Understanding Rock

[AdamTill]

Hi folks,

I'm new to the board (and to saddle design), but have really enjoyed reading everything so far.

I gather that there were previously a lot more threads that were lost in a server crash, so please excuse me if this topic has been discussed before, but I would really appreciate learning more about rock in saddle trees. I can understand flare and twist pretty easily, and the concept of rock itself is straightforward, but the details of rock are eluding me a bit. I also understand that every tree blends these concepts to a large degree, but for purposes of discussion, splitting rock out for now would benefit me immensely.

So, in no particular order:

1) If rock can be visualized as a curve, is the "low point of the bowl" under the low point of the riders seat (assuming jineta design)? In the middle of the bar? (assuming that point isn't one and the same)

2) If one were to micro-fit a tree to a particular horse, how does one determine the amount of rock? Is that designed for the case where the topline is fired, and the back dropped, or a worse case then that?

3) If looking at the horse from behind, is the "plane of rock" perpendicular to long muscles of the back, or parallel to the midline? So in the attached photo, if turquoise is the rough plane of the back, red would be perpendicular to that, and dark blue parallel to the midline. That may be overthinking things since the difference would seem to be small on most horses, but I'm curious.

Thanks tons...really appreciate any help anyone could offer.

Cheers,

Adam

[Rod and Denise Nikkel]

Adam,

Yes, there is information on this topic in the currently buried threads. But I believe they are getting closer to the surface all the time. Be prepared for a slug of reading when they do come back…

3) If looking at the horse from behind, is the "plane of rock" perpendicular to long muscles of the back, or parallel to the midline? So in the attached photo, if turquoise is the rough plane of the back, red would be perpendicular to that, and dark blue parallel to the midline.

One of the things that needs to be considered when getting a tree that fits a horse is the angle of the bars relative to each other. They need to match the horse’s back. A flatter backed horse needs a flatter angle to the bars. A more A shaped horse needs a steeper angle. This means that the bars will fit against the horse on the same “angle” as the horse is. So when you look at the bars, the curve from front to back (the rock) needs to be evaluated not compared to the ground, but compared to the horse. Thanks for the pictures. They help in explaining. The blue line would be cutting through the bar at an angle, and that angle would change with every difference in bar angles. Life is complicated enough without that, so red line on your drawing is how we evaluate rock. Does it make a difference? Actually, a little bit. Enough to worry about? Questionable. How the horse is standing and moving and using their body makes more of a difference, but at least this is one variable we can control.

1) If rock can be visualized as a curve, is the "low point of the bowl" under the low point of the riders seat (assuming jineta design)? In the middle of the bar? (assuming that point isn't one and the same)

The shape of the bottom of the bar is independent of the shape of the top of the bar. The bottom is shaped to fit the curves of the horse. It needs to fit with contact down its full length, yet with relief built into the bar edges and the tips of the bars front and back so that it doesn’t dig in and cause excess pressure anywhere. If there is too much rock, pressure is concentrated in the center of the bar, or else, depending on a number of factors, the tree tips forward and the whole front half has excess pressure while the back half isn’t doing its share of weight carrying. So rather than picturing a bowl, I guess I would see rock as more like a saucer. The curves are gradual and mild, and although I guess technically there has to be a “low point” somewhere, it is not an easily definable place. And when you put it on the horse, that spot would be in a different place than a bare tree sitting on a table, and may vary a bit from horse to horse as well. As to how rock relates to the low point for the seat, that isn’t necessarily correlated. Ultimately that part is determined by the saddle maker, though a tree maker can make it easier or harder for him to put in a good seat. (What a “good seat” is we will let the saddle makers fight about discuss.) Depending on the length of the seat and the conformation of the rider (e.g. how much “padding” they carry on their backside) where that low spot goes may vary a little along the bar while still forming a very good seat. The rock in the bars still needs to match the horse regardless of where the seat is placed.

