Odds are the spokes are too short.
Take a look and check whether the tops of the spokes reach close to the top of the nipple. If you're breaking nipples I'll bet it doesn't, and that's why your nipples are breaking.
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Think of the nipple as having two distinct parts. The head which is about 2-3mm thick and is basically like any other nut, and the shank of the nipple, which is a thin tubular extension allowing you to turn the nipple from the outside.
The head butts directly onto the rim or eyelet easily transferring the load the same way any other nut would if the spoke reaches into it. But if the spoke is short, ending below the nipples head, than the thin section shank becomes the last link in the structure transferring the spoke's tension to the head, then to the rim.
As they say a chain is as strong as it weakest link, and the nipple shank which was never intended to be a structural element - whether brass or alloy - will ultimately fail.
In fewer simple words - nipples used properly with spoke heads reaching the top of the head don't fail.
BTW- some time back someone posted 3 nice cross-section sketches of spokes in nipples at different heights, which nicely show the problem of too short spokes. If he sees this maybe he'll be so kind as to post them again.
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FB
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Last edited by FBinNY; 04-03-11 at 04:36 PM.