisphere was the absence of domesticated large animals which could be used to pull wheeled carriages. The closest relative of cattle present in Americas in pre-Columbian times, the American Bison, is difficult to domesticate and was never domesticated by Native Americans; several horse species existed until about 12,000 years ago, but ultimately went extinct.^[8] The only large animal that was domesticated in the Western hemisphere, the llama, did not spread far beyond the Andes by the time of the arrival of Columbus.

Early antiquity Nubians used wheels for spinning pottery and as water wheels.^[9][10] It is thought that Nubian waterwheels may have been ox-driven^[11] It is also known that Nubians used horse-driven chariots imported from Egypt.^[12]

The invention of the wheel thus falls in the late Neolithic, and may be seen in conjunction with other technological advances that gave rise to the early Bronze Age. Note that this implies the passage of several wheel-less millennia even after the invention of agriculture and of pottery:

• 9500–6500 BC: Aceramic Neolithic
• 6500–4500 BC: Ceramic Neolithic (Halafian)
• ca. 4500 BC: invention of the potter's wheel, beginning of the Chalcolithic (Ubaid period)
• 4500–3300 BC: Chalcolithic, earliest wheeled vehicles, domestication of the horse
• 3300–2200 BC: Early Bronze Age
• 2200–1550 BC: Middle Bronze Age, invention of the spoked wheel and the chariot

Wide usage of the wheel was probably delayed because smooth roads were needed for wheels to be effective. Carrying goods on the back would have been the preferred method of transportation over surfaces that contained many obstacles. The lack of developed roads prevented wide adoption of the wheel for transportation until well into the 20th century in less developed areas.

Early wheels were simple wooden disks with a hole for the axle. Because of the structure of wood, a horizontal slice of a tree trunk is not suitable, as it does not have the structural strength to support weight without collapsing; rounded pieces of longitudinal boards are required. The oldest known example of a wooden wheel and its axle were found in 2003 in the Ljubljana Marshes some 20 km south of Ljubljana, the capital of Slovenia. According to the radiocarbon dating, it is between 5,100 and 5,350 years old.^[13] It has a diameter of 72 centimetres (28 in) and has been made of ash wood, whereas its axle has been made of oak.^[14]

The spoked wheel was invented more recently, and allowed the construction of lighter and swifter vehicles. In the Harappan civilization of the Indus Valley and Northwestern India, we find toy-cart wheels made of clay with spokes painted or in relief,^[15] and the symbol of the spoked wheel in the script of the seals,^[16] already in the second half of the 3rd millennium BC. The earliest known examples of wooden spoked wheels are in the context of the Andronovo culture, dating to ca 2000 BC. Soon after this, horse cultures of the Caucasus region used horse-drawn spoked-wheel war chariots for the greater part of three centuries. They moved deep into the Greek peninsula where they joined with the existing Mediterranean peoples to give rise, eventually, to classical Greece after the breaking of Minoan dominance and consolidations led by pre-classical Sparta and Athens. Celtic chariots introduced an iron rim around the wheel in the 1st millennium BC. The spoked wheel was in continued use without major modification until the 1870s, when wire wheels and pneumatic tires were invented.^[17]

The invention of the wheel has also been important for technology in general, important applications including the water wheel, the cogwheel (see also antikythera mechanism), the spinning wheel, and the astrolabe or torquetum. More modern descendants of the wheel include the propeller, the jet engine, the flywheel (gyroscope) and the turbine.

Timeline[link]

• 

  Bronze Age disk wheel as depicted on the Standard of Ur (ca. 2500 BC)

• 

  Classical Greek four-spoked chariot-wheel (as a Linear B glyph), in use from the 15th century BC. Hittite and Egyptian chariots tended to have six spokes, Iron Age Assyrian ones eight.

