I caught on at the last minute when I mentioned short shod in the heels. lol It's kinda coming together for me. Discussions like this help piece together the puzzle in my head. So in a sense goes back to the 50/50 over the COA.
Two things strike me about this picture: 1. There is very little shoe protruding beyond the heels. 2. If you remove the shoe the natural foot is not centred around the coffin joint.
Gary, it's an imaginary hoof for illustration purposes. You won't find a "natural hoof" that is proportioned this badly. See below how the COA position over the ground changes with the toe angle.
[quote="Tom Bloomer, post: 3475, member: ] Shoe leverage ends at the coffin joint. [/quote] How did you come up with the coffin joint?
So the principle is that in a well conformed hoof, the centre of articulation of the coffin joint bisects the bearing surface of the hoof (longitudinally and laterally) and that we should be aiming to replicate this when we shoe?
IMO, when people start using phrases like this it means their theory isn't working out like they hoped.
As long as you're shoeing replicas this is valid in regards to A/P loading. The M/L weight bearing vector is not vertical in the standing horse unless you find a horse with 4 centers of gravity. The weight bearing center is in the front half of the foot. Statistically it has been demonstrated to be at roughly 60/40% favoring anterior. So the position of the breakover point (anterior ground bearing point) at the toe relative to the center of GRF has substantially more influence over GRF distribution at the heel than the heel length relative to the center of GRF itself.
So what is it in all of this that requires us to leave an inch of shoe behind the heels? Which is what this thread was started to establish. From what you've said, it seems to me that so long as you have your hoof prep right, all you need is sufficient length of shoe to cover the heels and allow for growth and expansion.
That seems to be what many consider an adequate minimum standard in a glass half full sort of way. Generally I fit heels to a plumb line dropped from where the heel buttress originates at the coronary band. I have some that I want to be that way at the end of the cycle, and so I give them more length to accommodate that goal. Which means, based on the above criteria, I shoe everything "out of specifications with too much" heel. OTOH, nobody has presented any evidence that in and of itself excess heel out the back of the foot is harmful to the horse when the mechanics of the influence of the toe are taken into account. Most shoeing specifications (the stuff of text books and competitions) seem to be more concerned about how the toe is fit - as such they're more aesthetic specifications practiced my many over a long period of time. That ain't the same as empirically derived, since that sort of thing would show some evolution over time. To reiterate, I see a lot of crippled horses as a result of short shoeing and so far I haven't seen ONE that was harmed by shoes with the heels extended "too long." So if you haven't tried it on a few hundred horses, how do you know?
Because it's a joint and that is a good place to draw the line when it comes to a theoretical discussion about the leverage of ONE COMPONENT in ONE PHASE OF THE STRIDE an extremely complicated system that makes up of a living organism.
If the center of pressure is at the center of the coffin bone, that sounds like one messed up see saw to move the fulcrum away from center.
How do you move the fulcrum? You can't move the connections of the DFT and SFT - they fix the fulcrum and load bearing center to P3. The coffin joint's center is fixed relative to that as well. So those are two fulcrums that you can't move. All you can do is move the ends of the see saw and/or change its angle.
LOL! Now THAT would really complicate things. OTOH, without gravity, the BUA would take over the planet.
Question. The most weight bearing phase-would that be where the coffin bone is empty?, Thus spreading any pressure to the surrounding sides? Is a COA in different places based on what the horse is doing? Is the most weight bearing phase prior to the COA since the weight starts to lighten up as well? Would long toes or long heels or underslung heels change the COA or most weight bearing phase? Since the weight is spread out to the walls of the hoof, or coffin bone, would a misplaced COA or most weight bearing phase show up on the hoof, and can that be a guide as well for an educated farrier? Sorry if my wording is incorrect, but I think farriers can decipher it, LOL
Karen, I'm not sure the coa is anything a horse owner really needs to know about. If the end of p2 was a circle, p3 would move (articulate) around it. If you put a dot in the middle of that circle it would be the center of articulation. Look at the red circle in Bloomer's pic. The center of it is coa and does not move. The center of weight bearing on the other hand.... that is the yellow line in bloomers pic. That should have a big dot in the center of the coffin bone. The center of weight bearing doesn't change but the structures around it change in relation to it, ie, raise the angle of the coffin bone (club foot) and the toe gets closer to the center of weight bearing. Decrease the angle (negative angle) and the heel gets closer. To try to answer your last question, the hoof capsule distorts due to different weight bearing and is something Farriers deal with on every horse.
This has turned into a great discussion. Thanks all for input. I'm going to go back and read it all again just to absorb everything properly. It seems that there are several views on the original question. I had been leaning towards Tom's description of a plumb line from the heels in my thoughts, just an overall impression I had gained from reading on HS. Eric horse owners may not need to know about COA but it sure doesn't hurt and some of us are kinda interested in knowing