2) If one were to micro-fit a tree to a particular horse, how does one determine the amount of rock? Is that designed for the case where the topline is fired, and the back dropped, or a worse case then that?

There are no specific terms or numbers to describe rock. The best thing we came up with on our own was making a pattern with a flexible curve along the horse’s back under where the bar runs and sending that to us. Dennis Lane’s system has the same idea, but he takes his 3 ½” from the midline, but with the card perpendicular to the horse, not the ground. We are still working out how we can use that information in the way we build trees, but it is a much more consistent way to measure than the flexible curves. However, you need to have the horse standing square with his head in “normal working position”, because as anyone who has ever taken back drawings knows, that head and body position affect what you get in terms of shape for the rock. Overall, unless you have a mule or a horse that is extreme on either end, the variations in rock are relatively small. You don’t want a saddle that consistently bridges, and you don’t want one that has so much rock it has constant high pressure in the center of the bar. Anything else will not hurt your horse.

There is a lot of variation in spinous processes and while there may be a correlation between topline and the shape of the side of the horse where the bar goes, I am not at all sure it is consistent. At least, we don’t have enough information to know what it is. So we have decided look at where the bar actually sits when figuring out amount of rock. We only use the topline for figuring out gullet and handhole height.

If the question is “What do you fit – a damaged horse or a healthy horse?”, that is a very difficult question. If you look at a horse with a “sway back” and say, “I can change that by working the horse differently”, and get a saddle to fit the not so swayed back you are hoping to achieve, you will be using a saddle that will bridge on that horse now. The pain in his loin will cause increased splinting of the back, etc. etc. etc. and perpetuate the problem. You can run into other problems if you make the opposite choice. If the difference is severe, it may take two saddles. This is why we don’t want to “micro-fit” for one horse. The horse moves under that tree constantly and if you could read the PSI as he moves it would be changing every microsecond under every part of the bar. And unless you have spots that have too much constant pressure or way too much intermittent pressure, you will not hurt the horse if you get the basics right. So we work to fit a type or style of horse rather than one particular horse at one particular point in time.

[AdamTill]

Adam,

Yes, there is information on this topic in the currently buried threads. But I believe they are getting closer to the surface all the time. Be prepared for a slug of reading when they do come back…

Hi Rod and Denise,

Thanks very much for the reply. I look forward to doing a bunch of reading - my workshop is packed up in preparation for putting the house on the market, and it's killing me to not be able to go down there and putter.

One of the things that needs to be considered when getting a tree that fits a horse is the angle of the bars relative to each other. <snip> The blue line would be cutting through the bar at an angle, and that angle would change with every difference in bar angles. Life is complicated enough without that, so red line on your drawing is how we evaluate rock.<snip>

Okay, I'm with you on all points there.

The shape of the bottom of the bar is independent of the shape of the top of the bar. The bottom is shaped to fit the curves of the horse. It needs to fit with contact down its full length, yet with relief built into the bar edges and the tips of the bars front and back so that it doesn't dig in and cause excess pressure anywhere. If there is too much rock, pressure is concentrated in the center of the bar, or else, depending on a number of factors, the tree tips forward and the whole front half has excess pressure while the back half isn't doing its share of weight carrying.

So rather than picturing a bowl, I guess I would see rock as more like a saucer. The curves are gradual and mild, and although I guess technically there has to be a "low point" somewhere, it is not an easily definable place.

By saying saucer rather then bowl, do you mean that it should be a more gentle curve, or do you mean that the rock curves at the bar tips are more upswept then the curves in the middle?

<snp> As to how rock relates to the low point for the seat, that isn't necessarily correlated. Ultimately that part is determined by the saddle maker, though a tree maker can make it easier or harder for him to put in a good seat. (What a "good seat" is we will let the saddle makers fight about discuss.) Depending on the length of the seat and the conformation of the rider (e.g. how much "padding" they carry on their backside) where that low spot goes may vary a little along the bar while still forming a very good seat. The rock in the bars still needs to match the horse regardless of where the seat is placed.

