Rim cracks
I built wheels for our tandem using Ryde Andra 35 rims, 36 Saprim butted spokes, and Saprim polyax nipples. Sadly….After 7000 miles, much of which was heavily loaded, 4 of the spoke holes in the rear wheel have developed 5 mm long hairline cracks in the rim (not easy to see or photograph). The spokes are not next to each other, but in very different parts. The wheel remains perfectly true with average tension of 105 kgf (both sides, Rohloff) with 15% variation.
The cracks are very fine and I wonder if these are just superficial cracks in the anodized surface coating, is that possible? If not superficial, I am surprised since the rims are stout. Should I have used nipple washers? I suppose it is not worth the time to save the rim by adding nipple washers after the fact. Thanks for your comments in advance.