[Andy Carson]

I read an interesting research paper entitled

“THE RELATIONSHIP BETWEEN MECHANICAL WORK AND ENERGY EXPENDITURE OF LOCOMOTION IN HORSES” by MINETTI, ARDIGÒ, REINACH, AND SAIBENE.

Perhaps those interested can google it and see for themselves, I do not feel comfortable copying things from it onto a public site…

When they measured the capcity of a horse to store and release energy at the walk, they found that the horse was between 10-40% efficient. This is pretty much in line with the 33% efficiency I had calculated from Tim’s force and velocity numbers. It seems there really is alot of wasted energy there!

[Andy Carson]

Thanks all, I am not looking for an extremely strong animal, just something slow, steady, reliable, and low maintainence. I have a horse I can use for heavy jobs. I met a few donkeys a little bit ago and seemed to get along with them well. That is an important factor to me, but was curious if I was missing out on something as to why they are not more popular. I had also read that they don’t do well in the wet, and pittsburgh gets alot of rain. I didn’t know if that mattered all that much though, because I sure see them (especially the mini-donks) in almost every region of the country. It seems that if given a small shelter and a dry place to stand, there shouldn’t be any other special care needs…

[Andy Carson]

I think Kevins point is a good one, there are many factors and situations these buffers could help with, but I am not sure how to go about testing each individually (or even if this is possible). The factors or situations that seem most intersting to me are:

1. Buffering the starting of a load
The Spring really seems to help with this and I think it makes sense that they should, as this is an areas with a great number of very high peaks in draft. Tim’s system should be able to tell us whether the spring helps here. Honestly, I had not been designing this to help start loads, but it seems that this would be useful to alot of people too.

2. Reducing maximum draft
By my measurements, max draft force is reduced by the spring buffer. I just don’t know if the consequences of this are important.

3. Substituting for the animals natural buffering system
It seems that the spring, over the long term, might act as a substitute for the natural buffering system of the animal. Steel springs are extremely efficient at storing and releasing these forces and I think the best an animal could do would be to tie the spring’s efficiency. Judging from the speed vs force data, it seems that animals are not very efficient and I am hoping this might be where improvements in working load can be achieved. If the animal has learned to use thier own weight and gravity as a buffer, than the speed with which that energy is released would likely be constant (gravity is more properly an acceleration rather than a force). If the speed of the load and the speed of energy release are very different, than it could lead to substantial losses that could be improved upon by the spring. Measuring the average draft would tell us if this is working.

4. Smoothing forward speed
The spring definately does this, and I think this could be useful for some applications.

5. Improving animal comfort or balance
Maybe the reduced head bobbing was due to improved balance when the spring was used, which it seems would lead to improved comfort. I’m not really sure how to measure this though…

6. Shock load protection
Even though the sprin I made is not designed for shock protection, it should provide some protection and is likely better than nothing. This is not unlike “reducing maximum draft” and could be tested int he same way.

[Andy Carson]

@near horse 17626 wrote:

So, I do appreciate Andy’s “preloaded spring” which I interpret to mean you have compressed the spring to one degree or another and the buffering effect would come from any force large enough to relieve some of the preload (allowing spring to extend towards its “normal position”). This is opposed to the alternative which is a spring in its “ready or normal state” (not compressed) which is stretched beyond the normal state when placed under a load. How do those 2 systems differ? Is it just easier to calibrate as well as have multiple settings from the preloaded spring or is there something more?

The spring I am using is a compression spring set up to be compressed upon loading (look up a few images of a “drawbar spring”). If there was no preload on my spring, it would move 1 inch if 119 lbs pull was applied. With “horse sized” forces, this means it would always be compressed to one degree or another and would change rapidly in response to changes in terraign, etc. As almost any force was responded to at low preload settings, the system buffers unpredictably. If the spring was compressed 4 inchs, than the spring would not more at all unless the force exceeded 476 lbf (4*119), and would move 1 inch at 595 lbf. In second run I did, the force exceeded the preload only during times when the terraign was at least a little rough and the horse was in the middle of the “powerstroke.” In short, it only stored energy from moments where force was very high, which are exactly the areas I was targetting. As “very high” will vary from one job to the next, adjusting the preload gives you a way to adjust the threshold at which the spring will begin to store energy. This is one advantage. The other advantage is that a preload means the spring is less likely to respond to the small chaotic changes in draft force can lead to “swinging” and unpredictable buffering. One might think that you could “dial in” a different preload for every different job, but the math doesn’t support this. If my spring I was using was compressed 6 inchs, for a 714 lbf preload, it would only have about one more inch to more before the spring bottoms out. This could be solved with a longer spring, but even if the spring didn’t bottom out, the energy would be released too slowly to be effective. The math supports the idea that you would need a different spring rate (like 200 lb/inch) for this application. So, for big differences, you would have to change springs, but the preload can be adjusted for small differances within a specific range.

