Draft buffers

[Andy Carson]

Tim,
I would love to run a demo with you! I don’t want to waste your time with something that doesn’t work at all though, so will try to get some of the technical details ironed out to set up a more convincing demo. I think the best demo would be the spring at optimal adjustment versus a chain of the same length. The spring I used was from McMaster-Carr (Part Number: 96485K436). It is not a progressive spring, and is sold as a spring with a rate of 127 pounds per inch. This is if we believe McMaster-Carr, but you are right that I ought to calibrate it with known weights. I just didn’t have a reliable 500 pounds lying around and was excited to see if the whole thing had potential. I’ll figure something out for next time. This run was more to determine if the preload on the spring mattered at all and if it does, what sort of preload would be best. In my subjective measurements, it seemed like getting the preload right is very important and it also seemed like my math was pretty close to right (if we trust the preload on the spring and my only somewhat accurate estimate of the weight of my sled). Objectively, I could see a reduction in max force, but again I’m not sure if this is important. The head bobbing thing is troublesome to me because I had not expected it and makes me worry I am not measuring the real phenomenon. I am very curious to speculate on what this is. I wonder if the raising of the horse head and front quarters (to a much lesser degree) might be it’s own way of storing some energy to be released during the relatively weak front leg movement. If so, than the fact that it went away with the spring buffering could demonstrate that my horse felt like she didn’t need to buffer the load herself anymore and could just focus on belting out strong hind leg thrusts. That’s what it looked like to me, but again, it’s very subjective. The reason I want to know is that I could try to optimize the system for minimizing head bobbing (if that seems attractive) OR to minimize max draft OR to maximize smoothness. These are probably not mutually exclusive goals, but it would help prioritize my designing if I knew which would probably give me the most “bang for my buck.”

[Carl Russell]

I am having a bit of a hard time determining what you are describing as head-bobbing, but the way I read your report is that with the light setting the horse could compress the spring easily which took away the effectiveness of the buffer on the high draft areas. This was also accompanied by head-bobbing.

With the second setting the spring was compressed more commonly around the draft leading up to the highest levels, giving the horse a more elastic buffer at the point where the increase in draft could have more affect on the horse’s movement. This setting had less head bobbing.

The third and tightest setting only compressed the load at the very highest drafts, which was just slightly more elastic that without a buffer, and the head bobbing was back.

Andy you described in action what I tried to offer before as my understanding of how the horse’s bio-mechanics create an elasticity within their own body. If you think of the spine and the hind leg as two sides to a triangle, with the line of draft being the hypotenuse, the tendons and muscles that go up the hind leg and along the spine will lift the front end with every step. The bobbing of the head is the indication that the tendon is compressing all the way up to behind the ears.

With the middle setting the buffer is apparently more effective and as you intuited, I think the horse started to take advantage of the buffer and used the elasticity of the spring to her benefit.

Carl

[Tim Harrigan]

Countymouse;17566 wrote:

It is not a progressive spring, and is sold as a spring with a rate of 127 pounds per inch. I ought to calibrate it with known weights…. The reason I want to know is that I could try to optimize the system for minimizing head bobbing (if that seems attractive) OR to minimize max draft OR to maximize smoothness. These are probably not mutually exclusive goals, but it would help prioritize my designing if I knew which would probably give me the most “bang for my buck.”

If you have some front tractor weights they are nice to use because you can add in increments of about 100 lbs. It is nice to know for sure what the spring constant is and how it responds over the full length. As you become more familiar with the buffer will you be able to make it more compact? If we put a pull meter with it as it is the two things would be quite long and create a lower than necessary angle of draft.

The work I did was with a nylon tow rope, the response of your spring buffer might be similar but it might not. The response of the spring is likely more rapid than the nylon. The challenge is to link animal behavior to measureable things such as average draft, uniformity of pulling forces or other things that translate to improved performance.

Good observation and comments, Carl. That is what I was hoping for in my previous comment.

[Andy Carson]

Thanks Carl, What I was refering to as “head bobbing” is the normal gentle up and down movement of the horses head at a walk when pulling a heavy load. I’m sure you’ve seen what I’m refering to, I just didn’t know what else to call it. My horse doesn’t do this with light loads and just keeps her head at a moderate height. When the load gets heavy, though, the head goes down a little more and starts to “bob” gently up and down a few inches in rythym with her stride. This is probably the same as the tendon tightening phenomon you are talking about.