My thought process in saying that the low point was in the center of the seat was that the seat (in my mind) should ideally be centered in the middle of the span of the bar which contacts the horse in most cases (ie, the section that isn't flared to clear the shoulder). That way the center of pressure (from the rider's weight) is centered over the center of support (middle of that span of bar). Equally ideally, the support center of the rigging on the tree should designed to straddle the center of pressure, so the saddle doesn't rock or shift.

Not sure if that's true, but it makes sense from over here.

There are no specific terms or numbers to describe rock. The best thing we came up with on our own was making a pattern with a flexible curve along the horse's back under where the bar runs and sending that to us. Dennis Lane's system has the same idea, but he takes his 3 1⁄2" from the midline, but with the card perpendicular to the horse, not the ground. We are still working out how we can use that information in the way we build trees, but it is a much more consistent way to measure than the flexible curves.

I don't know Mr. Lane's system, but I think I can picture what he's doing. I'll attach a few files to this message with my interpretation of that, though I'll pull off a profile 4" from the midline instead (assuming the midpoint of a 5" wide bar positioned 1.5" off centerline). The dark blue line is the midline profile, and the light blue line is the profile at the offset distance.

Now, in the photographs I've overlaid the profile lines onto the actual horse (the profile was done two years ago, though it still seems close enough today). The angled red lines are just an attempt to get a feel for the boundaries of the saddle fit region (saddle might be a scotch forward in the photos), and the angled green line is the center of the seat (or close to). The taped bits on the horse are meant to show the scapular spine, the rear edge of the scapula, the shelf of the ribs, the (very defined) edge of the spinalis muscle, and the edge of the last rib.

So, taking all that, if you go to the attachment that shows only the line drawing with the two profiles and the saddle fitting range, I'm picturing the line of rock being something like the pink line in relation to the back profile underneath the bar (standing, at rest) as the light blue line. Thus the rock measurements I'm trying to get an appreciation for are the ones marked forward (considering rock ahead of the scapular seems pointless, as flare would dominate), and rear (at the end of the bar).

Does all that make any sort of sense? (tough concept to show in drawings I'm finding).

There is a lot of variation in spinous processes and while there may be a correlation between topline and the shape of the side of the horse where the bar goes, I am not at all sure it is consistent. At least, we don't have enough information to know what it is. So we have decided look at where the bar actually sits when figuring out amount of rock. We only use the topline for figuring out gullet and handhole height.

Absolutely…midline profile along the tops of the spinous processes would seem to be almost but not quite independent of the profile in the region where fit occurs. The files I've attached back that up (in this case, anyway).

If the question is "What do you fit – a damaged horse or a healthy horse?" <snip> This is why we don't want to "micro-fit" for one horse.

I guess I had taken micro-fitting to refer to customizing a tree for a specific horse (as opposed to using a "standard" tree from a given range that comes close enough), but I take it the term actually more specifically refers to fitting a horse "as-is" with no consideration for whether that's where you actually want them to be in the future. I'll remember that for the future, and I'll also heed your warnings against doing so.

Edited February 13, 2008 by AdamTill

[AdamTill]

Sorry, the last graphic had an error on it (rock was being measured past the end of where the bar ended)...should be as follows.

[David Genadek]

Adam,

It might be helpful to you if you think of it in terms of a board with two hinges in it. When you engage the hind quarter

you will be effecting the rear hinge when you raise the base of the neck you will be effecting the front hinge.

David Genadek

[daviD A Morris]

Adam,

You sure have done a heap of work on this. What CAD program are you using? Personly I use AutoCad but only because when I first went to work as an engineer thats what the company used and I don't have time to learn anything else.

