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.
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.
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.