From Wikipedia, the free encyclopedia

A curb chain on a curb bit

A curb chain, or curb strap, is used on curb bits (including the pelham and kimberwicke) when riding a horse. It is a linked chain or leather strap that runs under the chin groove of the horse from one cheekpiece to the other. It often has a "fly link" in the middle to apply a lip strap (used to keep the horse from grabbing the shank and to keep the curb chain from unfastening).

The curb chain applies pressure to the curb groove under a horse's chin when the curb rein of the bit is used. When the curb rein is pulled, the shank of the bit rotates back towards the chest of the horse and the cheek (upper shank) of the bit rotates forward (since it is a lever arm). The curb chain is attached to the rings at the end of the cheek, so, as the cheek moves forward, the chain is pulled and tightened in the curb groove. Once it comes in contact with the curb groove of the horse it acts as a fulcrum, causing the cannons of the bit mouthpiece to push down onto the horse's bars, thus amplifying the bit's pressure on the bars of the horse's mouth.

The tightness of the curb chain therefore has a great effect on the action of the bit. If the bit is used without a curb chain, it loses its leverage action. If used with a loose curb chain, it allows the shanks to rotate more before the curb chain is tight enough to act as a fulcrum and exert pressure. This extra rotation can warn the horse before pressure is exerted on his mouth, so he may respond beforehand. If used with a very tight curb chain, the bit immediately exerts curb pressure and increased pressure on the bars as soon as pressure is applied to the reins. Therefore, a tight curb chain is harsher, and provides less finesse in signaling the horse than a looser curb chain would.

Most horsemen adjust the curb chain so it only comes into action when the shank rotates 45 degrees back.