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 (keeping it centered), and it controls the lever action of the reins. It keeps the bit shanks from bit from over-rotating. The curb strap 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 strap gives no leverage to the reins and no pressure to the chin groove.
When the rider picks back on the reins, the bit moves slightly, and the horse is alerted to impending action (signal). Past the first “pick up”, the balance between merely picking up the reins vs. “handling” the horse through the reins is where the curb chain is engaged. When the reins are “pulled”, “the shank of the bit rotates back towards the chest of the horse and the cheek (upper shank) of the bit rotates 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, the chain is pulled and tightened 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 (the bars) to push down onto the bars of the horse’s mouth, thus amplifying the bit’s pressure on the mouth.” Wickipedia Additionally, the “twisting” down on the cheek puts pressure on the horse’s poll by dragging the cheek straps of the bride down.
If the strap is too loose, it either doesn’t work at all or it slows down the action of the reins. When the rider pulls back on the reins, if the chain is too loose, it allows the bit shanks to rotate too far upward and the bit itself to rotate too far forward in the horse’s mouth before any curb action is engaged. It lengthens the signal time considerably, but, if used with a high port bit, may cause pressure on the palate that is both painful and damaging.
Because the curb strap keeps the bit from over-rotating, 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 movement instructions before uncomfortable pressure is exerted on his mouth. Additionally, it can cause constant pressure on the tongue, never allowing the horse any relief.
Common practice is to adjust the strap 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 strap. 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 (engaging the strap pressure action sooner) than a laid-back horse.
It has a buckle or hook attachment and English designs have a “fly link” in the middle to hold a lip strap. On English bridles the horse is bridled with the curb chain undone on one side, then connected once on the horse. On western bridles, the curb chain is kept buckled to both sides of the bit.
Caution: Single link curb chains (like dog chains) are very severe. Larger, flat chains or leather 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.
Curb Chain Size
The more narrow the chain is, the more intense is the pressure to the chin groove. Generally wider is better. Some horses can be irritated by the strap. In those cases, check to be sure the curb chain is being adjusted and used correctly. You can purchase rubber or plastic hose covers for the strap to lesson its irritation factor.
While this Quick Stop with Rope Noseband has no mouthpiece, it certainly has leveraged cheeks and a chin bar. The cheeks magnify the pressure under the horse’s chin to encourage a very fast whoa. Not for a novice rider or trainer, as it can be very severe.
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