A wheel is basically a round piece of, in our case mostly aluminum, tied with force to another piece that makes turning possible. The way the wheel turns (and keeps turning) is depending on the inner piece, called a hub. The way it can withstand forces from outside – or from your crazy riding – is greatly depending on the outer piece. Yes, you probably guessed right, a rim.
In other words, the strength of a wheel is mostly determined by the rim. Rims come in all different shapes and sizes, each with their intended aim of use and strengths. However, the rim itself is only one part of the story of strength. What’s probably more important is the way this rim is supported. A shield without someone to hold it up, is basically an inefficient doormat – I believe it was Sun Tzu who said that, not a 100% sure though…
Anyways, what I’m trying to say: the strength and stiffness are mostly a result of how a rim is tied with spokes to a hub. First, there should be enough tension. It will be an egg after the first speed bump if tension is low. However, a common misconception is that, the higher the tension, the stronger and stiffer the wheel. This is not true; there should be enough tension. When this threshold of tension is reached, the strength and stiffness, and durability, comes down to the way this tension is divided over all the spokes. To explain why this is so important, we should first have a quick look at how a wheel is built.
To build a wheel, one simply tightens every spoke in turn a little bit, until you reach a certain tension. What one will notice however, is that, especially when you reach higher tensions, the rim will not be perfectly round and straight anymore. It’s simply impossible to tighten every spoke just as much, rims aren’t flawless and hole drillings in rim and hub, as well as spoke lengths aren’t perfect down to the nanometers – oh in a perfect world…. So, some spokes need a little more tension than others to keep the rim round and straight. In the meanwhile, the rim needs to be in the center of a hub, and as the hub in mountain biking isn’t symmetrical, the spokes on one side need to be tighter than the other. These are factors that should be considered constantly from an early stage, and if you do, you end up with a high (or sufficiently) tensioned, round and straight wheel.
What I described for you above, is basically what a machine does as well. The spokes have enough tension to hold the rim in place, and the spoke tensions are adjusted so that it is straight as well. But now imagine we take these wheels for a spin on our local trails. We make a little jump and land on a root or rock. The impact this has on the wheel must be absorbed by that part of the rim and the spokes that are holding it in place. Now imagine, that the spokes that are absorbing the blow all have different amounts of tension. At first, after completing the wheel, this was no problem. Even with highly different spoke tensions, the wheel can still be straight and round. However, when dealt with a blow, the more loose spokes will absorb significantly more added tension by the blow than the higher tension spokes, simply because they can. When this happens enough, the looser spokes will become fed up with having to do all the hard work, and, without forming a labor union (because spokes simply don’t have this capability, they’re spokes…) they go on strike. As a result, they become even more loose, now being so loose they can even break, or just make your wheel go out of true (not straight or round anymore). In addition to this risk of losing trueness, and thus strength, even when it still was round, it wasn’t as stiff as to when the spoke tension was evenly divided over all the spokes.
So this is what a good wheel builder does. Besides from constantly making sure the rim doesn’t go to the right or left, OR up or down, AND that it stays in the middle of the hub, you also try to evenly divide this tension over all the spokes. As you can imagine, a lot of the times these are contradictory actions, what makes it an attentive and painstaking task. A wheel is like a story of which you have to try to remember every character in order to make sense of the whole.
However, it’s certainly no magic. You just have to take your time and do it well. And that’s exactly the added value of a hand built wheel. The time was taken to not just construct a well-shaped wheel, but also a well-constructed one, for you to enjoy for years to come. If it rhymes, it’s probably true…