Dr. Hilary Clayton has been studying the way bits act on horses' mouths for more than 20 years. But even after all those years of systematic research, she still says that "finding the right bit is more a matter of trial and error than a scientific process."
And in that time her two most significant all-purpose findings are that the size of a horse's mouth isn't directly proportional to his body size and that most horses prefer thinner bits to thicker bits.
Clayton has conducted numerous test on bits, but one of the most significant was a "radiographic study of bit position within the horse's oral cavity," published in July 2005. That study, funded partly by the U.S. Eventing Association, analyzed the effects of six bits (the single-jointed snaffle, the Boucher, the KK Ultra, the Myler snaffle, the Myler ported barrel and the Myler correctional-ported barrel).
Clayton said some horses don't have room in their mouths for large bits, usually because their tongues take up too much of the oral cavity. "So the size and the shape of the bit is individual to every horse, meaning you have to keep trying until you find a bit they're comfortable with," she said.
Similarly, loose-ring bits tend to encourage horses to chew the bit, "which we want," said Clayton. But some horses get pinched by the loose rings and prefer eggbutt snaffles or other bits. Still, "I always recommend starting with a loose-ring snaffle," she said.
How Bits Work
Clayton's studies have both confirmed and disproved some of our traditional and popular theories about how bits work.
For instance, her studies have shown that reins can't act independently of each other. She's found that "forces applied to one rein are always transmitted via the joint to the opposite side of the mouthpiece."
And her studies have found that the action of a single-jointed snaffle is not a nutcracker effect, as has long been believed. What really happens is that, when you apply pressure to the reins, it pulls the joint away from the roof of the mouth, or hard palate, to compress the tongue against the bars of the lower jaw. Horses, it turns out, like the snaffle because they don't like getting poked in the palate and because the tongue cushions their bars.
The research Clayton conducted at Michigan State University has shown that the effect a certain bit has on a horse depends on its shape, what it's made of, and how the reins are attached to the bit relative to the position of the mouthpiece and the height of the rider's hands.
Clayton's research has shown that the bars and the roof of the mouth are sensitive and vulnerable to injury. Clayton advised that bits should be about a half-inch wider than the horse's mouth, to avoid pinching the lips or the tissues between the teeth. And she added emphatically that the cheek pieces should be adjusted so that there are "two little wrinkles" at the back of the lips. She said she too often sees riders whose adjustment leaves the bit far too low in the horse's mouth, allowing them to raise the bit with their tongues to either avoid it or to relieve the discomfort of the bit banging into their teeth.
"I know it's a bit controversial, but my observation has certainly been that horses don't like the bit way down in their mouth. It causes them to fuss and shake their heads, trying to move the bit around to a comfortable place in their mouths," she said. "My feeling is that a horse with a quieter mouth is always a happier horse," she added.