The part that usually gets left out
When people say the wheel changed the world, they usually picture the disc. Yet a disc lying on the ground does nothing. A useful wheeled vehicle needs a wheel, an axle to carry the load, and some way for the two to turn against each other without grinding themselves to pieces. The wheel and axle is better thought of as a pair of parts that must work together, and the second part is harder to get right than it looks.
No inventor can be named, and the dates of the first working combinations are uncertain. The earliest evidence suggests solid wooden wheels in use during the fourth millennium BCE. The details of the evidence are covered in the article on the earliest wheels. This piece concentrates on how the mechanism functions.
The two ways to build it
There are two basic arrangements.
Fixed axle. The axle is attached to the vehicle body and does not turn. Each wheel has a hole through its center and spins on the axle. Wheels can turn at different speeds, which helps on corners. The load is carried by the axle, and the wear occurs where the hole meets the axle.
Rotating axle. The wheels are fixed to the axle, and the axle turns with them under the vehicle, held in place by bearings or brackets. Both wheels turn at the same speed. This is simpler in some ways, but it causes trouble in turns: the outer wheel needs to travel farther than the inner wheel, so one of the wheels must slip or drag.
The Ljubljana Marshes find, which includes both wheel and axle, is often discussed in this connection. Its axle was found with the wheel, which is rare, but how the two were joined and which arrangement was used on early wagons in general is uncertain, because wood survives so poorly.
Why the combination is hard
Making the pieces is only the first step. Several problems need solutions.
- Friction. Wood rubbing on wood generates heat and wear. A dry axle squeals, burns and eventually breaks or wears loose. Animal fat, tallow and plant oils were used as lubricants in ancient times, and the need to grease axles appears in historical sources.
- Fit. The hole in the wheel has to be centered carefully and the axle round enough to turn. A hole that is slightly off will cause a wobble that shakes the whole vehicle.
- Load. The axle carries the full weight of the cart and takes sharp shocks from bumps. A thin axle snaps. A thick one adds friction and weight.
- Keeping the wheel on. The wheel must not slide off the end. Early builders used pegs, linchpins or caps to hold it in place.
- Material. Harder wood or metal can be used for the surfaces that rub, but metal was expensive and hard to shape in the early periods.
It is a reasonable guess that these problems explain why the wheel arrived relatively late. Many cultures had pottery, sledges and rollers long before they had a wheel and axle that worked reliably, though that is an inference and not a documented fact.
Improvements over time
Solutions developed slowly. Hubs, the thickened center of the wheel, gave the wheel a longer bearing surface and made it stronger. Metal fittings, such as iron sleeves in hubs and metal axle ends, appear in Roman vehicles and later European wagons, and cut down wear. Greasing continued for centuries.
Ball and roller bearings, which replace sliding with rolling, are a much later development. Sketches of the idea appear in Renaissance notebooks, and practical versions became widespread in the nineteenth century, particularly with bicycles. A bearing in this sense is simply any part that supports a rotating shaft, and the earliest forms were wooden holes.
The problem of the two wheels on a turn did not disappear. Railway wheels fixed to a shared axle are shaped with a slight cone to help them cope with curves, and car axles use a differential gear, a solution of the nineteenth and twentieth centuries.
Why the pair mattered beyond transport
The idea of a wheel turning on an axle is the basis of many machines. Ancient builders used it in pulleys, windlasses, capstans, and treadwheel cranes, where a rope wound around an axle lifts a load. The potter’s wheel, the water mill and the mill gear all depend on it. In the history of simple machines, the wheel and axle is classed with the lever and the inclined plane, a way of trading force for distance.
Social effects, handled with care
Claims about what the wheel did to society are often sweeping, and caution is wise. What the evidence supports is modest.
- Wheeled carts allowed heavier loads to be moved on suitable ground, which helped farming and building.
- Wagons and chariots show up in elite burials and in military scenes, suggesting they were symbols of status and power as well as tools.
- The need for firm tracks pushed communities toward building roads and bridges over time.
- Where draft animals or good tracks were missing, wheels offered less, and some societies relied on boats or pack animals instead.
The wheel did not make civilization by itself. Cities and long-distance trade also depended on writing, agriculture, metals and rivers. A wheel and axle is a good example of an invention whose value depends on other things already being in place.