DARIVS ARCHITECTVS wrote:
One only has to imagine the forces of compression, tension, and shear in the 3-D volume of the tiller to understand what the effects of the geometry changes would be. This comes with study of stress mechanics.
A super heavy prod is really hard on the bindings, which in turn tug forward on the volume of wood between the binding hole and the rear face of the prod when the crossbow is fired. If you imagine a huge sudden pull on the binding hole in the forward direction, you can see why there would be two shear flat horizontal planes between that section and the wood immediately above and immediately below. Since this in along the grain, cracks form over time. Imagine you grabbing the front end of the tiller at the binding hole and tearing that volume of wood right out. As a result, the upper and lower volumes of wood in the nose of the tiller were tied to the central volume with a steel rod. The rod transfers the force on the central section to the upper and lower sections, so no shear cracks form.
Now this rod is required, but ANYTIME you drill a hole in wood, that GEOMETRICAL IMPERFECTION from a SOLID piece of material introduces a STRESS RISER in the material.
In this case, the drilling of a hole, installation of a rod, and the introduction of the resulting stress riser is a necessary TRADEOFF to eliminate the problem of the shear stress of that "tugging" on the binding hole which will crack the wood. Sometimes you have to add a negative design feature in order to get rid of a GREATER negative design feature. The trick is known WHEN to do this!
As with siege engines (which I LOVE to build), a crossbowmaker MUST have a good understanding of the strength of his materials, especially ones like wood which are stronger in some directions and not in others, and a good understanding of:
1) The directions and magnitudes of the forces in 3-D space within the materials,
2) How sharp changes in geometry, like holes, inside corners and outside corners, can concentrate stress.
3) How these more concentrate stress areas will reach the breaking point first, before other areas of the material do, as you apply more and more force on the object.
So if you want to really design good crossbows, study basic stress mechanics. Then you can choose the best materials, best shapes, and best dimensions to withstand the forces without overbuilding the shit out of everything like a novice.