A curb chain, or curb strap, is a piece of horse tack used on any type of curb bit. It is a flat linked chain or strap that runs under the chin groove of the horse, between the bit shank purchase arms. It has a buckle or hook attachment and often has a "fly link" in the middle to apply a lip strap. Normally the horse is bridled with the curb chain undone, then the curb chain is done up.
The main use of the curb chain is to control the lever action of a curb bit. Additionally, it also helps to keep the bit steady and in place within the mouth, and helps to hold a lip strap in place.
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. However, a very loose curb chain can be undesirable, especially if the port is high. It allows the bit to rotate in the mouth too much, causing the port to become more vertical and press against the palate, which is painful and can damage the mouth, and can cause the horse to gape. Additionally, it can completely nullify the correct action of the curb, making its use pointless.
There are two undesirable consequences when using a very tight curb chain. First, 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 much harsher, and provides less finesse in signaling the horse than a looser curb chain would, as the horse is never given a chance to respond before the bit pushes into his bars. Secondly, a very tight curb chain causes the mouthpiece to constantly push down on the sensitive tongue, never allowing the horse relief. In severe cases, the tongue can lose blood supply.
Most horsemen adjust the curb chain so it only comes into action when the shank rotates 45 degrees back. However, horsemen with skill and experience with the curb bit can adjust the chain to accommodate the needs of the individual horse and training situation. When in doubt, however, most suggest keeping the curb chain slightly looser.
The curb chain should be applied by twisting it clockwise on one hook until it is flat, and then attaching it to the other hook. A curb chain must NEVER be used twisted, as it can cause serious damage to the horse.
Differences in chains
Curb chains vary in width and linkage. Thinner curb chains are more severe, and can eventually cause sores if the chain is engaged consistently. Additionally, most people prefer a curb chain with links that run through two others, rather than one, as this decreases the chance the chain will pinch. For horses that are sensitive or that are rubbed by the chain, a cover made of rubber, neoprene, leather, or gel cover can be used, as well as a leather curb strap. However, it is important that the rider check that the curb is being used correctly and is not the cause of the rubs. It is recommended that the curb chain be 1.5 times the width of the bit.
Price, Steven D., ed.The Whole Horse Catalogue. New York: Simon and Schuster/Brigadore Press, 1977