NYC Velo’s Wheelbuilding Program with Spaceman Bill

Hey man, why so tense?

With so many budget machine-assembled wheels and high-end lightweight wheel systems on the market today, one has to wonder: Why invest in a custom hand-built wheel? Well, there are several reasons, and building a custom wheel allows the mechanic to control a number of variables throughout the assembly and tuning process. Some wheels need to be stronger and have more spokes than an off-the-shelf pre-assembled model, such as for a cargo or touring bike. Many pre-built wheel systems assume that the rider is heavier and rougher on components, but overbuilt wheels may not be necessary for lighter riders, or may be too stiff or harsh. Sometimes the desired performance characteristics of the wheel are not readily available in a pre-built model. Sometimes the color of rims and hubs need to be perfectly matched to the finish of the frame or to the whims of the rider. These concerns can all be addressed by individually building a wheel, one that’s designed to meet the rider’s needs.

There are many factors that go into building a spoked wheel, such as component (rim, hub, spokes, nipples) selection and spoke count, but tension is one of the most important. The tension of a wheel is crucial for longevity, because of the forces applied to the spokes on every revolution. As the wheel turns, every spoke is individually loaded and unloaded, a process referred to as a “fatigue cycle”, and many materials can undergo a finite amount of fatigue cycles before they fail. An example of which is a paper clip, if bent back and forth multiple times it will eventually break. A common misconception is that spokes break if they are tensioned too high, but in reality it is the opposite. Building a wheel with high tension limits the intensity of the fatigue cycles, and reduces the likelihood of spoke breakage.

With the importance of tension established, how do you know whether a wheel is under or over tensioned? There are several traditional methods, such as “feel” (givin’ it a squeeze) or “sound” (plucking the spoke to hear what note it plays). For the musically inclined or those with calibrated torque-wrench hands (don’t believe anyone if they tell you that) these might be fine, but for the rest of us, there is a much more scientific method available. A spoke tensiometer is a tool that measures the deflection, or how much a spoke bends, when force from a calibrated spring pushes against it. There are many different varieties and price ranges, but a tensiometer with a dial gauge provides a very exact and consistent measure of tension.

A tensiometer is the ideal way to make the tension of a spoke tangible, but where it excels is the ability to determine tension consistency across a selection of spokes, like those laced to the drive-side of a rear wheel. Surprisingly enough, a perfectly round and true wheel is no indication of consistent spoke tension. The tensioning of the spoked wheel is a delicate balancing act where spoke pairs do not need to be even close to identical tension to keep the wheel straight, and an under-tensioned spoke is more prone to breakage. Balancing the tension is only achievable with a tensiometer. Because a detailed tensioning process is rarely done by a wheel building robot, the careful execution of this process is just another reason why there is great value in a hand-built wheel.