How do air pressure and tire structure support a bicycle's load?

A bicycle tire contains mostly air, yet it can help hold up a bicycle and its rider. At the bottom, its round shape becomes a little flatter against the road. That small change of shape is a clue: support comes from air pressure working together with the tire's strong, flexible structure.

Air consists of tiny moving particles. They continually collide with the surfaces around them, producing pressure. Inside an inflated tire, that pressure acts in every direction. It pushes outward around the tire, not just downward toward the road. The surrounding air also pushes on the outside, but the pressure inside an inflated tire is greater.

The tire's casing contains strong threads covered with rubber. These threads resist being stretched, helping the tire keep its shape while allowing it to bend. The edges of the tire sit securely on the wheel's rim. Air pressure stretches the casing into tension, rather as the skin of an inflated ball becomes taut. Air, casing and rim work together; loose air without a container would simply spread out.

The rider's load reaches the wheel through the frame and axle. The tire deforms where it meets the ground, making a contact patch. The ground pushes upward there. Pressure forces and changes in the casing's shape and tension connect that upward support to the downward load on the wheel. For the same load and tire construction, lower inflation generally gives a larger, flatter contact region.

As the wheel rolls, successive parts of the tire enter and leave this flattened region. Its ability to bend lets the tire follow small bumps instead of acting like a solid stone wheel. The familiar rubber ring is therefore both a structure and an air container: something flexible can support a load because its shape, material and internal pressure cooperate.