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Spoke Over-Tension and Drifting Wheel Alignment
Last December I commuted 30 miles a day on an old Specialized Allez Sport
that increasingly suffered from cracked chain stays. As the bike fell apart, the cheap generic spokes broke frequently and the RX-100 components had degraded to near uselessness.With patience and greasy hands at the curbside, I was able to keep up with replacing the breaking spokes for several weeks, but with each broken spoke I got a close-up reality check on how fast the chain stays were giving way. Despite my crunch budget as a student, the specter of catastrophic frame failure was not only enough for me to keep my speed below 20 mph, but also to fork over $1500 at the LBS for a new bike. I depend upon the bike for transportation, and so this was the way to go. In January, I decided to buy a Redline Disc-R (disc-brake road bike) to replace my commuter/road bike and I walked into a LBS that was supposed to be receiving one in my size, and I told the staff I was there to buy. The shop promptly required a down payment, but I got a call later in the week and was told the bike was in the shop, assembled, tuned, and ready to roll. I showed up, saw the bike, and paid the balance. The sales person insisted on wheeling the bike back to the mechanics' shop for a "final check to be sure everything is adjusted right." Seconds later, the sales person rolled the bike back out to me on the showroom floor and announced the bike was all ready to go! I took the bike home, and the next day I returned the wheels to the LBS to have them re-tensioned because they were grossly out of true. Although with disc brakes the bike could function despite the wheel wobble, I was afraid that if I rode the wheels in that condition it would ruin them. The rear wheel actually had a 1/4" wobble. The mechanic was friendly, but dismissed the wheel wobble problem as a minor one. In a couple minutes, he made spoke adjustments that significantly reduced the wheel wobble, and he explained it was not necessary to re-tension the wheels because all the spokes were at a similar amount of tension. Four months later, while I sat on a park bench next to the bike, I heard a spoke snap. It seemed really strange for a spoke to break when the bike wasn't being ridden, but I recalled having just gone over a series of stutter bumps and I attributed thermal expansion of the rim to be the final straw since it was the first hot sunny day of the season. At the same time, the wheels had never been subjected to any hard collisions with curbs or potholes or anything like that, so I was concerned by the implications of this spontaneous failure. Coincidentally, I was only two blocks away from the shop where I purchased the bike. After buying the bike, in aversion to the original bright red finish and the many "steal-me" decals, I repainted the frame. When I wheeled it into the shop, nobody realized it was originally purchased there only a few months earlier. The shop's mechanic explained the spoke snapped because the spokes were badly tensioned and of cheap generic quality. The shop had no replacement spokes of correct length, and therefore sent me to a sister shop located several blocks away. At the sister shop, I got the same explanation: 'Bad work, bad parts.' The next day, I took the wheel to a different shop and learned the spokes are of brand quality (Sapim) but that they were all extremely over tightened. The mechanic at this shop expressed amazement that the wheel was surviving the high tension of the spokes. I asked him to re-tension the wheel, but the mechanic refused, saying the wheel was ruined and all he could do was replace the spoke and make some adjustments to minimize the hop caused by the wheel damage and help me get a few more miles before having to replace the wheel. I had truthfully told the mechanic that I'd never taken a spoke wrench to the wheel, nor hit anything with it, but the mechanic doubted my veracity. He insisted I must have hit something really hard with the wheel and then tried to fix it myself and caused further damage. Then he asked if I'd gotten the bike serviced since I bought it. I told him the truth and he decided that the spokes were too tight because I had not taken the bike in for a complete tune up after the first few hundred miles. I admit I privately disbelieved the statement that the spokes could have spontaneously self-tightened to the point of extreme tension, but the mechanic did provide convincing information that lead me to the following conclusions: One, the wheel had been poorly built because the spokes were way too tight. Two, this condition of excessive tension caused damage and resulted in reduced life expectancy for the wheel. Three, I'd likely have to replace the rim and spokes soon. As it has turned out, no more spokes have broken on this wheel in the 2,000 miles covered since the spontaneous spoke failure. Not all is hunky dory, however. Over time, it appears that the rim is actually drifting to the left, affecting the wheel alignment in the frame. When the bike was new, I observed the wheel was centered in the frame stays. Now, the bike has started to track a little bit to the right. Any ride of over 50 miles now causes a bit of pain on left side of my groin, as if I were sitting off-center over the nose of the saddle, and the bike no longer has the no-hands stability that is used to have. When I dismount and take a closer look, I can see things that don't add up right. The tire is visibly offset in the chain stays by an 1/8 of an inch; that is, the gap between the rim and the chain stay on the right is 1/4" greater than the gap on the left! At first, I was afraid the misalignment was a frame problem, but string-line measurements indicate the frame is actually pretty straight. Next, I was worried about the dropouts, and wondered if they are tweaked-out somehow. So I borrowed and installed a new wheel, just to see how the alignment looks with that, and observe no indications of alignment problems with the new wheel installed. Now, here is my theory: The spokes on the cassette side are the tightest, of course, since they are shorter to accommodate the dish of the wheel. In the alleged over-tensioned state, these spokes are stretching under the cyclic stressing of normal use. As the right-side spokes now yield to the excessive tension, the left-side spokes apparently are contracting as the yielding spokes effectively grow longer. The change in effective spoke length is enough to shift the rim to the left 1/8" and this is why the wheel is now 1/4" closer to the left chain stay, compared to its distance from the right chain stay. When I ride the bike, this offset of the rear wheel requires compensation by the direction of the front wheel or by the camber of tires exerted by leaning the bicycle slightly to the left. As such, the bike is generally tilted to the side and therefore I really do sit with extra pressure on the left side of my groin and the bike really is no longer as balanced and stable when underway. Comments? Recommendations? mykal in Seattle |
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