Curb bit
From Wikipedia, the free encyclopedia
A curb bit is a type of bit used for riding horses that uses leverage. It includes the pelham bit and the Weymouth curb along with the traditional "curb bit" used mainly by Western riders.
Kimberwickes are modified curb bits, and a curb bit is used in a double bridle along with a bradoon. A curb bit is, in general, more severe than a basic snaffle, although there are several factors that are involved in determining a bit's severity.
Contents |
[edit] The curb bit
The curb bit consists of a mouthpiece, curb chain, and a shank, with one ring per side on the top of the shank, and one ring on the bottom of the shank. Pelham bits also have a ring next to the mouthpiece.
[edit] The action of the curb bit
A curb bit works on several parts of a horse's head and mouth. The bit mouthpiece acts on the bars, tongue and roof of the mouth. The shanks add 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 act on the sides of the mouth and jaw.
[edit] The shank
A curb bit is a leverage bit, meaning that it multiplies the pressure applied by the rider. Unlike a snaffle bit, which applies direct rein pressure from the rider's hand to the horse's mouth, the curb can amplify rein pressure several times over, depending on the length of the curb's bit shank. Shank sizes vary from the Tom Thumb (2 inches long) to more than 5 inches.
[edit] Leverage principles
Overall shank or cheek length, from the top of the cheek ring to the bottom of the rein ring, usually cannot exceed 8½ inches for most horse show disciplines. The relation of the upper shank (purchase)—the shank length from the mouthpiece to the cheekpiece rings—and the lower shank—the shank length from the mouthpiece to the lowest rein ring, is important in the severity of the bit. The standard curb bit has a 1½" cheek and a 4½" lower shank, thus producing a 1:3 ratio of cheek to shank, and producing 3 lbf of pressure on the horse's mouth for every 1 lbf placed on the reins (3 newtons for every newton).
Regardless of the ratio, the longer the shank, the less force is needed on the reins to provide a given amount of pressure on the mouth. So, if one were to apply 1 lbf of pressure on the horse's mouh, a 2" shank would need more rein pressure than an 8" shank to provide the same effect.
A long lower shank in relation to the upper shank (or purchase) increases the leverage, and thus the pressure, on the curb groove and the bars of the mouth. A 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.
However, longer-shanked bits must rotate back further before applying pressure on the horse's mouth than shorter-shanked bits. Therefore, the horse has more warning in a long-shanked bit, allowing it to respond before any significant pressure is applied to its mouth, than it would in a shorter-shanked bit. In this way, a longer shank can allow better communication between horse and rider, without increasing severity. This is also directly dependent on the tightness of the curb chain.
[edit] Type of shank
Shanks come in a variety of types, which may affect the action of the bit. Some shanks are loose-jawed, meaning they swivel where the mouthpiece attaches to the shank. Others have a loose, rotating ring at the bottom of the shank for rein attachment. Both of these functions allow slight rotation before the bit engages, again providing a "warning" to the horse before the bit engages fully and allowing him to respond to the slightest pressure, thus increasing communication between horse and rider.
The cheek-shank angle also varies, with some straight up and down, others with the shanks curving backward, and some with an S-curve in the shank. The straighter the cheek-shank line is, the less signal is provided to the horse before the bit engages. Those that curve backward provide more of a signal to the horse. Therefore, the type of shank needs to be considered according to the use of the horse. Horses that maintain a more vertical head position, such as dressage horses, generally use a curb bit with straight shanks. Those that have a nose-out head position when working, such as cutting and roping horses, more commonly use a curved shank.
[edit] The mouthpiece
The curb bit's mouthpiece controls the pressure on the tongue, roof of the mouth, and bars. A mullen mouth places even pressure on the bars and tongue. A port places more pressure on the bars, but provides room for the tongue. A high port may act on the roof of the mouth as it touches. Some western style curbs, particularly the Spade Bit, have both a straight bar mouthpiece and a high welded port., thus acting on the bars, tongue and palate. In the wrong hands, such bits can be extremely severe, but on an exquisitely trained animal, they allow the rider to communicate with the horse with a simple touch of the fingertips to the reins.
Curb bits can also be purchased with a variety of jointed mouthpieces that are sometimes mistakenly called "snaffles", some of which (like the twisted wire) can further increase severity. Jointed mouthpieces increase the pressure on the bars due to the nutcracker action of the mouthpiece. In addition, the joint angle is altered by the shank leverate to tip the bit downward and into the tongue. These bits, sometimes called "cowboy snaffles" due to their popularity among western riders, are actually more harsh than a curb with a simple, solid, ported mouthpiece.
[edit] The curb chain/curb strap
The curb chain or strap applies pressure to the curb groove under a horse's chin. When the shank of the bit rotates back (due to rein pressure), the cheek of the bit rotate 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, it pulls the curb chain, tightening 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 amplifing the bit's pressure on the bars of the horse's mouth.
The action of the bit is therefore also dependent on the tightness of the curb chain. If the bit is used without a curb chain (very uncommon-- and dangerous), 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 the mouth, so the well-trained horse may respond faster. If used with a very tight curb chain, the bit immediately exerts leverage 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.
[edit] Fitting the curb bit
Curbs are generally placed lower down in a horse's mouth than snaffle bits, touching the corners of the mouth, or creating a single slight wrinkle in the lips. The lower the bit is placed, the more severe it is as the bars of the mouth get thinner and so pressure is more concentrated.
The curb chain should be adjusted correctly, lying flat against the chin groove and only coming into action against the jaw when the shank is rotated, but not so loose that the shank exceeds 45 degrees of rotation.
