The curb chain or strap is mounted in the chin groove under the horse’s chin between the bit shanks of the bit. It steadies the bit within the mouth, and it controls the lever action of the reins. The curb chain acts as an arm of a fulcrum, pushing the bit down onto the horse’s bars, amplifying the bit’s pressure directly onto the sensitive bars of the horse’s mouth. It also brings pressure up into the chin groove and “twists” the headstall down, causing downward pressure on the horse’s poll.
A bit with no curb chain gives no leverage to the reins.
When the rider picks up the reins, the horse is alerted to impending action (signal). When the rider pulls back on the reins, the bit moves. The balance between merely picking up the reins vs. “handling” the horse through the reins is where the curb chain is used.
When the rider pulls back on the reins, if the chain is too loose, it allows the bit to rotate to far forward in the horse’s mouth (pressuring the tongue or bars) before any curb action is engaged.
If it is too tight, it applies curb (chin) pressure from the reins immediately (as soon as the rider is on the reins), giving the horse no time to respond to impending movemenet instructions before uncomfortable pressure is exerted on his mouth. (Even more painful and damaging results can be caused by a curb chain that is too tight.)
Common practice is to adjust the chain so that it comes into action only when the shanks rotates about 45 degrees back. This is usually about two fingers of space between the horse’s curb area and the chain. However, each horse is different, and that adjustment is not set in stone. A strong horse who needs curb action to rate back may need a tighter curb strap than a layed back horse, so that the curb pressure is applied more quickly to the harder-to-manage horse.
Caution: Single link curb chains (like dog chains) are very severe. Larger, flat chains are better. Curb chain hooks can cause pinching. The Quick Link at the left is a very handy connector for curb chains. It makes it very easy to take a chain off or put one on a bit, and it won’t pinch.
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