Just a little something that "jumped out at me". Many years ago, back in the 1980's I went around making plaster casts of horse's backs and from them i made the fibreglass replica of the horses backs. This was to help me understand the true shape of horse's backs and one of the things I noticed which was different to what most people have stuck in their minds about what they think a horse's back is shaped like, is contour of that cyan colored line in this drawing or yours.

Notice where you have the numbers 7, 8, and 9 that the line is convexed almost like the horse has a bump there, It is only slight and I find is further down the side, where the ribs "spring out" than what appeas in your drawing, but that is the nature of 3d stuf. When i first noticed it i thought it was something to do with my casting method. interensting that you have the same thing. This begs the question, if you are "micro" fitting this horse are you going to make the tree-bar concaved in that area? personaly it did lead me to flattening the bar in that area. But when i say that, i had previously just copied the old factory made trees from the USA, which had a lot of "ball" to them. With a bar that is convexed placed against a convex shaped horse the preasure is increased. (Of course you know that, just adding for benefit of other readers) The benifit of bars with a lot of ball is that while they will not fit any horse extreemely well they not be extreemley "bad" on a greater range of horses. Unlike the flatter bars, which will fit some horses very well, but on a horse that they are not designed for, the edges, which ever edge it is that makes contact, will cause severe problems.

On another point - I am curious that you said ".... the seat should ideally be centered in the middle of the span of the bar..." It appears to me that the brown saddle on the horse in that photo has the lowest poiint of the seat way back towards the cantle, the bars appear to be quite short behind the cantle. Thus it appears, in the photo anyway, that your weight would be place a long way to the rear of middle.

Your post is very detailed, I'd like to have more time to discuss more with you. looking forward to yuor reply.

dam

Edited February 14, 2008 by daviD A Morris

[daviD A Morris]

I'm new to the board,.......

2) If one were to micro-fit a tree to a particular horse, how does one determine the amount of rock? Is that designed for the case where the topline is fired, and the back dropped, or a worse case then that?

Adam,

I only just noticed that you are new to the board. WELCOME, I'm looking forward to more of your posts.

Forgive me for asking such a dumb question - but what do you mean by "topline is fired" I have not heard the term before.

Acutaly the whole question "Is that designed for the case where the topline is fired, and the back dropped, or a worse case then that?" Lost me, but I am just a dumb saddlemaker

regards

dam

[AdamTill]

Adam,

It might be helpful to you if you think of it in terms of a board with two hinges in it. When you engage the hind quarter

you will be effecting the rear hinge when you raise the base of the neck you will be effecting the front hinge.

David Genadek

Hi David,

Okay, that helps, but I guess my next thoughts concern where the flex "points" of those hinges would be.

For example, from Dr. Deb's writings and the dissection course I attended, I don't really recall there being much flexibility to the spine along the ribcage. But that said, "much" is relative. Can the area from around T12 back to the last rib be fitted with a rock that roughly equals what the back would allow when dropped (spinous process gaps limiting that)? Then, when the horse rounds up and releases the muscles of the topline, do those relaxed long back muscles "absorb" and conform to the rock that isn't technically ideal for the rounded condition?

As a result, is the rock at the back of the bar mainly designed to keep the skirt of a "western" saddle off the horse's back? (and by extension, would an "english" saddle with bars like a western saddle need any rock at the back end of the bar, given no skirt). Is rock at the front of the bar in excess of what the dropped back requires designed to account for scapula movement?

Lots of questions, sorry, but any help would be appreciated.

Adam

[AdamTill]

Adam,

You sure have done a heap of work on this.

Hi David,

Less then it seems, actually...I did the back tracings for David G when I ordered a couple of saddles from him a couple of years ago.

What CAD program are you using? Personly I use AutoCad but only because when I first went to work as an engineer thats what the company used and I don't have time to learn anything else.

