So anyways, a little while ago I started to notice that the rear wheel on my daily commuter bike was out of true (had a slight wobble for the non-bike geeks out there). No problem I've trued it (straightened it by adjusting the tension on the spokes) before so no big deal. Well when I went to true it this is what I found.
So for those non-bike geeks out there that's a view of my rim and tire. Now here's a little bike terminology lesson. How a wheel is built: you have the hub -I hope you already know what that is, but it's the center where the axel rides in bearings. The rim and tire -the rim is laced to the hub via spokes. The spokes pass through holes in the hub and have a head on one end so they cannot go all the way through the little holes in the hub. Out at the rim the spoke is threaded. It screws into these little brass bits called nipples that are too big to go all the way through the holes in the rim -part of them does stick out of the rim, it's that part that you turn to tighten or loosen a single spoke. Now if your rim has a wobble to the right you'd want to tighten the spokes that go to the left side of the hub in the region of the wobble and possibly loosen the ones that go to the right (if they're significantly tighter than the ones that go to the left). It's sort of an art because there's a real feel to knowing when it's too tight or a little lose.
ANYWAYS, as you can see in the photos of the rim the nipples are pulling through the rim, the metal (it's an aluminum alloy) is ripping and the holes are elongating. So now if you thought the above discussion was geeky, I've only just begun. So being a nerd by trade I started to wonder how many load cycles those spokes had seen, because aluminum unlike steel has an endurance limit. An endurance limit for metals means that no matter how small the load there is some finite number of cycles that will make it fail (break). So this is where I wax really, and I mean really geekie. I'm just forewarning you. So the wheel size on my bike is 26 inches, this is actually measured to the outermost diameter of the wheel and tire combined. That’s the diameter so the circumference of a circle as I’m sure you all know is C=2*pi*r, where pi is a constant you know the one 3.14159265… (it’s an infinitely long number that doesn’t repeat so I think 8 significant digits to the right of the decimal place is sufficient…) r is the radius of the circle. Therefore for those of you without your calculators handy the circumference of my wheels is… 81.68 inches or 6 ft 9 inches. So every 6 ft that I travel on my bike each spoke is loaded and then the load is relaxed one time (assuming that it isn’t loaded when it’s on the opposite side relative to the reaction force from the ground on my tire, but I haven’t modeled that behavior yet so I will conservatively estimate that it’s only loaded once per revolution and not twice… Currently I travel about 6 miles per day by bike, in Sunnyvale it was about 12 miles per day. At BYU it was like 2 miles per day. Now lets assume that I bike on average of 4 days a week all year long (most of the time I ride every day so that should make up for holidays and vacations and what not). Therefore, over the last 8 years of primarily bike commuting I’ve ridden about 8,700 miles, which is approximately 46.4 million ft. Which means that each of those spokes and consequently each of those holes in the rim has been loaded some 6.8 million times. WOW! I know that is not near as many cycles as I would have expected it to last either. I was thinking like 20 or 30 million at least. So let's postulate some reasons for the low number...
a) It's loaded more than once per revolution
b) I bike more than I think
c) The holes in the rim act as stress concentration factors and reduce the number of cycles that can be attained.
Hum... Yeah I'd better stop or I'll have the longest post we've ever had. Sorry but it’s been on my mind and I had to share it. And trust me Carmen’s heard enough of this kind of stuff already…
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