The Physics of Bicycles

The Physics of Bicycles 1

Spokes

Even the earliest bicycles used spokes of one sort or another. In fact, even in ancient times many chariots and animal-drawn carts used spokes. A spoked wheel can be made as strong as a solid one and have only a fraction of the weight.While early spoked wheels were almost always made out of wood, the bicycle wheels and spokes of today are made out steel or aluminum or occasionally more exotic materials such as carbon composite or ceramics.

Minimizing the weight of the wheels is extremely important in bicycle design. Why does weight matter? Each time you push the pedals, you have to accelerate the weight of the wheel both forward and around its center. In other words, the wheel undergoes angular and straight motion simultaneously. You can see this when you ride--the front tire of your bicycle rotates while it moves forward along with you and the bike.

Tangential & Radial Spokes

There are many different ways to spoke a bicycle wheel. Most bicycles have tangential spokes, meaning that the spokes do not connect from the hub to the rim in a straight line, but at an angle. There are many different patterns of tangential spokes. Occasionally bicycles will have completely radial spokes. These spokes go straight from the hub to the rim of the tire. Wheels typically have tangential spokes. The way in which the wheels are spoked determines how they will perform.

You can spoke the front wheel completely radially, but the rear wheel had better not be spoked radially. There is no way to convey the twist of the wheel out to the rim to drive you forward. Tangential spoking helps transmit the torque from the hub out to the tires.

Not only would a radially spoked rear wheel be less efficient than one spoked tangentially--it would be significantly weaker. A bicycle wheel needs to be able to handle a variety of forces. Besides holding up the weight of the cyclist, a wheel must withstand the forces of pedaling and braking and the jarring effects of the road surface. The benefit of radial spoking has to do with the stiffness of the wheel (less deformation makes the wheel slightly more efficient).

The Physics of Bicycles 2