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Bit shank

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Bit shank

The bit shank is the side piece or cheekpiece of a curb bit, part of the bridle, used when riding on horses. The bit shank allows leverage to be added to the pressure of the rider's hands on the bit. Shanks are usually made of metal, may be straight or curved, and may be decorated in some disciplines. The headstall and curb chain or curb strap of the bridle is attached to the top of the shank, and the reins are attached at the bottom. Shanked curb bits are used in western riding for nearly all adult horses, and are seen in English riding disciplines primarily as part of the double bridle used by advanced dressage riders, and on the hybrid pelham bit that includes a ring for a second rein attached at the bit mouthpiece.

Direct pressure snaffle bits have no shanks, instead they have a single bit ring.

Bits that have shanks coming off the mouthpiece create leverage and place pressure on the poll via the crownpiece of the bridle, to the chin groove via the curb chain, and, especially with a "loose jaw" shank, may also touch the sides of the mouth and jaw. The shank and its leverage action is what defines a curb bit as a curb, regardless of mouthpiece. Though most curb bits have a solid mouthpiece, with or without a port, any bit with shanks and leverage is always a "curb" type bit, even if has a jointed mouthpiece. Shanked bits in the curb family include the Weymouth, which is the curb portion of the double bridle; the pelham bit, a single bit ridden with two sets of reins; and the single-reined curb bit.

The term shank is generally used interchangeably with the term "cheek" to describe the entire sidepiece of the bit, but shank also may refer just to the lever arm, the portion of the bit that extends from the mouthpiece to the rein ring. The purchase of the bit is the upper portion of the cheek that extends from the mouthpiece to the headstall rings. All shanks have a rein ring at the bottom for the curb rein and a cheek ring at the top to attach the headstall. Some shanks may also add rings or slots to attach a snaffle rein at the mouthpiece, allowing the bit to be used with two sets of reins, making it a pelham bit.

Some shanks, especially on the Weymouth, have small rings placed midway down the shank to attach a lip strap, a helpful addition to the bit for preventing a horse from grabbing at the shanks with its lips. Some shanks on western-style bits are "hobbled" together by a metal bar (sometimes called a "slobber bar" because saliva from the horse's mouth can drop onto it) or even a piece of leather, which has the dual effect of keeping anything from getting wrapped around the shank, such as a lariat, and can limit excessive motion in a loose-jawed shank.

The length of the shank determines the degree of leverage put on the horse's head and mouth. The leverage ratio for a typical curb bit is 1:4 (where the ratio of upper and lower cheek is 1:3), in that one ounce of pressure by the rider will result in four ounces of pressure on the mouth of the horse. Overall cheek length, from the top of the cheek ring to the bottom of the rein ring, usually cannot exceed 8½ inches for most western horse show disciplines. In Dressage the length of the lever arm portion of the cheek cannot exceed 10 cm. Cheek sizes vary from the Tom Thumb (about 4½ inches long) to bits that exceed the 8½ inch "show legal" maximum cheek length.

The relative ratio between the length of the purchase and the lever arm also affects the amount and type of leverage that is applied to the chin and poll of the horse (producing 1:3 ratio of rein to chin+poll forces in case of the typical curb bit). A long lower shank (lever arm) in relation to the upper shank (purchase) increases the leverage, and thus the pressure, on the curb groove and the bars of the mouth. This design is best suited for a longer-necked horse, as it encourages the horse to both drop its head down and bring its nose in. A somewhat long upper shank in relation to the lower shank increases the pressure on the poll, but does not apply as much pressure on the bars of the mouth. This design is often more helpful on a horse with a short and thick neck, as it encourages the horse to drop its head, but with less pressure to flex the nose in, an act that is physically more difficult for a horse with a thick neck.

Overall, a shorter-shanked bit is usually a milder bit, but also responds quickly when the rider touches the reins. Short shanked bits are usually better for a young horse transitioning from a snaffle to a curb because if the inexperienced horse gets into a place where bit pressure from the rider's hands becomes significant, there is less leverage pressure placed on the horse's head.

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