Shank Length and Poll Pressure

The difference between the distance from the mouthpiece to the rein attachment on the shank and the distance from the mouthpiece to the bridle attachment on the purchase determines the leverage effect. The longer the distance to the rein attachment compared to the distance to the bridle attachment, from the mouthpiece, the greater the leverage.

We did some basic testing of two different bits to illustrate how the leverage affects poll pressure with the same amount of rein pressure. We used two similar bits, one with a shank roughly twice the length of the other.

The shorter shank recorded an additional poll pressure of 3.3bs. With the same rein pull pressure, the longer shank recorded an additional poll pressure of 6.6lbs.

The longer shank, which was roughly double the length, recorded roughly double the amount of additional poll pressure the horse would feel.

Though these exact pressure amounts may vary slightly when used with a real horse, they can be extrapolated to prove that the rounding that you use can affect the mechanics of the bit and the additional pressure felt.