Bicycle spokes might appear to be simple gadgets that don’t need much attention, and overall this is true. But wire spokes do more than provide stems to clip on wheel reflectors or shine after a good polishing. Spokes that keep coming loose indicate certain problems that you can resolve either by yourself or by a professional, depending on the situation, by understanding the basics of bicycle spoke wheel construction.
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Wire spokes support the wheel rims of the bicycle. The inner ends fit into ports on the wheel hub, and the outer ends thread into spoke nuts at the rim. Spokes are designed to allow tension adjustments by tightening or loosening the spoke nuts. Looking at the spokes on a wheel, you'll notice that they consist of pairs that cross over in the middle, and that the right spoke nuts are offset from the left spoke nuts at the rim. The crossovers provide the necessary support to keep the wheel from collapsing. The offset nuts allow you to true the wheel, similar to aligning the front end of a vehicle.
Spokes can loosen from hitting bumps hard or from landings following jumps. If you hear tapping or clicking sounds coming from a wheel, you should reach for your spoke wrench and start jiggling to find the loose spoke. A common mistake among novice riders is tightening one spoke without considering the effect of tension, or pull, on spokes at the opposite side of the rim. If you tighten a spoke nut too much, tension pulls that portion of the wheel rim toward the hub, loosening spokes on the opposite side of the wheel. If you don't tighten the nut enough, the tension from the opposite spokes remains greater -- and the spoke nut loosens again as the novice continues to ride. Bicycle wheel builders use a tension meter to ensure the uniformity of spoke tension. But persistent tightening or sophisticated methods are ineffective if a wheel rim is damaged.
Dents and Twists
Spoke lengths are specific to the diameter of a wheel rim. A dent at one portion of a wheel rim can shorten the distance between the wheel and the hub, reducing the wheel diameter at that point. As a result, the spoke in the area of the dent is too long to be tightened. It’s rare that riding on rough terrain can impact a rim in a way that causes it to twist, but it can happen. Dents are relatively easy to spot. A twisted rim might be out of true by as little as two mm, which is difficult to notice, but is enough to affect the specific length of spokes. Attempting to tighten excessively loose spokes can strip the threaded end or the threads in a spoke nut. Experienced riders turn to the services of a qualified wheel builder to resolve more serious problems with spokes that keep coming loose.
Wheel truing is a precision craft that requires expertise. A good wheel builder can true a wheel in a relatively short time. A novice might spend hours without being able to get a wheel in exact true. To true a wheel, the tire is removed and the wheel is secured on a truing stand that has dropouts similar to those on front forks or the rear frame arms of a bike. Wheel builders understand that spokes cross over and pull the rim from opposite sides. Rotating the wheel slowly allows the builder to locate dents and adjust the appropriate spokes with a spoke wrench. A special alignment gauge on the stand is positioned across the outer surface of the rim. Measuring the clearance between the gauge and the rim reveals any twists. The final step is tightening each spoke to the recommended tension with a spoke wrench and a tension meter.