Join Date: Jan 2007
Location: Silicon Valley, CA.
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Although they look like rigid structures, wheels are dynamic in that their spoke tensions change and the rim deflects from circular with each load cycle. Thus, the spokes do move (albeit on a very small scale) and rub against each other during normal service. Over a very long time, the rub marks show up as notches on the spokes. The wheel does not need to be overloaded nor does the spoke tension need to be below a certain range; correctly built and spec'd wheels still exhibit this phenomenon.
Thankfully, these notches do not limit the life of the wheel. Quality, correctly-installed spokes had been reported to give service exceeding tens of thousands of kilometers, over many repacking of the hubs and over multiple replacements of the rims. In other words, don't worry about it.
Check out Jobst Brandt's The Bicycle Wheel
for a lengthy and thorough discourse on spoked wheels.