Trying to think outside the box since my standby (loose ring french link snaffle) isn't quite working out. He goes alright in it, but tends to lean/pull/evade especially in downward transitions. I try to keep my hands soft and forgiving, not ream him in the mouth, leg on. He can get rushy when going from canter to trot at times, worse in one direction than the other (I think most of that is fitness as he's starting to come back into full work). He does open/gape his mouth and I do have a loose flash on to try to help. He will also toss or raise his head when pulling doesn't work (a lot at reverse), I sometimes work him in a running martingale(after a weekend off or going outside) and he accepts it happily.
His teeth were done in January. Eats fine, weight is holding. He does like to "clack" his teeth when he's relaxed under saddle, like he holds his mouth slightly open, and seems to press his tongue to one side of his mouth (not hanging out, but you can see it). Current bit is a fat, hollow mouth loose ring. Tried medium french eggbutt, seemed too much. Medium french boucher had lots of head tossing. Rode him in a waterford loose ring once, for jumping, he seemed much more inclined to behave and wasn't leaning as much... leaning towards using a waterford again. My other thought was a Nathe.
I just tried mine in the Sprenger Duo last night (happened to be the d-ring, as I borrowed somebody's). I really, really liked it on him. Right from when I got on he was stretching down into the bit (even on the buckle) and I could feel him stretching over his back. He was nice and forward in it, continuing to reach, and felt responsive still.
I was most recently using a double-jointed oval mouth loose ring on him, and I did find a few of the same issues as you - sometimes wanting to lean and get heavy in downward transitions, and he did seem to mouth the bit quite a lot (I can hear him sort of slurping on it, chewing, which can be good but he seemed to be opening his mouth too).
With the Duo I also found that when I picked up contact from a longer rein, that I didn't get the usual bracing, tense feeling. I don't think I realized how much he was doing this until he suddenly wasn't! Normally I would pick up the reins, and then just add more leg to push him forward, but I think now he was resisting the contact somewhat.
I rode him in it again today to see if yesterday was a fluke, and he felt good again. I did some pole work with lengthening/shortening to see if I had any adjustability and he seemed to respond with less fuss or leaning on me.
Yes, they're pricey, but I'm going to suck it up and get one. They do come in a loose-ring version for about half the price, but I don't have one to try and our tack store with a Sprenger rental program doesn't have it to test. I don't want to buy the loose ring, then find out he doesn't like it as much and have to buy the d-ring anyway.
I've spent most of my life riding horses. The rest I've just wasted.
He doesn't know what you want; no tack will fix that. You need to teach him. That is your job in the partnership.
"No tool is a sleeping pillow for poor riding skills. Every aid given must be taught to the horse as there is no natural reaction to it. It is the wrong idea to measure the possible physical influence of a tool. The horse is not a machine that reacts on a certain push-button, but a living being, learning in a pedagogical process. All kinds of aids are a language between man and horse, that both must learn. Once he has understood the aid, it is not the amount of grams or kilos in the hand that makes the reaction, but the understanding of the message and the attention he pays." -Bent Branderup
What groundwork have you done with him? He would benefit from walk halts on the wall, graduating to trot walks, and longing (but only if you are skilled in how to implement these).
Duo, Nathe, straight bar Happy Mouth, straight bar rubber. Any of those would be my go to on a young horse, especially an OTTB. If not that, then a jointed Happy Mouth (SINGLE. Some smaller mouthed TBs don't actually seem to appreciate french links, in my opinion, unless you can find one with a really narrow link...and even then, not always). Or possibly a jointed rubber. Any of the above or my go tos for youngsters.
Then soft, gentle contact with BIG gives when he responds correctly to an aid. Make the aids SUPER simple. Grab the breastplate or a neck strap and bury your hands if he's pulling so he can only pull against you.
I find a lot of young TBs like a steadier sense of the bit, so my go to is the Sprenger D ring that acts like a mullen when they pull, an actual D-ring mullen (happy mouth or Sprenger Duo) or a Myler Comfort Snaffle D ring.
We only work with TBs from birth to track to OTTB. Ours and others. I have found this is not that uncommon in the beginning. Especially with ones we have gotten from others. Like most we have our big box of tricks/bits most of which hardly every get used. Just the steady eddies. IME in the end the simple standards are the ones they end up with. Snaffle, egg butt, D and full or half cheek. Happy mouth Snaffle, or rubber. Happy mouth jointed or solid elevator, Happy mouth shaped Mullen elevator, Pelham, straight bar Happy mouth. Just about all end up in a Snaffle at some point. A gag if the rider has good hands and understands how they work and the negative effects they can have in the wrong hands. Double reins or side. My personal favorite for OTTBs that lean and or have head carriage issues is a German martingale with a soft bit. The horse will/should dictate how long to use it. A couple of weeks seems to work for ours and then go back to a snaffle. If they haven’t picked up on what’s being asked/expected alternate days. I also feel that owners of fresh OTTBs can be at a disadvantage if all they have to work/school with is a ring. Too much constant turning and checking from what they are used to. Long hacks, walk, trot, canter outside the ring. In our case we have a decent size property the perimeter of one field is 1 mile in circumference. We can do large to small figure 8s. And wooded trails and long off property hacks. Some come around quickly, some many months.
As always to each their own.
I agree with petstorejunkie, mine has only gone in a KK Sprenger loose ring but the leaning on the bit is something my trainer and I have to resolve. I half halt and drive forward when he leans. He is doing it less and less especially now that he is more fit. My trainer uses transitions when he leans. It has gotten much better.
He goes alright in it, but tends to lean/pull/evade especially in downward transitions. I try to keep my hands soft and forgiving, not ream him in the mouth, leg on.
I think that maybe it's something that you need to teach him? Being too forgiving might be reinforcing the lack of balance in the transition?
For horses who are too light in the contact I go to the rubbers and mullens.
For horses who lean I would use KKs. My favorite is the KK conrad. I like D rings too, especially for young horses. The first bit I put on everyone is my D ring KK conrad. LOOOOOVE it. : )
the mouth piece is shaped like this: http://www.bitofbritain.com/v/vspfiles/photos/743-2.jpg
I would try a rubber D or rubber eggbut - single joint - he may not like the loose-ness that comes with the loose ring and/or the french link but it's hard to tell which aspect he doesn't like. Some horses much prefer stability with a bit and while we think a KK loose ring is nice and soft, it doesn't give the precise signal they need. Same concept that many TBs like the feeling of a secure leg all the time - he wants to feel your hand "THERE", not wiggly/inconsistent all the time. A rubber mouthpiece will make it less severe. He also probably raced in a single joint D so it will be familiar.