The curb chain is an essential part of leverage bits like pelhams and Kimberwickes or double bridles. Adjusting them properly is key to having the bit work the way it is intended. Curb chain fit into the groove under a horse’s chin. When the rider pulls on the curb rein, it causes the shanks to rotate backward and the mouthpiece to move forward. As the shanks come back, the curb chain will come into contact with the horse’s chin and apply pressure. If the curb chain is too loose, the shanks will come back farther, magnifying the pressure; if the curb chain is too tight, the curb action is felt almost immediately.
The most common rule of thumb is for the curb to come into play when the shanks are rotated 45 degrees. On most bits that means that there should be about two fingers’ width of space between the curb chain and the curb groove.
Most of the time the curb chain is just that — a length of chain. In some cases, a leather strap is used. This is a milder alternative and you can skip the first step mentioned below.
To begin, attach the curb chain to the hook on the right hand side of bit and untwist it until it lies flat. If the chain is twisted, it will increase the intensity of the pressure against the bit groove.
Once the chain is flat, twist it one extra half twist and attach it to the left hand hook.
You should be able to fit two fingers between the curb chain and your horse's chin.
Make sure you can fit two fingers in between the curb chain and the curb groove.
Some horses prefer the curb chain to be slightly looser or slightly tighter. Always start with the two fingers measurement and then make minor adjustments (one link at a time) and see how your horse responds.
Remember that a leverage bit exerts considerably more pressure on a horse’s mouth than a snaffle. With a snaffle the amount of pressure you apply to the reins is directly felt by the horse. With a leverage bit, the shanks magnify that pressure. With a short shank bit, liked a Kimberwicke, it might only be a small increase. For example, if you apply 5 pounds of pressure the horse might feel seven and a half; with a long shank bit, the amount of pressure might be several multiples of the direct pressure.