January 03, 2008 1 Comment
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.
This is a huge difference, so wood is going to make a very different rim.
Because wood is so light, its resistance to bending is necessarily less than metals. Compared to the other materials, wood needs more frequent spoke support. So, we use traditional spoke numbers like 32 and 36 per wheel. In fact, wood's long reign as premier high performance rim is a major reason for these particular spoke counts. Even three decades after switching to aluminum alloys wheel makers retained these numbers. In the face of aerodynamic evidence, spoke numbers have come down dramatically. However, research shows that the wind resistance of larger spoke numbers only becomes a liability at high speeds rare outside of competition.
So, given more spoke support, what kind of wheel does this solid but very light material make? First, the lower spoke tensions that wood prefers allow it to move around more. This additional degree of motion allows it to absorb shock, to attenuate the vibrations of the road; the same as a lower pressure tire. But the actual deflection of a wood rim during riding is tiny, so the bicycle's quickness is not impaired. What seems to disappear are the higher frequency vibrations of pavement that can tire the body over time and make joints ache. An aluminum rim, built to lower tension, would also move around. Unfortunately, aluminum does not absorb energy to the degree of other materials like steel, wood or composites. So the comfort benefit would be small.
In addition to shock absorption, wood is harder to dent. Its low density means that a pot hole will create only local damage: a nick rather than a generalized dent that might interfere with braking. So, wood rims are legendary for resisting dents; a valuable asset in a world of poorly paved roads. One further advantage is the heat resistance of wood. Rim braking dumps large amounts of heat into the brake caliper and rim, in order to slow the vehicle. Aluminum rims eagerly accept this heat which, when excessive, can melt the tire or tire cement, causing failures. Wood rims refuse to accept this heat preferring, instead, to burn superficially at their surface. A wood rim pushed to braking extremes will create a barely detectable burning odor, but its tires remain cool. The flip side of this tendency is higher heat that brake pads see. Unable to hand off the heat to the wood rim, traditional brake pads will melt on wood. This characteristic can be managed.
On first glance, the thermal characteristics of wood seem similar to carbon fiber: neither readily accepting heat. However, the similarity is superficial. A carbon rim accepts heat slowly, a wood rim nearly not at all. During a demanding descent, brake pads can feel overheated with either material, but slowly and relentlessly the carbon rim becomes hotter and hotter. It dissipates the heat too slowly, so can reach melting temperatures. Wood, on the other hand, might burn a bit on the surface but as a bicycle rim will not reach elevated temperatures. Bottom line, no rim material is ideal for braking. Aluminum or carbon, wood or magnesium, dealing with thousands of watts and trying to protect an inflated tire is a tough and hazardous job.
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