[Carl Russell]

Countymouse;17574 wrote:

Thanks Carl, What I was refering to as “head bobbing” is the normal gentle up and down movement of the horses head at a walk when pulling a heavy load. I’m sure you’ve seen what I’m refering to, I just didn’t know what else to call it. My horse doesn’t do this with light loads and just keeps her head at a moderate height. When the load gets heavy, though, the head goes down a little more and starts to “bob” gently up and down a few inches in rythym with her stride. This is probably the same as the tendon tightening phenomon you are talking about.

Yes that is the same thing. My uncertainty was around how it was demonstrated during your experiment. I tried in my own words to describe what I thought you were seeing.

When pulling heavy loads it is important that horses have enough room at the throat of their collar to allow for the bobbing of the head without restricting the wind. Too much room though will make the collar too big and won’t fit the shoulder adequately for appropriate draft. This is true for oxen also, although the bow is tight into the throat, it can’t be too high, nor too low.

The same thing is true for check lines that may also restrict free movement of the head to get that lifting action.

It is somewhat counter intuitive because these animals carry 60% of their weight over the front legs, that there is so little power there. I don’t mean so little, but comparatively little power. The front legs need to stabilize the animal’s weight. There is very little lifting mechanism in those muscles so for the animal to carry its front end forward, the rear end must apply a lifting action so that the front legs can move forward.

Carl

[near horse]

I know I’m on the fringe in this discussion and this could be way off base but could the head bobbing be more of a response to a changing enter of gravity (or balance point) as they walk – factor in a draft load and …. The reason I say this is I know some birds like geese and ducks extend or retract their necks to alter the center of gravity for take offs and landings.

Again, this might be splitting hairs, but I don’t remember tendons and ligaments being able to contract – only muscles. And muscles can only shorten (contract) or relax – no lengthening per se. That said, the biomechanics of how the muscle works a joint based on its point of attachment etc is how the work gets done. It is really incredible the series of levers (muscle/joint combinations) that work in unison to provide motion.

I think some of what we see in changes in movement based on load (or speed) is a result of utilizing additional muscle/joint combos as needed. For example, at a basic unloaded walk, it is primarily leg muscles doing the “work”. But when the load is increased I think some of the core musculature (particularly the back or back strap or tenderloin – your choice) comes into play in addition to the driving leg muscles. While not the same thing, that is how most quadripedal mammals run fast. Drive off the rear legs, extend the front through contracting the back muscles, and reach forward and anchor the front with the front limbs – repeat.

Sorry if I take us off topic – perhaps I should just listen 🙂

[Andy Carson]

I weighed everything used in the previous experiment to calibrate the spring and load data. I used a bathroom scale and weighed the sled at 290 lbs, me at 210, and the 12 cinder blocks at 420 lbs. That meant my total load was 920 lbs (not 1000). To measure the response of the spring, I suspended the spring buffer from a beam and put a large plastic barrel below it to fill with cinder blocks, which were weighed prior to adding. I added in groups of 3, which together were very consistent at 105 pounds per group. After I added each set of blocks, I measured the length of the spring. The graph is below. It starts at 35 lbs, which was the weight of the plastic barrel, chains, etc, which were I used to hold the cinder block weights. It is a very linear response in the spring, but the constant is 119 lbs/inch by my measurements, not 127 lbs. This means that it was preloaded to 238 lbs in the first run (not 254), 476 in the second run (not 504), and 595 in the third (not 635). These are pretty small differences in measured vs expected values and I do not think they change the “take home” messages. It was still an important thing to do. Kinda lucky, really that my spring constant was about 6% under expected and the weight of the sled was about 8% under. Almost the same amount of error in the same direction… Geoff, I do not think I am any more of an expect in on this topic than you are and I, for one, really appreciate your thoughts. I do not know if ligaments or tendons can extend like springs to a large degree either. That why when Carl first mentioned that ability of the animal to buffer the load by itself, I was a little skeptical. I did not think of the animal being able to store energy by elevating itself (or parts of itself). This seems to be an efficient way to store energy and makes some sense to me. A horses head and neck weighs alot, I’m sure, and elevating it a few inches could store a substantial amount of energy… Even more, or course, it the front quarters are involved in this too. I don’t see why it could not be a a way of balancing though. Have any of you out there notices larger degrees of head movement with loads that are more unbalanced or unpredictable? If this is a balance tool, the spring seems to make the horse feel more balanced and that is probably a good thing too, although more difficult to measure…

[Tim Harrigan]

Well it certainly is a linear response. If you make inches the dependant variable (corrected 4/18, not independant) the predictive equation would be more useful for set up. I have been revisiting some of the work I did with starting forces, I think I will start a new thread.

