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A wheel is a circular component that rotates around an axle. It is used to transmit torque, support loads, and enable motion with reduced friction through rolling rather than sliding. In a wheel-and-axle system, the wheel acts as the rotating member and, together with the axle, changes the distribution of force, enabling easier movement and higher mechanical advantage in many applications.

History and development: The wheel first appears in the archaeological record around 3500 BCE in Mesopotamia, initially in pottery and then in wheeled vehicles. By about 3000 BCE wheeled carts spread across cultures, transforming transport and trade. Early wheels were solid wooden discs; later innovations added iron rims or steel tires for durability and wear resistance. Spoked wheels provided strength with lower weight, and wheels were made from increasingly varied materials, including metal and composites, to suit different loads and speeds.

Design and components: Modern wheels typically include a hub, spokes or a solid disc, a rim, and a bearing arrangement inside the hub. Tires or treads mounted on the rim enhance grip and ride quality. Wheels can be designed for various purposes, such as disc or drum brakes, and may feature different materials, including wood, steel, aluminum, or carbon fiber, depending on strength, weight, and cost requirements. The wheel-and-axle configuration is a classic simple machine that reduces friction and enables controlled motion, with the axle length and wheel diameter influencing speed, torque, and stability.

Applications and variants: Wheels are central to land vehicles such as bicycles, motorcycles, cars, and trains, as well as to machinery, cranes, and power systems where rotation is required. They also appear as flywheels in engines and as pulley wheels in belt drives, illustrating the broad utility of the wheel concept.