The Spoked Wheel

A spoked wheel, as seen on a bicycle, is a familiar sight to all. You've used them, but perhaps you haven't thought much about how they work. It may not be what you expect.  

Let's think it through: The weight of the bike and rider (the red arrow) needs to get to the ground. The most obvious path for the that load would be though the spoke below (red). We would say that red spoke is compressed, pressed in from either end. But that spoke is very thin, and very long compared to it's thickness. If you think about compressing anything similar, such as a thin strip of paper, you know that it immediately buckles out to the side without taking much force at all. This is true of the spoke as well. So that must not be how it works.

Actually, the weight is hanging from the top of the wheel. The blue spoke is holding it up by tension. It is being pulled at each end. Even your thin strip of paper can stand quite a bit of force pulling at each end. Now pulling down on the top of the wheel like this would tend to squash it into a flatter oval, but the green spokes are there to hold the sides in; they, too, are in tension. The wheel is a tension structure, with the top spokes under the greatest tension, the sides under less, and the bottom carrying very little load at all.