The thickness of the material in the rims also affect the longevity of the rim, because eventually the brakes abrade the rim wall to where it becomes too thin for safety.
Hi Andre!
<nods> You raise a good point I think I can answer from my "roadie-wheelbuilder" background.
What can happen (has, in fact, a number of times historically) is the rim can become so thin, the spoke nipples (and the ferrules they're seated in) pull through the rim -- even when the rim "should" be at proper tension. I once ran into a set of tubular (sew-up) rims that had too-thin walls in places at the spoke sides (an extruding error, apparently where the flow-pressure was reduced) and the spokes pulled through. I have no doubt the rim sidewalls would have eventually split at the braking track, as the rim was much thinner there as well (as I found after cutting and measuring sections during a post-mortem).
Sometimes, the rim needn't be "too thin" for this to happen. For example, one of my all-time favorite rims is the old Mavic MA-2 in a polished finish. This 700C rim has withstood countless miles of my fully-loaded tours on rough logging roads. The same rim -- except for being hard anodized and designated the MA-40 -- was known to have a very short life. Why? When the double stainless spoke ferrules were clinched into the hard-anodized rim at the factory, they caused micro-cracking of the anodic structure at the surface, creating stress risers. Once that happened (and one never knew if it had...till it did), the rims would crack through around the spoke-hole ferrules.
theoretically a point arrives where a rim might be so light that it could affect handling by not being stiff enough. I say "theoretically" because in practice what happens is that people reduce the spoke count to save weight and that in turn causes the wheel to pretzel before handling errors can be observed.
Yehkinda. Pretzeling is largely a function of either a) uneven spoke tension or b) excessive side loads where lateral loading pushes spoke tension beyond equilibrium. Spoke count plays a huge role, but more about that in a moment. You see pretzeling far more often on dished rear wheels where the "flatter" (cassette) side is under higher tension at shallower bracing angles than the non-drive side.
I've built-up some wickedly light sewup rims (that were lighter 'cos they had no rim sidewalls, being essentially tubular themselves) that were as stiff laterally in normal riding as their box-section clincher counterparts ceteris parabis (in this case, holding constant for spoke count, crossings, spoke gage, rim width, etc).
Lateral rigidity of a completed wheel depends greatly on spoke count. If you're running 48 spokes and break one, you'd hardly notice. If you're running, say, a 16-spoke wheel, oh yeah; you'd notice a broken spoke immediately. Rolf Prima wheels are built here in Eugene, and they employ low-count, high-tension pairs of spokes in their wheels -- and rims and hubs designed to work as an integral part of the overall wheel design. So much of their success depends on materials design *and* high, even spoke tension. Remarkable stuff, but not so well-suited to touring.
In general, light rims are less forgiving of builder errors (and uneven spoke tension) than are heavy ones. A wide touring rim with lots of "meat" in the structure is one of the most forgiving rims to build, giving reasonably good results for amateur and pro alike (and hurried roadside rim replacements in torrential downpours). Still, good technique is always rewarded by producing a wheel with high, even spoke tension that will stay nice and true over the course of a long, long time and heavy use without the need to retrue frequently.
All the best,
Dan.
