A wheel is a rotating component (typically circular in shape) intended to turn on an axle bearing. The wheel is one of the key 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 transportation, supporting loads, and performing work in machines. Wheels are also used for other purposes, such as a ship's wheel, steering wheel, potter's wheel, and flywheel. Common examples can be found in transport applications. A wheel reduces friction by facilitating motion by rolling together with the use of axles. For a wheel to rotate, a moment must be applied to the wheel about its axis, either by gravity or by the application of another external force or torque.
Terminology The English word wheel comes from the Old English word hwēol, from Proto-Germanic *hwehwlaz, from Proto-Indo-European *kwékwlos, an extended form of the root *kwel- 'to revolve, move around'. Cognates within Indo-European include Icelandic hjól 'wheel, tyre', Greek κύκλος kúklos, and Sanskrit chakra, the last two both meaning 'circle' or 'wheel'.
History The wheel undoubtedly evolved from pre-historic use of rollers (round tree logs) to move heavier objects such as rock slabs and bigger logs. The invention of the solid wooden wheel (disk on axle) dates to the late Neolithic and may be seen in conjunction with other technological advances that gave rise to the early Bronze Age. This implies the passage of several wheel-less millennia even after the invention of agriculture and of pottery, during the pre-pottery Neolithic.
4500–3300 BCE (Copper Age): invention of the potter's wheel; earliest solid wooden wheels (disks with a hole for the axle); earliest wheeled vehicles 3300–2200 BCE (Early Bronze Age) 2200–1550 BCE (Middle Bronze Age): invention of the spoked wheel and the chariot; domestication of the horse
The Halaf culture of 6500–5100 BCE is sometimes credited with the earliest depiction of a wheeled vehicle, but there is no evidence that Halafians used wheeled vehicles or pottery wheels. Potter's wheels are thought to have been used in the 4th millennium BCE in the Middle East. The oldest surviving example of a potter's wheel was thought to be one found in Ur (modern Iraq) dating to approximately 3100 BCE. However, a potter's wheel found in western Ukraine, of the Cucuteni–Trypillia culture, dates to the middle of the 5th millennium BCE which predates the earliest use of the potter's wheel in Mesopotamia. Wheels of uncertain dates have been found in the Indus Valley civilization of the late 4th millennium BCE covering areas of modern India and Pakistan. The oldest indirect evidence of wheeled movement was found in the form of miniature clay wheels north of the Black Sea before 4000 BCE. From the middle of the 4th millennium BCE onward, the evidence is condensed throughout Europe in the form of toy cars, depictions, or ruts, with the oldest find in Northern Germany dating back to around 3400 BCE. In Mesopotamia, depictions of wheeled wagons found on clay tablet pictographs at the Eanna district of Uruk, in the Sumerian civilization are dated to c. 3500–3350 BCE. In the second half of the 4th millennium BCE, evidence of wheeled vehicles appeared near-simultaneously in the Northern (Maykop culture) and South Caucasus and Eastern Europe (Cucuteni–Trypillia culture).
Depictions of a wheeled vehicle appeared between 3631 and 3380 BCE in the Bronocice clay pot excavated in a Funnelbeaker culture settlement in southern Poland. In nearby Olszanica, a 2.2 m wide door was constructed for wagon entry; this barn was 40 m long with three doors, dated to 5000 BCE, and belonged to the Neolithic Linear Pottery culture. Surviving evidence of a wheel-axle combination, from Stare Gmajne near Ljubljana in Slovenia (wooden Ljubljana Marshes Wheel), is dated within two standard deviations to 3340–3030 BCE, the axle to 3360–3045 BCE. Two types of early Neolithic European wheel and axle are known: a circumalpine type of wagon construction (the wheel and axle rotate together, as in Ljubljana Marshes Wheel), and that of the Baden culture in Hungary (axle does not rotate). They both are dated to c. 3200–3000 BCE. Some historians believe that there was a diffusion of the wheeled vehicle from the Near East to Europe around the mid-4th millennium BCE.
Early wheels were simple wooden disks with a hole for the axle. Some of the earliest wheels were made from horizontal slices of tree trunks. Because of the uneven structure of wood, a wheel made from a horizontal slice of a tree trunk will tend to be inferior to one made from rounded pieces of longitudinal boards. The spoked wheel was invented more recently and allowed the construction of lighter and swifter vehicles. The earliest known examples of wooden spoked wheels are from the Sintashta culture, dating to c. 2000 BCE (Krivoye Lake). 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 BCE. In China, wheel tracks dating to around 2200 BCE have been found at Pingliangtai, a site of the Longshan Culture. Similar tracks were also found at Yanshi, a city of the Erlitou culture, dating to around 1700 BCE. The earliest evidence of spoked wheels in China comes from Qinghai, in the form of two wheel hubs from a site dated between 2000 and 1500 BCE. Wheeled vehicles were introduced to China from the west. In Britain, a large wooden wheel, measuring about 1 m (3.3 ft) in diameter, was uncovered at the Must Farm site in East Anglia in 2016. The specimen, dating from 1,100 to 800 BCE, represents the earliest and most complete of its type found in Britain. The wheel's hub is also present. A horse's spine found nearby suggests the wheel may have been part of a horse-drawn cart. The wheel was found in a settlement built on stilts over a wetland, indicating that the settlement had some link to dry land.
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