Banktoturn is very correct. You must have good contact between the webing and the spars or the webing is nearly useless. Jim is also correct, the maximum stress is at an angle; neither perpendicular nor horizontal. For maximum strenth the grain should run at 45 degree angle but; it shifts 90 degrees when the load shifts from positive to negative. For that reason, horizontally grained sheer webbing as as good as vertical grained webbing. For max strength, you would have to have a ply webbing with the grain of each ply at 45 degrees to horizontal, one ply to the left, the other to the right. Take a look at any mechanical engineering book and you will see what really happens.