The science of wheelbuilding: understanding the importance of spoke tension
There is an art to building a reliable wheel, and while most wheelbuilders are reluctant to share the secrets of their trade, there are some, like Zak Smiley, that will. In this article, Matt Wikstrom talks to Zak for some insight on what makes for a strong and reliable wheel build.
To many, the construction of a bicycle wheel seems complicated. Dozens of spokes must be perfectly ordered and balanced otherwise the wheel will suffer a horrible bias. However, there’s no need to contend with all the spokes at once. Building a wheel is an act of repetition, concentrating on one spoke at a time.
I’m not going to attempt to explain how a wheel is built—there are books for that—but the whole process can be broken down into four phases:
1. Assembly: spokes are inserted into the hub flanges according to the desired lacing pattern (radial, two-cross or three-cross patterns are the most common for road wheels) and attached to the rim with the nipples.
2. Tensioning: The nipples are gradually tightened in a step-wise manner to slowly and evenly increase spoke tension in the wheel. Wheelbuilders typically count the number of turns of the spoke key as they tension a wheel to ensure every spoke is tensioned to the same degree.
3. Rounding and truing: As each spoke is tensioned, the rim is pulled towards the centre of the hub, as well as in the direction of where the spoke originates. By using a wheel jig that measures the rim’s deviation from round and true, the wheelbuilder is able to adjust the tension on each spoke until essentially all deviation disappears.
4. Dishing: The dish of a wheel is a measure of how far the rim is displaced from the end of the hub axle and should be identical for both sides of the wheel. Adjusting the balance of spoke tension between the left and right sides of the wheel adjusts the dish of the wheel.
If the rim is round—perfectly round—and the hub has a perfectly centred axle with truly round flanges and precisely positioned spoke holes, then the wheel will be round and true once assembled with evenly tensioned spokes. However, there is no such thing as a perfect hub or rim, and minor deviations must be compensated for, either at the expense of the wheel’s shape or the evenness of spoke tension.
The wheelbuilder’s job is therefore one of managing compromise, striving to achieve a round and true wheel while preserving even spoke tension. It’s an iterative process that takes time and patience as the wheelbuilder works their way around the wheel, again and again, until a perfect balance is achieved.
The importance of spoke tension
Most riders will judge the quality of a wheel by how round and true it is, however, a wheelbuilder’s primary concern is spoke tension.
As a wheel turns, the spokes are loaded and unloaded and it is the repetition of this strain that takes its toll, especially at the head of the spoke. Under-tensioned spokes are especially susceptible to this strain. While there is nothing that can be done to avoid spoke fatigue per se, even spoke tension ensures that it develops at the same rate for every spoke.
Where once wheelbuilders had to rely on their senses to judge spoke tension, now there are sophisticated tension meters that provide an accurate measure. And according to Zak Smiley, the owner and operator of Skunkworks Bikes in Sydney, they have become indispensable for wheelbuilding.
“Using a meter to balance out the spoke tensions takes more time,” said Zak, “but it reduces the chance of the wheel coming out of true for no reason and ensures the wheel has increased stability over its lifespan.”
Zak uses a digital tension meter made by Wheel Fanatyk. The resolution of this meter affords him enormous precision that he uses to diligently tension the wheel. While it is not possible to eliminate all variation, he routinely achieves less than 10% variation for each side of the wheel (Figure 1).
A good spoke tension meter not only removes a lot of guesswork from wheelbuilding, Zak has also found it invaluable for diagnosing and remedying problem wheels. An example of one such wheel is shown in Figure 2 below. Not only was it over-tensioned, there was also too much variation in the spoke tension, so Zak set to work righting both. Over the course of an hour, he loosened then gradually re-tensioned the spokes while keeping an eye on the roundness and lateral true of the wheel.
Asymmetrical spoke tension: a special case
Most riders rarely have any problems with loose or broken spokes in the front wheel. By contrast, the rear wheel can suffer terribly, and the problem stems from its asymmetrical design.
Any conventional front wheel (i.e. non-disc-equipped) is symmetrical about its longitudinal axis. Spoke lengths are identical for both sides of the wheel, as is the final spoke tension (see Figure 1, left). In contrast, the freehub body spoils the symmetry of the rear wheel, displacing the right flange of the hub such that it is much closer to the middle of the hub. As a result, shorter spokes are required and extra tension develops as the wheel is dished (Figure 1, right).
Over the years, wheelbuilders have experimented with a variety of strategies to address the asymmetry of the rear wheel, such as different lacing patterns and offset rim designs, and while some have alleviated the difference, none have ever eliminated it. Wheelbuilders continue to argue the best approach but all agree that there must be even spoke tension on each side of the wheel.
The difference in spoke tension also has practical ramifications since the extra tension makes the drive-side spokes difficult to work on. More torque is required to turn the nipple, which can lead to spoke windup (the spoke twists rather than the nipple turning) and/or damage to the nipple. Lubrication of the nipples is crucial however Zak has found a way to build the rear wheel that makes it easier to contend with the extra tension on the drive side spokes.
Building from right to left
It is not strictly necessary to wind the drive side spokes up to a final tension of ~120kgf when a lot of tension can be added simply by dishing the wheel. So rather than tensioning both sides of the wheel at the same time, as is done for a front wheel, the non-drive spokes are left loose until the final stages of the build.
Zak has been using this approach since discovering it in Wheel Fanatyk’s library of articles.
“While there are many different and equally valid ways to build a wheel, this method has helped me to produce strong, well-balanced wheels,” said Zak. “Separating the drive side and non-drive side tensions is also helpful for streamlining the truing and repair of wheels that have been built elsewhere.
“The only major drawback of this approach is that it does not work for symmetric wheels so I revert to more traditional methods for building a front wheel.”
To start with, Zak laces the wheel as required and tightens all the nipples until the last thread disappears on each spoke. “Then I quickly use lots of turns of the spoke key to bring up the drive side tension to around 70-80% of my target (ie ~80kgf). This part is relatively quick and dirty as I’m not chasing the radial or lateral true at this stage.”
At this point, Zak uses his tension meter to make sure all the drive side spokes have the same exact tension.
“When the tension is exactly where I want it, I use the drive side spokes to obtain the radial true. I check the tensions as I go and make sure I don’t have any spokes pulling substantially more tension than their neighbours.”
In the next step of the process, Zak uses the non-drive spokes to centre the rim and equalise the dish of the wheel. Once again, he plays close attention to spoke tension, this time on the non-drive side of the wheel. Once the wheel is centred, he uses the non-drive spokes to true the wheel laterally.
“Using this system allows me to completely separate the radial and lateral true from the high tension in the drive side and the lower tension non-drive side. The added bonus of this technique is that there is no need to manhandle the drive side nipples.”
The whole process can take up to 90 minutes, where the final adjustments to tension, roundness and lateral true are the most labour-intensive. Zak uses a simple online app to plot the final spoke tension measurements, which he supplies his customers and keeps for his own records.
Wheel science versus art?
It has often been said that there is an art to building great wheels, but as can be seen above, a great deal of science has developed around the subject. Indeed, the construction of a wheel can be viewed solely as an engineering challenge, and provided the wheelbuilder has enough data on the challenge (i.e. the weight of the rider and expected power output), an appropriate structure can be designed and built to satisfy the demand.
However, to argue for science alone is to deny a place for imagination and creativity in the process. At the very least, there is an art to its execution, but I can also see that the development and refinement of wheelbuilding practices actually depend upon imagination and creativity.