Originally Posted by
donalson
outside of some very exotic wheels I've never seen a wheelset ridden on the road fail catastrophically... it's usually the spoke that goes and that is on the rear wheel drive side... MTB trail riding is another story and those tend to be failures because of landing on a wheel at to high of an angle, wheels are strong vertically but not nearly as strong torsionaly (which is one reason you can bend the rim back and forth by adjusting one spoke).
next... the spoke failure is VERY VERY VERY rarely anywhere but at the J end on the hub... bend a hanger back and forth and it work hardens, becomes brittle and eventually snaps, a similar thing happens when there isn't enough tension on the spoke, the J bend ever so slightly bends ever time it is brought into tension and then released, eventually this will fail... on much rarer ocasions a spoke can pull out of the rim, the rim cracks around the nipple area, or the hub flange itself will brake... all of these tend to be either bad materials, poor design, or overzealous wheel building tension
A spoke that breaks is a catastrophic failure of the spoke. Since the spoke is a the most important part of the wheel, that's also a catastrophic failure of the wheel. True the wheel probably won't fold in half but a single broken spoke has a dramatic impact on the rest of the spokes of the wheel. The fewer spokes, the worse the impact of the a single broken spoke.
Spokes do fail because of fatigue. How the fatigue develops at the bend can be due to a number of factors that aren't all related to the spoke tension. A wheel is a very dynamic thing. As you rotate a spoked wheel with weight applied to it, the rim deforms a little at the bottom of the cycle. While it is true that the rest of the wheel takes responds to the deformation, the spokes that are at the bottom of the wheel can detension enough to move slightly in the hub. If you are using a spoke that is 2.0mm to fit in a 2.3mm (and a little bit) hole at the hub, each spoke can move that 0.3mm. That puts a lot of strain on the j-bend. If you use a spoke with a thicker head, the spoke is more resistant to breaking and it fits tighter in the hole so the spoke can't move much on each cycle of the wheel.
Rim cracking and spoke pull out can be have two causes. If the spoke has too high a tension, the rim can crack. However, if the spoke has too low a tension, the spoke moves in the rim as well which can lead to spoke cracking and pull out. It analogous to your bending of the hanger except you are bending aluminum (usually) which won't tolerate nearly as many cycles before it breaks.