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is 2 cross strong enough for a disc brake wheel?
Apparently so (see also loads of factory 29er wheels from Mavic and Shimano). It does depend a bit on the flange diameter and ERD, but you'd probably need to be building some ancient tiny flanges into 28" rims for the pulling spokes to be far enough away from tangential for it to be a problem.
4x a bad idea if spokes will lie across the hub flange?
Yes
So the spokes I bought working off internet measurements are a few mm too long.
I have shorter spokes in stock but more like 10mm+ shorter.
Before I bother measuring stuff and punting it all into calcs, is 2 cross strong enough for a disc brake wheel?
4x a bad idea if spokes will lie across the hub flange?