The simple answer is that a bicycle has two wheels because this is the most efficient design. Bicycles with one wheel do exist, but they are not particularly efficient or safe to ride. One popular model of a single-wheeled bicycle looks more like a unicycle than a traditional bike. The rider sits on a platform that is supported by a single, large wheel beneath them. To make this type of bicycle work as well as possible, the wheels are usually very large and the frame is quite sturdy to compensate for the lack of a second wheel.
A more common variation on this design is called a penny-farthing or high-wheel bicycle. In this model, the large wheel is at the front and a much smaller wheel is at the back. To get this type of bicycle to work well, it needs to be very tall, and again it has to be quite sturdy since there is only one wheel supporting the rider’s weight.
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The classic two-wheel design we’re used to seeing on most bikes today is the result of thousands of years of trial and error. While there were many early design challenges with building bicycles, eventually people realized that two smaller wheels worked much more efficiently than single larger ones or one massive wheel. One benefit to using smaller wheels was that they allowed the frame to be made out of thinner tubing, which made it lighter and easier to maneuver.
These days, most people who ride bikes regularly prefer the classic two-wheel design because it is lighter, simpler, and more efficient than other types of single-wheeled or uni-wheeled bicycles. While there are still some rare exceptions (like the British Moulton bicycle), almost all bicycles have two wheels.