A statics analysis of the equine curb lever bit and bridlework

The force distribution properties of the class of equine lever bits that includes the curb bit is analysed using the principles of static mechanical equilibrium. The force acting on the equine mouth tissues by the curb bit mouthpiece is shown to be a multiple of the magnitude of the rein tension. Whilst the force in the mouth due to the mouthpiece of a single ring snaffle bit is simply the magnitude of the rein tension, in curb bits it is predicted to rise above the magnitude of the rein tension in proportion to the ratio of the lengths of the upper and lower parts of the curb cheek shank. The relative position of the equine head and the rider’s hands can also influence the magnitude of the force in the mouth. The force through the cheekpieces creating poll pressure also rises in the same proportion but is moderated by the force distributed to the curb chain. The theory provided here is offered to guide future experimental work, advise equipment designers, assist riders’ choices of equipment and serve the governing bodies going forward in their efforts to optimise horse welfare. A worked example using the theory is provided for inspiration and illustrates the value of considering the action of the curb bit in this formal way.