Wheel

A wheel is a rotating part, usually circular, designed to turn on an axle bearing. It forms a key part of the wheel-and-axle system, one of the six simple machines. When paired with an axle, wheels make it easier to move heavy objects, aiding transportation, supporting loads, and doing work in machines. They also serve other purposes, such as in ship's wheels, steering wheels, potter's wheels, and flywheels.

Common examples appear in transport. A wheel reduces friction by rolling along with an axle. For a wheel to rotate, a force or torque must be applied around its axis, whether by gravity or another external source.

The English word *wheel* comes from Old English *hwēol*, tracing back through Proto-Germanic *hwehwlaz* to Proto-Indo-European *kwékwlos*, an extended form of the root *kwel-* meaning 'to revolve, move around'. Related words in other Indo-European languages include Icelandic *hjól* ('wheel, tyre'), Greek *κύκλος* (*kúklos*), and Sanskrit *chakra*, all meaning 'circle' or 'wheel'.

The wheel likely evolved from prehistoric use of rollers—round tree logs—to shift heavy objects like rock slabs and larger logs. The solid wooden wheel (a disk on an axle) appeared in the late Neolithic, linked to other advances that led to the early Bronze Age. This means many millennia passed after the invention of agriculture and pottery, during the pre-pottery Neolithic, before the wheel came into use.

During the Copper Age (4500–3300 BCE), the potter's wheel was invented, along with the earliest solid wooden wheels (disks with a hole for the axle) and the first wheeled vehicles. The Early Bronze Age (3300–2200 BCE) followed, then the Middle Bronze Age (2200–1550 BCE) saw the spoked wheel, the chariot, and the domestication of the horse.

The Halaf culture (6500–5100 BCE) is sometimes credited with the earliest depiction of a wheeled vehicle, but there is no evidence they used wheeled vehicles or potter's wheels. Potter's wheels are thought to have been used in the 4th millennium BCE in the Middle East. The oldest surviving potter's wheel was once believed to be from Ur (modern Iraq), dating to about 3100 BCE. However, a potter's wheel from western Ukraine, belonging to the Cucuteni–Trypillia culture, dates to the mid-5th millennium BCE, predating the earliest Mesopotamian use. Wheels of uncertain dates have been found in the Indus Valley civilization (late 4th millennium BCE), covering parts of modern India and Pakistan.

The oldest indirect evidence of wheeled movement comes from miniature clay wheels found north of the Black Sea, before 4000 BCE. From the mid-4th millennium BCE onward, evidence appears across Europe in the form of toy cars, depictions, or ruts, with the oldest find in Northern Germany dating to around 3400 BCE. In Mesopotamia, depictions of wheeled wagons on clay tablet pictographs from the Eanna district of Uruk (Sumerian civilization) are dated to about 3500–3350 BCE. In the second half of the 4th millennium BCE, evidence of wheeled vehicles appeared nearly simultaneously in the Northern and South Caucasus (Maykop culture) and Eastern Europe (Cucuteni–Trypillia culture).

A depiction of a wheeled vehicle appeared between 3631 and 3380 BCE on the Bronocice clay pot, excavated from a Funnelbeaker culture settlement in southern Poland. In nearby Olszanica, a 2.2-meter-wide door was built for wagon entry in a 40-meter-long barn with three doors, dated to 5000 BCE and belonging to the Neolithic Linear Pottery culture. Surviving evidence of a wheel-axle combination comes from Stare Gmajne near Ljubljana, Slovenia (the wooden Ljubljana Marshes Wheel), dated within two standard deviations to 3340–3030 BCE, with the axle dated to 3360–3045 BCE. Two types of early Neolithic European wheel and axle are known: a circumalpine type where the wheel and axle rotate together (as in the Ljubljana Marshes Wheel), and a type from the Baden culture in Hungary where the axle does not rotate. Both are dated to about 3200–3000 BCE. Some historians believe the wheeled vehicle spread 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 were made from horizontal slices of tree trunks. Because wood has an uneven structure, a wheel made from a horizontal slice tends to be inferior to one made from rounded pieces of longitudinal boards. The spoked wheel was invented later, allowing lighter and faster vehicles. The earliest known wooden spoked wheels come from the Sintashta culture, dating to about 2000 BCE (Krivoye Lake). Soon after, horse cultures of the Caucasus region used horse-drawn spoked-wheel war chariots for much of three centuries. They moved deep into the Greek peninsula, where they joined with existing Mediterranean peoples, eventually leading to classical Greece after the decline of Minoan dominance and consolidations 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 Longshan Culture site. Similar tracks were found at Yanshi, an Erlitou culture city, 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 about 1 meter in diameter was uncovered at the Must Farm site in East Anglia in 2016, dating from 1100 to 800 BCE.