Curb bit

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.

The standard curb bit has a 1+1⁄2" purchase and a 4+1⁄2" lower shank, thus producing a 1:3 ratio of purchase to lower shank, a 1:4 ratio of purchase to full shank, thus producing 3 lbs of pressure on the chin groove and 4 lbs of pressure on the horse's mouth for every 1 lb placed on the reins (3 and 4 newtons respectively 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.

A horse has more warning or pre-signal, 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, but ultimately it is the straightness or curve of the shank which translates to the abruptness of response.

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.

Those that have a nose-out head position when working, such as cutting and roping horses, more commonly use a curved shank.

The curb bit's mouthpiece controls the pressure on the tongue, roof of the mouth, and bars.

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.

In addition, the joint angle is altered by the shank leverage 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.

The lower the bit is placed, the more severe it is as the bars of the mouth get thinner and pressure is more concentrated.

A Western style curb bit with leather curb strap
An English style Weymouth curb
Parts of a curb bit
A decorative fixed shank on a western Salinas-style curb bit
Bit shanks, such as those on this spade bit, work as a lever
A western curb bit with a loose-jawed shank
A medium port curb offers considerable tongue relief. The addition of loose-jaw short shanks make this a relatively mild bit.
English curb set up with curb chain and lip strap
A carriage team's wheeler, in a Liverpool bit set up with minimum leverage. The leaders' two pairs of reins can also be seen being joined into one balanced pair.