I completely concur with what Jur is saying. The wheel does not derive its strength from upper spokes hanging or lower spokes standing like columns. All the strength of a wheel comes from ALL the spokes together in tension holding the centre in its place. Fine that there is a variation in that tension as the wheel rotates, and it does seem that the lower spokes at BDC may go negative for a moment as the wheel takes them over that position, but others tighten up as the rim distorts microscopical amounts out of round. In the end, all the forces are held in tension by the spokes as a whole.