I agree it can be tricky as it can be very much up to personal preference of the horse. There are a few things you can go by though just from logic of how they work. It helps to think of the bit as it would feel pulled against your wrist (or to actually do this).
Thicker bits are generally milder than thinner bits as they disperse the pressure over a greater area versus all in one area…the exception being a horse who has a low pallet or not a lot of room in his mouth, in which case a thinner bit might be more comfortable… But still, the thickest bit this horse can comfortably fit in his mouth will be milder than a wire bit.
Smooth bits are gentler than twisted bits for similar logic… A smooth but makes contact with the horses mouth it’s entire length but a twisted bit makes contact at the edges, creating pressure points.
French link bits or bits with more than a single joint are often milder (with the exception of the Dr. Bristol) because they conform to the shape of the horse’s mouth better than a single jointed bit, therefore distributing the pressure over a greater area – and no longer a single spot in the roof of the mouth where the center joint would hit.
Leverage bits are always harsher than snaffle bits as the amount of pressure on the horses mouth is multiplied… The longer the shanks the greater the pressure
Rubber is gentler than metal because it is softer and a little forgiving. Other materials are mostly preference of the horse… Copper is often thought to promote salivating.
From that I know at least which bits I don’t want to try, and I can experiment with the ones that should be gentle in theory until I find one the horse seems to be comfortable and relaxed in.
As far as cheek pieces go… I don’t really think one is harsher than another but more have different functions.
My horse went in a full cheek Dr. Bristol with a slow twist before I got him and he was extremely anxious and always went with his head in the air. They had him in it because he was fast…but I found he just tried to run away from the bit. I wanted to switch regardless because it is much harsher than I want to use.
I first tried a full cheek Waterford as it is supposed to me hard for a horse to take a hold of since it’s so flimsy and appeared to be gentle because the balls were wide and smooth and with lots of joints it would conform to the mouth well. Initially he seemed to like it a little better, but I decided to try something else because he still seemed to be evading it, and started being more difficult to put the bridle on.
I went with a single jointed smooth loose ring snaffle, my reasoning being he couldn’t grab ahold of it as easily, he might appreciate the little warning it gives before full pressure is applied since he seems so defensive with the bit, and the single joint since the multi joint of the Waterford seemed like too much “noise” for him. He was definitely more relaxed in it and after the first ride I added bit guards to help him a little with steering as he was used to the full cheek, and to prevent the bit from being pulled through his mouth. He was much more willingly bridled as well.
I kept this one for a few months and we made a lot of progress but we stopped progressing I decided maybe I could find something even better to communicate with him, so I tried the same bit in a French link. He was more relaxed in that so that is what I stuck with. I briefly tried a Herm Sprenger loose ring with the losenge mouthpiece because of how it was contoured and the augrian metal to see if he’d like it even better – but he was nervous and evaded it so we stuck with the simple French link.
Hackamores put pressure on the horse’s nose, poll, and under the chin… and although don’t have a bit are NOT necessarily milder. People often mistake them to be gentler because there isn’t a bit but they use leverage on sensitive areas of the horse’s face and can be harsher if not used with very light hands. I am not overly familiar with all types of bitless bridles but they all create pressure on a horse’s face in some way and whether this is milder or harsher would depend on what areas of the face, the width and material of the noseband for example, if it puts pressure on the nose, and the horse’s preference…he might prefer pressure in his mouth over on his face.
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