The loose-ring rings slide through the mouthpiece of the bit.

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.

The mouthpiece of the loose ring bit, top, is thicker than that of the eggbutt snaffle, below, making the eggbutt preferable for some horses.

Thicker bits
Traditionally, trainers and riders have considered that the thicker the mouthpiece, the less "severe" a bit is, believing that a thicker mouthpiece increases the area over which a horse will feel the bit’s pressure.

Clayton has studied the way several different types of snaffles work, using a hollow-mouthed snaffle as the standard. She’s determined from studying snaffles that "one of the things horses really dislike is when the bit is pushing against the roof of the mouth."

That’s why many horses like the KK Ultra, many of the Myler bits or straight-bar snaffles. None of them touch the roof of the mouth. (The KK Ultra is one of dozens of bits manufactured by the German company Herm Sprenger. The KK bits are made of a metal called Aurigan, a patented nickel-free alloy made of copper, silicon and zinc. The KK bits range in price from $99 to more than $250, but replicas of the design, not made with Aurigan, can be purchased for half to one-third of the price. )

Since pressure on the reins of a single-jointed snaffle pulls the joint away from the roof of the horse’s mouth, that may explain why some horses lean on the bit or fall behind the bit. They’re trying to escape the discomfort caused by the joint bumping up against their palate. These horses often prefer a double-jointed snaffle, which riders have often tried because they thought it was "less severe" than a single-jointed snaffle. Really, it’s just more comfortable for some horses.

But picking the right-sized plate to use on a double-jointed snaffle is, once again, a matter of trial and error, because it depends on the size and shape of your horse’s mouth. Clayton said that X-rays show that the distance between the bars on each side of the mouth is only 2 to 2.5 inches. So, if the plate is wide, the joints could be right on top of the bars, "which would be quite painful," she said.

That’s why Clayton prefers double-jointed bits with narrow plates, like the KK bits. The KK plates are also rotated 45 degrees so that they lie flat on the tongue, not digging into the tongue on an angle. "They’ve accommodated the way the bit sits on the tongue, so it’s more comfortable and, therefore, more effective," she said.

She said that simple comfort may be the reason some horses clearly prefer plastic bits to metal bits. But she allowed that she hasn’t done any studies involving plastic bits yet.

Clayton observed that riders and trainers have long believed that horses "lean on the bit," or travel on their forehand, because they’re naturally (or have become) insensitive to the pressure or effect of the bit. But her studies have shown that the opposite is really true, that horses lean on the bit to relieve the pressure on the soft tissues of their palate.

And that’s just one more reason why, despite her years of continuing studies, Clayton still believes that finding the right bit for your horse is a matter of simply trying different bits.