• 

  Bronze Age "wheel pendants" of the Urnfield culture (ca. 1200 BC), found in Zürich (Swiss National Museum)

• 

  An Early Iron Age spoked wheel from Choqa Zanbil (ca. 1000 BC, National Museum of Iran)

• 

  Wheel of the Etruscan chariot (ca. 530 BC)

• 

  The classic spoked wheel with hub and iron rim, in use from about 500 BC (Iron Age Europe) until the 20th century AD

• 

  Penny-farthing bicycle (1882)

• 

  Modern motorcycle alloy wheel with inflatable tire and disc brake

• 

  Michelin's "Tweel" airless tyre (2005)

Mechanics and function[link]

The wheel is a device that enables efficient movement of an object across a surface where there is a force pressing the object to the surface. Common examples are a cart pulled by a horse, and the rollers on an aircraft flap mechanism.

Wheels are used in conjunction with axles, either the wheel turns on the axle, or the axle turns in the object body. 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 help reduce friction at the interface. In the simplest and oldest case the bearing is just a round hole through which the axle passes (a "plain bearing").

Example:

• If a 100 kg object is dragged for 10 m along a surface with the coefficient of friction μ = 0.5, the normal force is 981 N and the work done (required energy) is (work=force x distance) 981 × 0.5 × 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, for wood, μ = 0.25, 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 × 0.25 × 0.5 = 123 joules; the friction is reduced to 1/40 of that of dragging.

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

A wheel can also offer advantages in traversing irregular surfaces if the wheel radius is sufficiently large compared to the irregularities.

The wheel alone is not a machine, but when attached to an axle in conjunction with bearing, it forms the wheel and axle, one of the simple machines. A driven wheel is an example of a wheel and axle. Note that wheels pre-date driven wheels by about 6000 years.

Stability[link]

For unarticulated wheels, climbing obstacles will cause the body of the vehicle to rotate. If the rotation angle is too high, the vehicle will become statically unstable and tip over. At high speeds, a vehicle can become dynamically unstable, able to be tipped over by an obstacle smaller than its static stability limit. Without articulation, this can be an impossible position from which to recover.

For front-to-back stability, the maximum height of an obstacle which an unarticulated wheeled vehicle can climb is a function of the wheelbase and the horizontal and vertical position of the center of mass (CM).

The critical angle is the angle at which the center of mass of the vehicle begins to pass outside of the contact points of the wheels. Past the critical angle, the reaction forces at the wheels can no longer counteract the moment created by the vehicle's weight, and the vehicle will tip over. At the critical angle, the vehicle is marginally stable. The critical angle Failed to parse (Missing texvc executable; please see math/README to configure.): \theta_{crit}

can be found by solving the equation:

Failed to parse (Missing texvc executable; please see math/README to configure.): \theta_{crit} = \tan^{-1} \left ( \frac {x_{cm}} {y_{cm}} \right )

where

Failed to parse (Missing texvc executable; please see math/README to configure.): x_{cm}

is the horizontal distance (on level terrain) of the center of mass from the lower axle; and

Failed to parse (Missing texvc executable; please see math/README to configure.): y_{cm}

is the vertical distance (on level terrain) of the center of mass from lower axle.

The maximum height Failed to parse (Missing texvc executable; please see math/README to configure.): h

of an obstacle can thus be found by the equation:

Failed to parse (Missing texvc executable; please see math/README to configure.): \ h = w \sin \theta_{crit} = w \tan^{-1} \left ( \frac {x_{cm}} {y_{cm}} \right )

where Failed to parse (Missing texvc executable; please see math/README to configure.): w

is the wheelbase.

In the Unicode computer standard, the Dharmacakra is called the "Wheel of Dharma" and found in the eight-spoked form. It is represented as U+2638 (☸)

Construction[link]

Rim[link]

An aluminum alloy wheel

The rim is the "outer edge of a wheel, holding the tire."^[18] It makes up the outer circular design of the wheel on which the inside edge of the tire is mounted on vehicles such as automobiles.^[19] For example, on a bicycle wheel the rim is a large hoop attached to the outer ends of the spokes of the wheel that holds the tire and tube.

In the 1st millennium BC an iron rim was introduced around the wooden wheels of chariots.

Hub[link]

The hub is the center of the wheel, and typically houses a bearing, and is where the spokes meet.

A hubless wheel (also known as a rim-rider or centerless wheel) is a type of wheel with no center hub. More specifically, the hub is actually almost as big as the wheel itself. The axle is hollow, following the wheel at very close tolerances.

