More charts and graphs today! I finally figured out how to illustrate a point that's been banging around in my head for a long time, which is how rim strength affects lacing. We've talked about lateral stiffness in rims a bunch of times (enough to earn ourselves the clearly affectionate sobriquet of "fun sponges"), but radial strength is as big a deal.
I took 2 rims, 2 built wheels, and a large clamp, and simply applied pressure at 12 and 6 o'clock to show what happens to rims and wheels when they are loaded this way. The two rims were a Rail 34 and a Pacenti SL23. The two wheels were a Pacenti/Miche 28h rear, and a Pacenti/WI 24h rear, both laced 2x/2x with CX Rays. I chose these rims because they are both stiff relative to their category, plus I already had the Pacenti wheels built up for something else.
Wheel test
Rim test
To do this test to lab standards would require time and fixtures/equipment not currently at our disposal, but as an illustration of the concept this worked beautifully.
Part 1 - Compress the rims: With a rim loaded into the clamp as shown above, I applied clamp pressure by the trigger until I couldn't squeeze anymore. With the Rail rim, that point was reached once the clamp's pads compressed. The Pacenti rim was much easier to squeeze. I didn't want to ruin the rim but compressing it 1/4" was easy. Once unloaded, it snapped back into roundness. These results were completely as expected. A heavier and/or deeper alloy rim would have resisted compression better, but is that worth it? Read on..
Part 2 - Compress the built wheels: The exact same protocol was used for wheels as rims - squeeze the trigger until I couldn't, this time measuring the change in spoke tension that resulted. This admittedly inexact loading was used simply for lack of the equipment to precisely repeat the loads, but since a large part of my life is spent squeezing and plucking spokes I'm exceptionally well calibrated for this kind of thing, and, well, this loading was pretty close.
As expected, the 24h wheel's spokes both gained more tension in the 9 o'clock - 3 o'clock axis and lost more in the 12 to 6 axis, along which the load was imparted. This is simply the axiom of "many hands make light work" in action - with the adjacent spokes closer to the spokes most directly affected to the load, they were able to help the most affected spokes carry the burden. If I was a graphics ninja I would graph this but instead you can interpolate the graphs below.
28h DS spoke tension loaded v unloaded
24 DS spoke tension loaded v unloaded
As you can see, the spokes in the 24h wheel saw a bigger change than the spokes in the 28h wheel. The NDS spokes acted exactly the same way, to no surprise. In both cases, the NDS spokes at 12 and 6 o'clock went completely slack, but the 3 and 9 o'clock spokes gained less tension in the 28h wheel than they did in the 24h wheel.
The more spoke tensions are cycled in this manner, the more quickly the wheel will wear out. This will manifest as either non-drive spokes breaking (usually at the hub), or drive side spoke holes in the rim cracking from stress (or, less likely, spokes breaking at the hub). This whole dynamic is why we are generally in favor of more spokes than others when it comes to alloy rear wheels. A heavier rim, as mentioned, would counteract this behavior - hence the number of 20h rear wheels (OEM and otherwise) that don't explode under riders. In our opinion, adding weight at the rim is a bad trade there. Adding 4 spokes makes an easily seen difference at a cost of 20 grams - to us, that's the best trade.
This isn't a perfectly precise mapping of everything that goes on in the wheel (wheels get loaded between the wheel perimeter and the hub, for instance), but it's a great illustration.
6 comments
very nice qualitative trial/study illustrating what seems intuitive: more spokes make a stronger wheel, all other things being equal. as a cyclist who geeks out on the data (like, i suspect, a fair number your customer base), i'd like to see even more along this vein. 1. how about comparing 2 different 24 spoked rear wheels, one with 2:1 driveside to non-driveside lacing—ie 16 spokes laced 2-cross driveside, with 8 radially laced non-driveside spokes), to another wheel laced traditionally 2-cross with 12 spokes equally per driveside and non-driveside? 2. sure, as you increase spoke count, strength and/or radial stiffness increases. but does one see similar improvements in lateral stiffness? 3. is there a point where increased spoke count increases wind resistance for a given wattage output, such that higher spoke counts are detrimental? this might require even more quantitative ninja gymnastics, plotting cyclist power required to maintain a certain linear speed as a function of wind resistance, yaw angles, spoke counts, etc for a given rim profile/rim weight…that last question is one i've thought of over the years, and excuse the ancient example because although it likely started in the Hinault-Lemond era for road racing (and maybe the purple anodyzed era for XC racing) the general concept can be extrapolated to contemporary ones: 32x 14 gauge (straight guage) spokes are heavier than 36x 14/15/14 gauge double-butted spokes, but conventional wisdom says more spokes generates more wind resistance, so there was/is at least a perceived benefit in building a wheel with fewer spokes (even if they're heavier). and fewer spokes = less wind resistance is probably at least partly responsible for the current standard road wheelset with 20F / 24R spoke pattern that's been around for a while now. but as you've shown, a few more spokes makes for a stronger wheel (at least radially), so where's the break point when adding more spokes yields diminishing returns?
