Greetings Thornoids!
I trust you're all well and those of you in northern climes are enjoying your springtime.
I know this question has been batted around before, but although I've done a reasonable amount of researching past discussions on this and other forums ("fora"?), I am still none the wiser. Let me describe my dilemma:
When cycling a rail trail in central Victoria week before last, I broke a spoke in Attila the Nomad's rear wheel (32h Andra 30 which I had laced myself only last July with 238mm Sapim Race black spokes). When last I checked, the wheel was running quite true, with spoke tension between about 80 and 105 Kgf (average about 94) as read on my Park Tool TM-1 Tension Meter. The spoke was more or less opposite the valve hole, and broke right in the middle, after hitting a sharp bump (buckled pavement) while I was going downhill at speed with lightly loaded panniers. I must say I was surprised, because I thought the wheel would be far stronger than that.
Later in the week, clumsy old me actually tripped over the bike, inadvertently stepping on, and breaking another spoke (same side, next adjacent to the original broken one). Also snapped in the middle.
No worries, says I, as I had 6 spokes left over for each wheel from my original build. Upon returning home, I pulled the wheel, replaced the spokes, and whilst tightening them up to tension, a third spoke snapped!, this time opposite the two others (valve side). This bothered me, as these spokes ought to be able to withstand at least double the tension, especially while static.
My question is this (apologies for being long-winded): As the Andra 30 is considered one of the strongest rims on the market, and the TM-1 tension chart shows a range up to 167Kgf for 1.8mm diameter steel spokes, why shouldn't I be able to tension up the wheel to say, 130 Kgf, +/- 20% (104 - 156)? I would think that would give me a stronger wheel, no? But if that is true, why did the third spoke break so easily?
I also note that the tension measured on my wife's Raven (same wheels, silver spokes, presumably Sapims?) built by Thorn in August are the same as mine (all very close to 94Kgf, or 20 on the TM-1 scale). Making the obvious assumption that Thorn knows a lot more about building this wheel that I do, I'm wondering what I'm missing? My wheel was true, the tensions were pretty tightly grouped around the average, so why did a spoke break on what should be the strongest wheel going?
I will try to take a macro-photo of the third spoke break surfaces, to see if the spokes were defective, but I would think there's a fairly low chance of that having been the case. I'm just trying to ensure I get good reliable service out of these wheels when fully loading them up on future tours.
Thanks in advance for your always well-considered responses,
Sam