I'm an AutoCAD user myself, for the same reasons (I'm an engineer as well by day...used to do mechanical design, though I'm now in land development). I've had a copy of Rhino3D for about 3 years from when I vowed to get into 3D modelling for some personal CNC milling projects (molded model sailplane wings), but haven't had time to learn how to do much with that.

Just a little something that "jumped out at me". Many years ago, back in the 1980's I went around making plaster casts of horse's backs and from them i made the fibreglass replica of the horses backs. This was to help me understand the true shape of horse's backs and one of the things I noticed which was different to what most people have stuck in their minds about what they think a horse's back is shaped like, is contour of that cyan colored line in this drawing or yours.

Notice where you have the numbers 7, 8, and 9 that the line is convexed almost like the horse has a bump there, It is only slight and I find is further down the side, where the ribs "spring out" than what appeas in your drawing, but that is the nature of 3d stuf. When i first noticed it i thought it was something to do with my casting method. interensting that you have the same thing.

I noticed it myself, and didn't know if it was a true profile or just a legacy of transfering flexible rulers to paper, scanning those, and tracing them in CAD. I tried to be careful and did the tracings twice to verify, but who knows what creeps in. I'd need to go back out and check on Marshall's back to see what the actual case...coud be the spring of the ribs locally, could have been a stance thing...who knows...

I actually used a line of best fit on the other sketch, FYI.

This begs the question, if you are "micro" fitting this horse are you going to make the tree-bar concaved in that area?

Not sure...it would seem to depend on whether that bump is a result of a skeletal/structural shape or a muscle/stance shape.

personaly it did lead me to flattening the bar in that area. But when i say that, i had previously just copied the old factory made trees from the USA, which had a lot of "ball" to them. With a bar that is convexed placed against a convex shaped horse the preasure is increased. (Of course you know that, just adding for benefit of other readers) The benifit of bars with a lot of ball is that while they will not fit any horse extreemely well they not be extreemley "bad" on a greater range of horses. Unlike the flatter bars, which will fit some horses very well, but on a horse that they are not designed for, the edges, which ever edge it is that makes contact, will cause severe problems.

Not sure what "a bar with a lot of ball" is referring to, sorry...new to this area.

On another point - I am curious that you said ".... the seat should ideally be centered in the middle of the span of the bar..." It appears to me that the brown saddle on the horse in that photo has the lowest poiint of the seat way back towards the cantle, the bars appear to be quite short behind the cantle. Thus it appears, in the photo anyway, that your weight would be place a long way to the rear of middle.

I noticed that it seemed oddly rearward, but I think it's just a weird photo angle. That's one of David G's excellent endurance saddles, and he wouldn't design the low point of the seat to be back too far based on what I know of his design philosphy. This is a link to David's website of the same sort of saddle:

http://www.aboutthehorse.com/secure-web/html/ls2d.htm

I don't want to speak for something I didn't design, but it seems to me that the low point of the seat is about in the middle of the section of bar that contacts the horse most often. Happy to stand corrected if I'm misreading that, however.

Your post is very detailed, I'd like to have more time to discuss more with you. looking forward to yuor reply.

dam

These sorts of conversations are why I'm here - happy to participate.

Cheers,

Adam

[AdamTill]

Adam,

I only just noticed that you are new to the board. WELCOME, I'm looking forward to more of your posts.

Forgive me for asking such a dumb question - but what do you mean by "topline is fired" I have not heard the term before.

Acutaly the whole question "Is that designed for the case where the topline is fired, and the back dropped, or a worse case then that?" Lost me, but I am just a dumb saddlemaker

regards

dam

Hi again David,

Thanks much for the welcome - very nice board thus far.

No such thing as dumb questions...probably just a terminology thing. A "fired" muscle in my world is just slang for a contracting muscle. So a fired topline would be when the muscles of the back contact, and the spine drops. So my thinking is asking that was to say that when the back drops, there would be the greatest degree of downward curve to it, and thus the greatest need for rock in the bar.

Cheers,

Adam