Yesterday's blog inspired a comment question so involved that the answer requires its own post. We've never done this before, but it gives us a chance to articulate how we tie all of the testing that we do into better recommendations and, hopefully, better wheels for you. I've (severely) edited/paraphrased the questions for brevity, but you can read the whole comment at yesterday's post. Incidentally, we probably have to spend some time figuring out how to catalog all of this info.
Q: How would 2:1 lacing affect the parameters that you describe here?
A. We’ve actually previously laced a wheel 16:8 on a 32 hole rim and tested it for tension balance and lateral stiffness. The tension balance was exceptional, the lateral stiffness was exactly the same as if it had been a 24 spoke wheel laced 2x/2x (it was compared directly to one of those). For radial stiffness, we haven't measured it. Presumably it's got a bit of an advantage because there are more drive side spokes, and so they're in closer proximity to one another. The quantity and bracing angle of non-drive spokes seems to affect wheel lateral stiffness as much/more than anything else. I think that wheel is still built, if it is I will give it a check when I have a chance. Not sure that my measuring system is sensitive enough to parse out that difference. It's also built on a Kinlin rim, so that would be noise in the answer. Here is where we talked about that setup
Q: Does spoke count affect lateral stiffness?
A: We've done much more measuring of lateral stiffness than radial. Here is the most recent post about how spoke count affects lateral stiffness. Lateral stiffness is more affected than radial stiffness by hub selection, and is also affected by rim stiffness. In the post prior to this one we quantified the lateral stiffness of all the rims we currently use - those aren't guesses they are quite precise measurements. The short answer is that every rim/hub combo has a point of diminishing returns for lateral and radial stiffness. For lateral stiffness, that depends on the “recruitment point” as I think of it – given the rim’s stiffness, how far can the support imparted by one spoke span. This is easy to see in building wheels – on carbon rims, tightening one spoke will loosen the adjacent spokes. That’s recruitment in action, as that spoke’s tension is spanning past its neighbors.
Q: How about quantifying the tradeoff between aerodynamic penalties and structural gains from more spokes?
A: This would be an incredibly involved piece of measurement, and I don’t think you could ever make it totally accurate. I’m going to do what I rarely do here, and break my arm patting us on the back here. We’ve done tunnel tests of a front wheel at 20 spokes versus an otherwise identical wheel with 24 spokes (a small but measurable difference. We’ve published tunnel tests of bikes with 34/34, 34/52, and 52/52 wheel combos,which is instructive on the relative importance of front wheel aerodynamics versus rear (front wheel is way more important). We’ve done lateral testing up the wazoo, and now radial testing. So we’ve tested every discrete component to that answer. I don’t see where anyone else has published even one of those component tests, and quite frankly I doubt they do them. The rub is that you have to credibly quantify performance impact of radial and lateral stiffness in order to gauge that versus aerodynamic gains and losses. Given the tests we’ve done, I have no problem claiming that we are better prepared to say that adding four spokes to a rear wheel does precisely jack squat to your system’s aerodynamic performance, so that there is no value in trading for fewer spokes until you reach the point of diminishing stiffness returns. In effect, every wheel recommendation we make to a customer or inquiry already encompasses this info. That’s why we do as much testing as we do. Admittedly though, we are making that tradeoff in light of durability vs performance. I don’t know how to do it as performance vs performance, at least in a way that will reach consensus.
On alloy wheels, the 20/24 convention is fashion, pure and simple. There is no objective support for this convention at all, but it does engender a lot of positive eyeball aerodynamics estimates, and I suppose helps bikes look cool on web sites. 20 spoke fronts are often desirable and defensible because they are adequately stiff and strong as is (front wheel structure is better), so we often recommend them. For carbon rims, 20/24 is very often at the sweet spot anyway. For very very deep rims we’d go fewer spokes, but we have no plans for a rim that deep.