[Tim Harrigan]

Geoff, I appreciate your comments and observations in all the dicussions on this site so don’t think of yourself being on the fringe of the discussion. It seems to me we have all had experiences and abilities that add to the collective knowledge of this community of interest. It works well because so many folks contribute to the discussion. We could all keep to ourselves or have private discussions, but we make an effort to lay things out as clearly and completely as possible because we all have a role and responsibility in the education and advancement of those seeking to advance animal power as a reasonable choice for small farms and other sustainable enterprises. I have learned a lot from contributors to this site and I appreciate the effort from all.

[Carl Russell]

Yes Geoff, I agree with Tim. I appreciate getting challenges to try to clarify myself. I actually did not intend to infer that the tendon was constricting. There is however a tendon, or bundle of tendons that run along the spine, and the muscles attached constrict which allows for the hind end to lift the front end, all the way up to the ears.

I also didn’t not mean to infer that it was the tissue that was providing the “elasticity” that can buffer the load. Rather, it is the infinitely variable nature in which the power can be applied through the tissue that provides the bounce.

I think it is important to point out that the changes in measurable draft, are not just increases in resistance, but a measure of the horse, or ox, applying increased power. Andy pointed that out from his observations. The spring not only compressed due to changes in terrain, but was noticeably compressed by the action of the horse.

If there was a geared machine moving forward at a given speed, then the increased draft would be purely a representation of the change in terrain, but in this case we can see the animal responding to the demand. It is the reserve in the muscles and the ability of the animal to sense that, combined with the act of storing some energy with every step in the elevated from end that gives the animal the elastic ability to overcome the changes in demand.

That is not to say that the spring buffer doesn’t augment that.

If you look at Geoff’s description of how the four-leggeds run, you can see the animal lifting the front end and not only pushing it forward, but positioning the body weight so that is has to fall forward. Truthfully it only needs to lift the front a small amount to be able to lift and reset its front legs without stumbling, but at the same time, if it needs to, it can raise the weight higher to provide more forward-moving ballast.

As far as balance, I agree. We all know that energy can be lost easily trying to keep from falling over. While the body weight is suspended, or in other words abscent of gravity, then it takes less energy to balance. Some times it is harder to walk slowly then to run, because when running our body weight is mostly not touching the ground, and we only need to touch down at intervals to keep the weight moving where we want to go.

Carl

[mitchmaine]

carl, a good example of what you are saying about easier running than walking is a horse in the woods. takes a lot of training to make a horse walk with a hitch.
its hard for me to see an animal as a constant force. i’m not sure yet if i don’t see the buffer as an impedence. i have an old hitch spring from pioneer here somewhere. if i can find it, i’ll put it in front of our walking plow and see how they go. i appreciate all the forces and numbers presented and have read each one and follow the thread. it’s when i get to the collar, i start to wonder. carry on. let’s see what happens. mitch

[Tim Harrigan]

Mitchmaine, we have already seen instances earlier where the nylon rope buffer seemed to be more of an impedance than an asset, at least the average draft was higher in several instances. The key is that the spring constant and rate of response has to be right for the task at hand. That is why Andy is interested in an adjustable buffer. While the concept is interesting the tasks that most of us do are too variable in draft to make a single buffer a practical tool, other than perhaps for shock protection or if you are doing repetitive and predictable work.

Just because the nylon rope was an impedance in some cases we can not say the same about the compression spring until we can evaluate it and see how it is performing. The least we can do is learn something.:)

[mitchmaine]

hey tim, i agree. the reason its so hard finding that spring is the distance i flung it trying to use it last time. it was clumsy to me and mostly the horse. added 20 lb. dead weight to the already stiff load. increased the distance to the point requiring more adjustment. when all is said and done the solution must be practical. keep at it, we’re listening. mitch

[Carl Russell]

How would you guys feel about me approaching RH or SFJ about publishing this thread?

Carl

[blue80]

Don’t know if it’s been mentioned, I haven’t seen it, but I came across “draught springs” on http://www.carthorsemachinery.com they are available in different tensions for different work; again as mentioned the springs are mainly listed for sudden shock absorption I believe.

Kevin

[Author]

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