JANUARY 21, 2012

When Rims Find Glory

Rims get discarded when they're taco'd beyond repair in crashes, or simply put out to pasture along with bad hubs and neglected bikes, but most folks will go through their entire bicycling lives without ever wearing an actual rim down to the nub.

This is what the end of a rim's long life looks like: Due to brake-caused wear on the surface, the thickness of the rim's sidewall diminishes to the point that the air pressure begins to force it outward, and it cracks. (Rims can also crack around the spoke holes when the spokes are over-tightened, a different phenomenon.)


The first symptom of this disease, perhaps even prior to any actual crack, is the thump-thump-thump that can be felt under braking, when the pads go past the bulge in the rim. This can feel very similar to other types of rim damage, or a bad seam on a non-machined rim, so it's pretty easy to remain in denial. 

Soon, a hair-line crack, lengthwise in the middle of the sidewall, becomes visible to the naked eye. Nasty. There's no denying that. At this point the rim is kind of like a bomb ready to go off. With high pressure underneath, and the force of twisting and turning the wheel, the crack can break apart, allowing the tube to explode through it in a spectacular catastrophic failure. Obviously this can be extremely hazardous when cornering or descending at high speed, especially if it's the front rim. (The photo shows a rim about to go off like a bomb.)

If you want to push your luck and keep riding on your broken rim, as I did the other day in order to make it home, let air out of the tube to relieve the pressure as much as possible, then ride in an extremely gingerly fashion at no more than 10 mph. Try not to, like, turn.

"My rim just exploded." No it didn't. You wore it down to the nub, and it cracked and broke apart.

Regular dry rim-braking would take a very long time to wear through a rim sidewall. Riding in the rain, or the ice and snow, will wear these things out somewhere on the order of 10 times as fast.

To all those who have worn through rim sidewalls: I salute you.

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