A curb bit is a type of bit used for riding that uses leverage. It includes pelhams along with the traditional "curb bit" used mainly by Western-style riders. Kimberwickes are modified curb bits, and a curb bit is used in a double bridle along with a bradon. 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.
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.
The Action of the Curb Bit
A curb bit works on several parts of a horses mouth.
The bars: applied by the mouthpiece, and directly related to the length of the shank of the bit. The curb chain and a high port can also increase the pressure placed on the bars of the mouth.
The tongue: applied by the mouthpiece, with the pressure related to the thickness and type of mouthpiece.
The roof of the mouth: only applicable if the mouthpiece of the curb bit has a high port.
The poll: severity is directly related to the length of the upper shank in relation to the lower shank. All curb bits apply at least a little pressure on the poll.
The chin groove: pressure applied by the curb chain when the curb rein is used.
Side of the jaws: a curb bit may add some pressure to the sides of the jaw due to its length.
A curb bit is a leverage bit, meaning that it multiplies the pressure applied by the rider. Unlike a snaffle bit, the curb rein can amplify the rein pressure several times over, depending on the length of the curb bit's shank.
The relation of the upper shank--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. A long lower shank in relation to the upper shank 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. Shank sizes vary from the Tom Thumb (2 inches long) to more than 5 inches, although most are less than 4 inches.
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, and provide room for the tongue. A high port may act on the roof of the mouth as it touches, and will act as a fulcrum, amplifying the pressure on the bars of the mouth. Jointed mouthpieces increase the pressure on the bars as it breaks.
The Curb Chain
The curb chain applies pressure to the curb groove under a horse's chin. It amplifies the pressure on the bars of the horse's mouth, because when it tightens it acts as a fulcrum.
Fitting the Curb
Curbs are generally placed lower down in a horse's mouth than snaffle bits, near or in the corners of the mouth. 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 curb ring is rotated 45 degrees.