[Andy Carson]

Oh, Tim, do you know if oxen vary in thie draft loads to the same extent that horses do? If they have a variable pull as well, maybe the spring could be tested on your team at your place. I wouldn’t mind mailing the spring to you for testing and we wouldn’t have to wait until June to see what happens… I think I’m going to take your suggestion and shorten the thing too.

[Andy Carson]

Publishing is fine with me too… I think it’s a fascinating discussion. Like alot of reasearch, it seems that a simple question led not to a simple answer, but to many more questions. The jury is still out on whether this whole spring buffer provides any positive benefit, but if it does, than I do not see the variability of the work load being a huge problem. I think that adjusting the preload lets it work in a decent range of loads (the one I made should work from about 300-500 pounds). In my mind, pulls under this might be easy enough that there is little reason to buffer. If there was only a slight increase in efficiency, you might not want to bother buffering loads over this weight, as the horses are probably not going to pull them for that long. Of course, there might be more of a benefit… It seems to me that the main question is whether the spring or the horses weight shifting is more efficient, and if there is much of a difference. Again, the jury is still out on that…

[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…

[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.

[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.”

[Andy Carson]

That was a very interesting experiment and I learned a lot! To test the draft buffer, I made three 20 minute runs with a 1000 lb sled with different preloads on the spring.

First run: 2 inches @ 127 lb/in = 254 lb preload
This preload is substantially lower than the predicted effective preload and should not increase efficiency. I thought I would give it a try anyway and just see what happened. The first thing I noticed is that the load was easier to start from a standstill, but I wasn’t really that surprised at that. After that, I was struck with how smooth the ride felt. Normally, I prefer to stand on the sled rather than rest on the rail, but I quickly found myself leaning into the rail. No quick changes in velocity. Watching the compression of the spring, I could easily see that there are two nearly equal factors that compress the spring. One is the natural movement of the horse, with spring compressions every time the rear legs move from nearly vertical backwards to the end of the stride. I would have expected that this would have been the “power” part of the cadence, but was suprized to see just how powerful this part of the stride is, and just how weak (comparatively) everything else is. Watching the spring, it appears that the front legs can only “hold on” until a back leg stroke gets into the “power zone.” Another factor that extends the spring is the natural variation in the terrain. Interestingly, the variation in the terrain has about an equal effect to the natural variation in stride power. I had expected the terrain to dominate the spring extension. Definitely wrong there… Sometimes, the compression of the spring due to stride occurred at the same time as a bump in the terrain. This caused a movement of several inches in the spring. When the energy was released, it caused a swinging effect that was too slow and subtle to feel, but you could see on the spring. I was not sure whether the sled was speeding up and slowing down or the horse was speeding up and slowing down, but one of the two was. Once the “swinging” started, it continued for several seconds causing wide variations in draft that nearly bottomed out the spring and annoyed my horse greatly. Over the 20 minute test run, the effort seemed to be about the same as without the buffer, except during those times when the “swinging” was going on, when it was a noticeably more difficult pull.

Second run: 4 inches @ 127 lb/in = 508 lb preload
This was what I would have predicted to actually help with this load. Watching the spring, I basically only see compression at times when the terrain is moderate to difficult AND it’s in the middle of the powerstroke of the horse. This still produced a smooth ride on the sled. The spring only failed to come back to the resting point after the “powerstroke” if the terrain was very difficult or we were going up a hill. The preloading eliminated the swinging effect. Interestingly, this system greatly reduced the “bobbing” head motion during the high draft areas. It’s a pretty dramatic difference (with respect to the head bobbing) but I would be speculating a lot to guess why… Perhaps others out there have ideas? Overall, I could see a reduction in draft compared to the first run, but I am not sure about how this compares to a non buffered system. It’s not a huge difference, that’s for sure. At maximum, I measured a 7/8 inch displacement, which corresponds to a maximum draft of 619 lbf.

Third run: 5 inches @ 127 lb/in = 635 lb preload
In this run, I compressed the spring beyond the maximum compression seen in run 2. If the spring is compressed at all in this run, than the draft is higher than seen in run 2. I counted 58 times that the spring was compressed in this run, which demonstrates that the spring settings in run 2 were able to reduce the maximum draft force compared to this setting and probably compared to an unbuffered setting as well. Every time the spring was compressed, it was very brief, and I am not sure about how tiring 58 0.2 seconds compressions can be over a 20 minute run… Maybe I’m wrong, but I don’t think that simply reducing the magnitude of the maximum draft force is going to be a game changer… The larger head bobbing in areas of high draft was back though, and this was a dramatic difference that occurred for larger periods of time. This might be a game changer in itself or indicate that a “game changing” phenomenon is occurring that I don’t understand… Overall, I though that this run was maybe a little more tiring than run 2, but I am biased. It’s not a night and day difference, and would require some sort of more careful analysis to convince myself or others.

