It is important to be able to predict the instantaneous forces and moments at the bit during drilling, but the rock/bit interaction process is not thoroughly understood. It is commonly accepted that the forces acting on a drag bit should be balanced to avoid excessive bit vibrations called whirling and uncontrolled well deviation resulting from non-symmetric side cutting. However, the relationships between drilling forces and bit design features are uncertain because of the difficulty in obtaining reliable, quantitative measurements of cutting forces at the bit. A series of single cutter PDC (Polycrystalline Diamond Compact) experiments were conducted in the laboratory; in which three components of drilling force were measured. A rock was cut at the speed and load corresponding to the same parameters while PDC bit drilling. The variations in the value and direction of the resultant force were compared with the variations of the depth of cut and the simulated wellbore pressure. The relationship between the cross-section of cut rock and the measured forces at a PDC cutter were analysed and used in the rock/bit modelling. A model for the resultant force at the bit was developed by integrating the effects of individual PDC cutters into geometry of a given PDC bit design. The model describing kinematics of the PDC bit, as well as the imbalance force resulting from the non-symmetric distribution of drilling forces was developed. Then, simulations of the rock/bit interaction forces were run for the different depth of cut and a given rock described by the index of rock strength and coefficient of friction between the rock and PDC cutter. The predictions obtained of imbalance force and moments at the bit are presented for the conventional and bi-centre PDC bit designs. The results clearly indicate the differences between these two designs, as well as the ability to predict the magnitude and orientation of the imbalance force generated while drilling with different PDC bit designs in different rock lithologies.