To understand how a wood rim functions, we need to talk about density and the stiffness of shapes and materials. A bicycle rim resists bending according to the stiffness of the given material and shape. However, material near the rim’s exterior does most of the work. Why? When the rim bends, this exterior undergoes the greatest deformation. For example, with a bend to the left, compression is felt on the left and tension on the right. These forces are greatest on the surface, furthest from the rim centerline. As it bends, the magnitudes of compression and stretching are greatest on the surface and this area puts up the greatest resistance. If the rim were solid, material in the center would barely detect the bending. For every degree of bend, internal deformation is smaller than that on the surface.
This principle favors tubes, whose mass is concentrated in their perimeter, far from the centerline. Therefore, hollow shapes are efficient and we certainly get our money’s worth from the metals. Composites, likewise, end up imitating metals to produce efficient structures. How does a wood rim resist bending forces? After all, it’s a solid that, according to the previous evidence, makes an arguably inefficient structure. Wood is much lighter than metals or composites, and this low density is what it leverages as a wheel rim.
- carbon fiber = 1.7
- aluminum = 2.7
- wood (beech) = .7
This is a huge difference, so wood is going to make a very different rim.