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June 18, 2007 3 Comments
Three wheelbuilding tips that are often overlooked:
(1) Set spoke elbows
Spokes will give maximum service life and strength if the elbow is firmly set against the hub flange during building. If not, spokes may appear to "loosen up" and their fatigue life may decrease, resulting in premature breakage. When a spoke is set loosely into a hub, it should lie about 15 deg. above the angle directly to the target rim hole. That's the amount that the spoke will need to be bent to conform to its position.
When designing a spoke there are two factors to consider: 1) strength, meaning reliability and fatigue life, and 2) hub fit. Unfortunately, some shapes that seem to improve hub fit can actually decrease strength and fatigue life. Therefore, there must be a compromise in the final spoke design to account for these two factors.
The spoke design is complicated by the fact that many hubs have different shapes and proportions. More importantly, even if all hubs were identical, there would still be two different ideal spoke shapes. The spoke whose elbow is on the inside of the flange would require a different shape from the spoke whose elbow is on the outside. The best compromise is a spoke whose elbow bend is "incomplete," creating the 15 deg angle mentioned above. This is best because spokes that are "overbent" compared to the hub fit, whose elbows are being opened out in the finished wheel, are more prone to fatigue failure. The secret is within the metal crystal structure, but well known to material science.
If a spoke does not fit a particular hub perfectly (ie. does not lie flat against the flange and aim directly at the rim), it can be "set" during the building process. If the spokes are not set, then the spokes will be held in place (partially set) by tension alone. But they will not fully conform to the hub flange. With full tension, the spokes will be flat against the flange but as the wheel is ridden, normal fluctuations in spoke tension will cause the spokes to flex at the elbow. This will shorten their fatigue life, resulting in breakage.
To set spoke elbows, try the smooth shaft of a screwdriver in the opening between two crossed spokes against the edge of the flange, over the outside spoke, under the inside spoke. Gently lever down to set both inside and outside elbows at the same time. With practice, this process will take only a few seconds.
(2) Prevent spoke loosening
Nipples can loosen from the vibration caused by regular hard riding. This is a more common problem with lighter rims, smaller and higher pressure tires, and rough roads or trails. Nipple loosening is an even more common problem in because the hollow section of modern rims prevents the inner tube or rim strip from pressing against the nipple head and holding it in place.
Building with high uniform spoke tension helps because any one spoke is less likely to reach very low tension during riding, and that is when loosening from vibration is most likely to occur. But this alone is not enough. If you imagine unwinding the threads inside the nipple you essentially have a long ramp at a slight angle. The tension on the spoke is constantly trying to pull the spoke down this ramp inside the nipple.
One answer is to use a mild thread locking compound. Wheelsmith SpokePrep™ is one such solution because it provides both lubrication and thread locking action. The locking action is just like the nylon ring in a locking nut (also called aircraft or nylok nuts). Since it is not an adhesive, future adjustments are easier and will not destroy its thread locking or anti corrosion properties. One complication: the spoke threads must be clean before applying this compound.
Gently crimping the nipple (first experiment on loose nipples and spokes) or using adhesives such as Loctite can provide the needed stability. Try the "after assembly" or "wicking" Loctite's, such as 220 or 290 (almost too strong). Linseed oil on spoke threads or tubular rim cement leaking onto the nipple heads also limit wheel loosening but these methods are best remembered, not used in the 21st century.
(3) Monitor tension
Spoke tension is one of the wheel’s most elusive properties. As a wheel supports various loads its spokes are constantly changing tension to absorb, distribute, transfer, and just plain endure the forces of riding. The base tension with which a wheel is built has a strong effect on its ultimate strength and longevity. Just remember, more is not better. Generally speaking, high (but not excessive) and uniform tension serves a wheel best. Now that tensiometers are commonplace, at least in bicycle shops, you can have your tensions measured. Don't leave such an important characteristic unsupervised.
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