BSMS, so now what I want to know is: Why is it so essential to change a horse like that out of the curb...? ...If it isn't broken, why fix it?
Why transition Mia to a snaffle bit?
My reasons certainly have nothing to do with any dislike of curb bits. The conventional wisdom is that a horse should be fully finished and completely obedient in a snaffle before transitioning to a curb. I reject that. In fact, I was banned from a horse forum for arguing that an average rider could transition an average horse to a curb bit without causing the horse to rear and flip...the English riders on that forum rejected it as heresy.
I actually think a good curb bit is a great bit for the average horse and rider, provided the rider uses the western approach of keeping slack in the reins unless giving a cue. From what I've seen, most western riders - the ones I respect, anyways - seem content to GIVE cues with some slack left in the reins, and almost never take all the slack out.
I actually think curb bits can be great bits for beginner riders, provided their instructor pulls them out of the saddle and makes them do pushups in the dirt every time they take the slack out of the reins. I think most beginners use the reins too much, not for balance, but out of fear that the horse will go too fast. Someone who is confident their horse will stop every time, right away, is free to let go of the reins and concentrate on leg and seat. All IMHO, but I think a good western curb bit is a great bit. Mylar makes a number of good designs. Billy Allens also seem gentle but effective. A smooth Jr Cow Horse - not one of the twisted wire ones - also seems like a good design.
So I like curb bits. But snaffles also have some strong points. I think a snaffle is the most intuitive design for a horse who needs to learn to flex laterally, or to tip his nose in during a turn (one of Mia's faults). It drops some of the advanced notice of a curb. In a Billy Allen curb, for example, the initial pull back on one rein starts to rotate that side of the bit with little back pressure. You have to pull further before the bridle tightens and the horse feels any pressure against the corner of its mouth. That advanced warning is an advantage of a curb design.
But for Mia, who believes the world is full of straight lines, and who has horrible lateral flexibility (she spins well but doesn't turn well), a snaffle should help me train her to use her body better in a turn. It also gives me a chance to see how well trained she is getting at stopping. I define training as building a habit of obedience so strong that the horse will obey the cue without thinking, even when scared or reluctant. By alternating between very similar style curbs and snaffle, both Billy Allen style designs, I hope to find out the training has carried over and she will stop reliably straight ahead in a snaffle. If she will, then that would give me even more confidence while riding her in a curb.
The "Old and New bits":
I dislike stopping a bolt by turning the horse hard in a snaffle. I've been able to do it with Mia on the trail if I jump in her mouth fast enough and hard enough...but there is nothing gentle about that, and the trauma of the struggle confirms her in her fear. That is why I've come to love a curb.
However, another bad habit Mia has is backing up. If she decides she really dislikes what lies ahead, she throws it in reverse. Smacking her hard on the rump with a heavy leather strap will not change her mind...she only accelerates backwards. Been there, done that. So when she throws it in reverse on a trail, where bad things may lie behind her, I want to be able to spin her around and at least get her looking where she is going. We can always turn around and try the bad thing again...usually with Trooper taking the lead long enough to show her it isn't bad.
I can do that in a curb, but I think a snaffle would be a better bit for that specific situation - getting a horse to do a 180 when you have no more than 5 feet of space to do it in. If she truly develops a solid stop in a snaffle, as in right now and straight ahead, then a snaffle would become a good choice for a trail ride.
If not, she can remain in a curb for the rest of her life. My birthday is next month. For #56, I might splurge and buy myself one of the Mylar bits with both a medium port and a roller. Used properly with a horse who understands it, a bit like that can be very gentle, very forgiving and a very good communication tool between horse and rider.
But snaffles can also be great bits. Ultimately, a few years from now, if everything goes right, I'd like to transition Mia to a high-quality leather sidepull, preferably custom made for her face. Not because curb bits are mean, but because success would mean she had been trained to listen and respond to her rider's wishes...that 'willing submission' that dressage talks about. Willing or (better still) enthusiastic submission is the goal of most good riders. If Mia & I ever get there, it will be after a long hard road. But it would also mean a horse [Mia] who was a horrible choice for a beginner rider [Me] had become a good choice for one...and that would put a big smile on my face!