I am open to suggestions of tests that would be more convincing…

[Andy Carson]

A picture of the much debated spring type draft buffer… I will test it tomorrow. It was pretty easy to put together, and only took about an hour once the spring was here. I have to admit, I am having doubts about the math to come up with this spring rate now, because the spring “feels” kinda light without a preload. With a good preload, though, I can’t move it… I’ll know if I was way off tomorrow.

[Andy Carson]

Sorry, when I wrote about the “suspended and swinging” log, I meant suspended and swinging on just one end. The other end would be dragging. I didn’t explain that very well… I agree that there would be little buffering advantage in a completely suspended log. I think a bunk cart would be a good comparison, provided the attachment point on the log was roughly the same as with a chain type logging arch. If the log was moved up onto the bunk cart much, there would be less dragging weight than with the arch and it wouldn’t be a fair comparison anymore. If the dragging weight is similar, how does the effort compare between a bunk cart and a logging arch using a chain? Is there a noticable difference in effort?

[Andy Carson]

Carl, I couldn’t agree with your logic more. On the arch, you are probably mostly storing the energy by elevating the log. I have no doubts that the swinging chain could store all the excess energy produced during the “spikes” and think it’s likely that it would be released at a correct rate to be useful. Maybe if the chain is nearly horizonal, you might get a “pop” in energy release (as opposed to a “push”) but I don’t know how often that happens. Also, if the log is suspended from a nearly verticle angle (like if it was winched up), the rate of fall is close to zero and the system might fail to effectively store energy until the chain and log swings back to a “ready” angle, which might take a little time. Logs being hauled (as least from what I’ve seen) usually reside in between these two extremes and are probably in a position to immediately act as an effective buffer. Doing a more detailed analysis of the rate of fall for a log on a swinging chain requires calculus (I’m pretty sure) and I’m a bit too rusty to do that in public… The real beauty of the system, at least to me, is that the buffering capacity AND the draft load are both proportional to the weight of the log. In fact, the example is so similar that is might provide an effective demonstration of the usefulness of the buffering principle. Has anyone had to hitch to a log in such as way that half the log is elevated but the log was chained so as not to allow a swing??? I would guess that this would be more tiring to the horses and illustrates how effective buffers can be… I don’t have a logging arch, but someone out there might want to do a little experiment and compare the effort to pull a suspended and swinging log to the effort required to pull a suspended but not swinging log… It would just take a chain, a couple chains, a come-along, and a little “conditioning” time. The weight of farming implements is not proportional to the draft they produce, so some other energy storage system (like a spring) is needed. Preloading the spring is analogous to swinging the logging chain back to a “ready” position where it is ready to store and release energy in a timely manor. The maximum load is analogous to the load that would make a the chain on a logging arch nearly horizonal, which also represents the maximum that the logging arch can buffer. Not really that dissimilar when you think about it…

[Andy Carson]

I think Carl’s comments give not only a good way to think about this but also good way to mathematically model draft power loss and Tim’s force and velocity charts provide some data to do this with. Forgive the switching back and forth to metric and english measurements, I have an easier time doing physics in metric and I’ll dual list any important units… I see that in Tim’s example, the draft forces for the 1200 lb sled fluctuates between about 400 and 600 lbf, that is a differance of 200 lbs (890 Newtons). Force=Mass*Acceleration, so an 890 newton force should result in an acceleration of 1.63 m/s/s (F=MA, 890=545*A). Applied over 0.2 seconds, this would yield a velocity increase of 0.327 m/s (1.63*0.2) and a final velocity of 0.886 m/s (0.559 initial velocity + 0.327). 0.886 m/s is 1.98 mph, and the sled clearly does not reach this speed. This represents at least one source of power loss, but is it significant? The sled does fluctuate in velocity, with several fluctuations between about 1.25 and 1.5 mph (0.559 to 0.671 m/s) occuring in the 0.2 second range. This is a difference in velocity of 0.112 m/s (0.559-.671), and is a result of an applied 305 newton force (F=MA, F=545*0.112/0.2). So, as little as 37% of the peak force (305/809) in these high draft peaks is applied to the sled, and fully 63% might be wasted! As even the highest differences in draft force yield nearly the same speed, very high peaks in force are even more wasteful. These are the areas I am targetting. This analysis assumes alot (including that the sled/ground friction forces are constant and are overcome by the low draft forces) and probably represents a “best case” scenario. But still, it is nice to see that are there are large amounts of power that can be recovered from the high draft peaks.

[Andy Carson]

Yes, I forgot to mention their driver! Shame on me. He said he has had them from birth and has put a lot of work into them in the past and has taken them to several shows. Still though, I am impressed that they remembered their training so well and can maintain their composure. I can’t imagine what my horse would be like in that crowd if I hadn’t worked her since last November (or even if I had worked her the day before)…

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