The old idea box was looking a little thin and then two topics that merit discussion jumped up, so this week has a good mix of pretty rich editorial content which we think you'll enjoy.
The final part of the wheel build selection process is deciding how many spokes, and of what type. This is sort of the black art of wheel building, where experience helps out quite a bit.
How many spokes, to me, is the leading part of the question. My go-to phrase for this is that you shouldn't ask spoke gauge (thickness) to do what spoke count should do instead. There are times when you're painted into a corner - someone has a rim they really want to reuse with a new hub, or a hub they really want to reuse with a new rim, or they want to absolutely maximize front wheel aerodynamics, or they're just absolutely stuck on getting away with as few spokes as possible (a case where sometimes we recuse ourselves from the process - we don't want to ship time bombs). Absent factors like those, though, you want to decide about spoke count. It has the greatest effect on the wheel's stiffness, stability, ability to stay true, and ability to be ridden in a "what if" scenario, so it goes first.
The way we decide how many spokes anyone needs is... we have this awesome set of dice from an old backgammon set, and we roll them. No, no, no, just kidding. We take a look at a lot of factors from the rider - weight, intended use (race day wheels, bike packing, every day, etc), history (what's worked or not worked in the past is a great guide) - and that becomes one side of the equation.
Then we look primarily at the rim. To some degree the hub, but mostly the rim. First, you look at the ERD (effective rim diameter) of the rim, which is the inner diameter of the rim (actually not precisely but close enough). A relatively shallow 650b rim and a mid-depth (~38mm) 700c wheel will have the same ERD. And what we do with that is we multiply that by pi, and divide that by the number of spokes and get some sort of "distance between spokes" equivalency. So a rim with a bigger ERD using 28 spokes could have the same distance between spokes as a rim with a smaller ERD using 24 spokes. A 540 ERD rim (~All Road 50) with 24 spokes has a distance between spokes of 70mm, more or less, with a 590 ERD rim (~HED Belgium+ or Eroica) with 28 spokes has a distance between spokes of about 67mm. The 590 rim with 24 spokes has a distance between spokes of about 78. That gives us a great starting point.
Then we look at the rim depth. The shallow 650b rim and the mid-depth 700c rim will have quite different properties, even though they have the same ERD. They're also likely to have much different use patterns, but let's keep that aside for now. The thing we're mostly looking for here is compression - how much spoke tension drop will installing and inflating a tire cause? You can watch this little old video that we made to see what I'm talking about on that. Carbon rims in general can be made to compress a little less than alloy ones, but there is by absolutely no means a valid "carbon rims always compress less" statement. Nope.
A good example here would be comparing a Boyd Altamont with a Boyd Altamont Lite build. The Altamont has a 580 ERD, the Lite a 590 ERD. The Lite is 5mm shallower than the regular Altamont. The Lite has a somewhat greater distance between spokes for any given sake count, compresses more, and needs more support because it's less stiff of a rim. For a 140 or 150 or 160 pound person, the minimum 20/24 in a rim brake build is usually just fine. But when you get up into 175 or 185 rider weight, if we're building with Lites you're going to get a 24/28 recommendation where there's still plenty of running room left if that rider wants to stay 20/24 with Altamonts. A 24/28 set of Altamont Lites is still notably lighter (by almost 100g) than a 20/24 set of Altamonts, so for the person who wants "a light wheel for climbing" (which is like 93% of our inquiries) but is in that grey range there, the 24/28 Lite set is a wonderful choice. And a good climbing wheel is as much about handling torque as is it about being light - NEVER forget that, although no one thinks about it in the first place so it's not even there for them to forget in the first place.
Extra spokes can't help with spoke tension drop - the rim doesn't compress less because you have more spokes. But there's a "many hands make light work" effect that goes on. As the wheel goes round and round, the spokes at the bottom of the wheel get more slack and the other ones around the rim get more tight. It's hopefully a small effect, or else you've got big problems headed right to your door, but it's there and that, over time, is why non-drive side spokes break. Cycle fatigue. The more spokes you have, the more this effect "tapers" - the unloaded spokes unload less, the loaded spokes load less. Take an extreme case of an 8 spoke wheel, with paired spokes at 12, 3, 6, and 9 o'clock. When the wheel is loaded with bike and rider, the spokes at 6 o'clock are taking all of that unloading between just two spokes. The spokes at noon are getting bone tight. Then imagine the same wheel but with 20 spokes. More spokes in the "unloaded zone" will unload, but that amount will be distributed over more spokes, so each one feels less unloading. That's what that's all about.
The 8 spoke imaginary wheel also shows us about "unsupported span," Rotate that 8 spoke wheel 45*, so it's just unsupported rim at noon and 6. Put enough load on that rim and it's Taco Tuesday. Not that we or anyone else builds wheels to that extreme (well, Spinergy did), but the extremes highlight the effects in play.
Asymmetric rims mitigate the danger of unloading spokes because they give the less tensioned spokes of the wheel more tension to start with, but we'll talk about offset rims more either tomorrow or Friday.
If we get through that process with about the right answer, then the type of spoke should matter relatively not that much. We use CX Rays for most of our builds, because our answer to the first part usually leaves us in a place where CX Rays are a great tool for the job. And, quite honestly, from a sales perspective swimming upstream sucks and you never have to apologize for using CX Rays. We've worked hard to lower our cost on them and reduce the premium we charge for them to a point where they're easy to justify. For builds where you're trying to hit someone's budget number or where it just doesn't make as much sense to use CX Rays, then there are others. When we know we've got plenty of spokes as it is, we can build with Lasers. D-Lights are a bit more spoke than Lasers, but they're still light and great to work with. Race spokes (2.0/1.8mm double butted) are the heaviest spokes we use with any regularity, and they're the least expensive. They're easy to build with (though CX Rays are the easiest) and can add some "oomph" to a wheel's strength, but as a builder I prefer working with lighter spokes which I can't fully explain.
Yannick Noah was the most compelling tennis player of the modern era. Fact.
Even if we don't do the math on this for every single build (which we don't), those are the methods behind the madness. And having put that out there, I sort of weep for my inbox's future.