Hi Dave – See today's post. – Dave
Why run this test with the wheels loaded in this way? The rim never sees loads like this, at least while the spokes remain under tension. And of course if they're not under tension under all real-world loading conditions, then you have an unsafe wheel – and that's a different scenario. The hub does not "hang" from the rim (Jobst Brandt has does this test). To verify that, put a built wheel in a bike. Measure spoke tension at 6 and 12 o'clock. Then climb on the bike and repeat. You can see the load path by the change in spoke tension. If the wheel is "hanging", then the top spoke will increase in tension. If it doesn't, then you'll see no change in the top spoke and the lower spoke will de-tension, indicating compression loading.
Galen – First, this is far more of an illustration than a test. Second, the illustration was significantly designed as a counterpoint to a video which had been going around where a mountain bike wheel builder was making it out that a rim's radial stiffness was a huge consideration. As his example, he did dips supporting his bodyweight on two carbon rims that were standing up, and then said "I couldn't do that on alloy rims without folding the rim." Maybe so, but spoke the wheels and not only could you do it on alloy wheels, you wouldn't feel the difference between an alloy and carbon build if you did that. So that was the primary thing we wanted to illustrate.Whether the hub hands or is pushed up is immaterial to the other main point. The closer the spokes are, the less any rim deformation that does happen will affect any one spoke – many hands make light work.A third thing is that carbon rims deflect less than alloy rims. What was unimportant in the above example of unspoked rims becomes important now. If you take Jobst Brandt 100% at his word, that spoke tension change happens exclusively to bottom spokes and is ONLY caused by rim deformation, then spoke count does get wicked important on alloy wheels – that small deflection, which you can't feel as the rider, is felt by the spokes. Any deflection felt by the spokes will increase cycle stress and decrease their life span.The brief bit about spoke tension as relates to spoke count on page 14 of the edition of The Bicycle Wheel that I have talks about this.We'll get around to an actual test of these concepts before long, but the illustration is valid and clear. Other companies are stating that 24 spoke alloy rear wheels are suitable for riders over 200 pounds. Simple illustrations make it clear that there are better choices for those guys, and the sum of the testing that we do that few others do shows how little a price there is to be paid for those 4 extra spokes. But 4 fewer spokes are the fashion, and fashion dies perhaps hardest of all prejudices.
Dave,Thanks for the reply. The mtb wheel builder was illustrating a poor understanding of wheel engineering, so while you successfully counter argue his points isn't it better to educate readers about proper engineering and what actually matters in a wheel build?It is both intuitive and demonstrable in practice and theory that fewer spokes mean higher loads per spoke and greater deflection for a given spoke size. However, I take issue with the argument that it necessarily means that a wheel with fewer spokes will wear out faster. First, steel does not fatigue in the same way aluminum does. within the right parameters it can be cycled indefinitely (http://en.wikipedia.org/wiki/Fatigue_limit). A correctly sized spoke will have a lifespan that outlasts the rim even on a low spoke count application. In other words, not every spoke is doomed to break. Second, rim design matters both in terms of material and cross section. There's no reason a low spoke count carbon wheel can't be designed that outlives its brake track, or its rider.Rolf Pima is a good example of a company building low spoke count, high life span wheels. Their in-house testing is pretty incredible and they have some high-quality engineering bench strength. So it can be done, given good engineers who understand material science, structures, and dynamics.Keep up the good work. Despite my criticism of this post, I really enjoy what you guys are trying to do and that you're exploring what makes wheels work.