Curb strap

A curb chain, or curb strap, is a piece of horse tack required for proper use on any type of curb bit. It is a flat linked chain or flat strap that runs under the chin groove of the horse, between the bit shank's purchase arms. It has a buckle or hook attachment and English designs have a "fly link" in the middle to hold a lip strap. On English bridles the horse is bridled with the curb chain undone on one side, then connected once on the horse. On western bridles, the curb chain is kept buckled to both sides of the bit.

The main use of the curb chain is to enhance and control the lever action of a curb bit. Additionally, it helps to keep the bit steady and in place within the mouth. On English Pelham and Double bridles the curb chin is attached by a ring ("fly link" (UK)) to a lip strap, which helps keep the lip strap in place while the lip strap in turn prevents the curb chain from being lost if it becomes unhooked.

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 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. Conversely, a very loose curb chain can be undesirable, allowing the bit to rotate in the mouth too much, causing the port, especially a high port, to become too vertical and press against the palate, which is painful, can damage the mouth in extreme cases, and can cause the horse to gape. Additionally, it can completely nullify the correct action of the curb, making its use pointless.

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