Abstract: A method of predicting the walk rate of a design of rotary drag-type drill bit of the kind where the leading face of the bit body is formed with a central depression, or cone, comprises the steps of: calculating the cone volume V.sub.cone of the bit; calculating that ratio ##EQU1## of the total torque generated by the cutters on the bit which is attributable to those cutters which are mounted in said cone; calculating a Walk Factor ##EQU2## and then determining the predicted walk rate W of the bit from a graph, or other database, relating previously established values of F for other drill bits to the respective walk rates W for those drill bits.

Abstract: A rotary drill bit comprises a bit body having a leading face and a gauge region, cutting elements mounted on the bit body, fluid channels extending outwardly across the leading face towards the gauge region, and nozzles for supplying fluid to the channels. At the outer end of each alternate channel, in the gauge region, is an outwardly facing junk slot which extends across only a part of the width of the channel, and a bearing surface which extends across the other part of the channel and bears against a wall of the borehole being drilled. In the channel, inwardly of the bearing surface, is an opening into an enclosed passage which passes internally through the bit body to an outlet. The bearing surfaces at the end of the channels increase the peripheral bearing surface area of the drill bit, thus improving stability, but fluid can still escape from all parts of each channel.