Rotary drill bits and methods of designing such drill bits

A rotary drill bit for drilling holes in subsurface formations, comprising:

• a bit body having a shank for connection to a drill string, a plurality of cutter assemblies mounted on the bit body, and a passage in the bit body for supplying drilling fluid to the surface of the bit for cleaning and cooling the cutters, wherein certain cutter assemblies on the bit body are higher volume factor cutter assemblies adapted to exhibit a volume factor which is significantly greater than the volume factor of other cutter assemblies on the bit body, with increase in rate of penetration, wherein the cutter assemblies are mounted on a plurality of blades extending generally outwardly from the central axis of rotation of the bit body, there being provided blades which carry cutter assemblies which are all substantially of higher volume factor and other blades which carry cutter assemblies which are substantially all of lower volume factor, and wherein at least the majority of said higher volume factor cutter assemblies are better adapted for cutting softer formations than at least the majority of said other cutter assemblies.