For this weeks Workshop Wednesday, we’re going to look at why spokes break and how correct spoke tension is the most important element in building a wheel that will stand up to the continual stresses and strains placed upon it.
The strength of a wheel comes mostly from the spokes being tightened to the correct tension, and that tension being as evenly distributed across all the spokes in the wheel. The rim also contributes to the strength of a wheel, but to a lesser extent than correct spoke tension. This is why the skill and quality of the wheelbuilder is far more important than the quality of the parts. Anyone can build a wheel using the highest quality components available, but if they fail to understand what gives a wheel its strength, then even the most expensive hand built wheels can quickly resemble a Pringle and fail.
Most people think that wheels fail due to an impact breaking a spoke. However in reality when your wheel buckles and you inspect the spokes, 9 times out of 10 they’re all intact. This is because wheels buckle when one of more spokes lose tension as a result of an impact.
Spokes usually break when you’re just riding along as a result of fatigue. If your spokes are under tensioned, then each time the wheel rotates, the spokes are flexed as the load changes. This continual flexing of the spokes caused by insufficient tension is what breaks a spoke. A wheel which has covered 1,250 miles has been subjected to a million of these load changes, this means a typical road wheel with 28 spokes has been subjected to 28 million load changes! Therefore although the spokes may have started out as perfect fit, they begin to move and flex, and more and more as time passes, until eventually they will fail. The most telling sign of an under tensioned wheel is one which has had many spoke breakages.
The solution then lies in the wheel being built with the correct spoke tension. If your spokes are tensioned correctly, then the spoke doesn’t move as the wheel rotates, meaning they last much longer. Thus a correctly tensioned wheel is one that will last often well after the rim has worn out.
Spoke tensions can be roughly measued using the Park Tool TM-1 (seen above), to check that the spoke tensions are equal between spokes, before checking absolute spoke tension with the more accurate Sapim spoke tensionmeter and making any fine adjustments thereafter. Although it is important to achieve equal spoke tension around the wheel, due to imperfections in the rim, there will usually be a very slight difference between the absolute tensions of each spoke.
If you want to learn more about hand built wheels, how to lace them and the processes required to achieve a well balanced wheel, then the book, “The Art of Wheelbuilding” by Gerd Schraner is worth a read. It also contains much of the content for the Cytech Level 2 wheel building module.