What method does CHA recommend for fitting curb chains to horses for curb bits and kimberwicke bits? How do you properly test the pressure of the curb chain?
There is information on this very subject in our Composite Horsemanship Manual, level 1, page 14. A curb chain or curb strap should be adjusted so that there is two finger's width (fingers stacked on top of each other) between the horse's jaw and the strap/chain. One way that you can check to see if the curb strap is tight enough is to gently pull back on the shank of the bit and the strap should engage (come in contact with the jaw) when the shanks are at 45 degrees.
If the strap is too loose, the bit will not work properly. If the strap is too tight, it will be putting undue pressure on the horse's mouth. The curb strap/chain for the kimberwicke is adjusted the same way as the curb bit. Also, you must make sure that if you are using a chain that it is twisted so that the chain lays flat against the horse's jaw.
By the way, the principles of what makes a bit harsh or mild also applies to curb straps and nose bands. The smoother the bit, curb strap or nose band, the milder it is. The wider the bit, strap or noseband, the milder it is. Texture and a narrow diameter make the device harsher. In other words, a curb chain is harsher than a leather curb strap because it has much more texture and points that focalize the pressure. A twisted wire bit is very harsh compared to a regular smooth snaffle because it has texture and a narrow diameter. A narrow nose band is harsher than a fat padded one, again because the pressure is focalized in the narrow piece and spread over a larger area on a wider noseband (or mouthpiece or curb strap).