Spokes[link]

Rod[link]

A spoke is one of some number of rods radiating from the center of a wheel (the hub where the axle connects), connecting the hub with the round traction surface.

A spoked wheel on display at The National Museum of Iran, in Tehran. The wheel is dated to the late 2nd millennium BC and was excavated at Choqa Zanbil.

The remains of a pair of cartwheels with metal axle assembly.

An Ox-wagon in Aliwal North, South Africa. Note the three missing spokes and the metal tyre.

Metal spoke wheel from a bicycle.

The term originally referred to portions of a log which had been split lengthwise into four or six sections. The radial members of a wagon wheel were made by carving a spoke (from a log) into their finished shape. A spokeshave is a tool originally developed for this purpose. Eventually, the term spoke was more commonly applied to the finished product of the wheelwright's work, than to the materials he used.

Wire[link]

The rims of wire wheels (or "wire spoked wheels") are connected to their hubs by wire spokes. Although these wires are generally stiffer than a typical wire rope, they function mechanically the same as tensioned flexible wires, keeping the rim true while supporting applied loads.

Wire wheels are used on most bicycles and still used on many motorcycles. They were invented by aeronautical engineer George Cayley and first used in bicycles by James Starley. A process of assembling wire wheels is described as wheelbuilding.

A 1957 MGA Automobile with wire wheels

Tire[link]

Stacked and standing car tires

A tire (in American English and Canadian English) or tyre (in some Commonwealth Nations such as UK, and Australia) is a ring-shaped covering that fits around a wheel rim to protect it and enable better vehicle performance by providing a flexible cushion that absorbs shock while keeping the wheel in close contact with the ground. The word itself may be derived from the word "tie," which refers to the outer steel ring part of a wooden cart wheel that ties the wood segments together (see Etymology below).

The fundamental materials of modern tires are synthetic rubber, natural rubber, fabric and wire, along with other compound chemicals. They consist of a tread and a body. The tread provides traction while the body ensures support. Before rubber was invented, the first versions of tires were simply bands of metal that fitted around wooden wheels to prevent wear and tear. Today, the vast majority of tires are pneumatic inflatable structures, comprising a doughnut-shaped body of cords and wires encased in rubber and generally filled with compressed air to form an inflatable cushion. Pneumatic tires are used on many types of vehicles, such as cars, bicycles, motorcycles, trucks, earthmovers, and aircraft.

Alternatives[link]

While wheels are very widely used for ground transport, there are alternatives, some of which are suitable for terrain where wheels are ineffective. Alternative methods for ground transport without wheels (wheel-less transport) include:

• Being raised by electromagnetic energy (maglev train and other vehicles)
• Dragging with runners (sled) or without (travois)
• Being raised by air pressure (hovercraft)
• Riding an animal such as a horse
• Human powered:
  • Walking on one's own legs
  • Being carried (litter/sedan chair or stretcher)
• A walking machine
• Caterpillar tracks (although it is still operated by wheels)
• Pedrail wheels, using aspects of both wheel and caterpillar track
• Spheres, as used by Dyson vacuum cleaners and hamster balls

In semiotics[link]

The flag of India

The Romani flag

The flag of Mahl Kshatriyas

The wheel has also become a strong cultural and spiritual metaphor for a cycle or regular repetition (see chakra, reincarnation, Yin and Yang among others). As such and because of the difficult terrain, wheeled vehicles were forbidden in old Tibet.

The winged wheel is a symbol of progress, seen in many contexts including the coat of arms of Panama and the logo of the Ohio State Highway Patrol.

The introduction of spoked (chariot) wheels in the Middle Bronze Age appear to have carried somewhat of a prestige. The sun cross appears to have a significance in Bronze Age religion, replacing the earlier concept of a Solar barge with the more "modern" and technologically advanced solar chariot.

The wheel is also the prominent figure on the flag of India. The wheel in this case represents law (dharma). It also appears in the flag of the Romani people, hinting to their nomadic history and their Indian origins.

Gallery[link]

See also[link]

References[link]

Wikimedia Commons has media related to: Wheels

Wikimedia Commons has media related to: Automobile wheels

External links[link]

• The Oldest Wooden Wheel. Virtual exhibition. City Museum of Ljubljana. Accessed 31 January 2012.