The invention of the wheel is commonly considered trivial, the source of jokes about cavemen carving stone tires. However, it was not the round tire that was the trick, it was the axle bearing.

It is virtually impossible to make a bearing from wood alone. Even with liberal use of grease, turning wood against wood at high speed is a good way to invent fire, not the wheel!

To build carts, such as the Sumerians, Egyptians and Romans used, the bearings must employ metal. The first image shows the remains of iron journal bearings from a Roman racing chariot — a challenging problem requiring light-weight tires and durable low-friction bearings. By Roman times, carriages had spoked wheels, iron bearings and shock-absorbing suspensions. This technology remained in use up until the days of the Conestoga wagons of the American West.

Modern bearing technology is remarkably sophisticated, including hard ferro-alloys, synthetic lubricants and precision machining impossible even a few decades ago. The most sophisticated bearings do not even allow the axle and sleeve to touch at all — such as the gas-wave bearing or the magnetic bearing. These are essential in devices like gyroscopic inertial guidance systems, where gyro wheels may turn at 60,000 rpm.