Draft buffers

[near horse]

I agree Andy and also think that we are interested more in power rather than speed. Also, my understanding was that because “resting metabolic rate” was relatively constant regardless of activity level, it would therefore makeup a larger portion of overall energy expenditure for walking over galloping. That would make a lower energy activity “appear” to be inefficient – maybe not the best word to describe it. At least that’s how I understood the article (good choice BTW!).

[Andy Carson]

I reread that paper and do not see the resting activity being a serious limitation of this work… Geoff, perhaps you noticed something I didn’t see… I also learned a couple more things of relevant to this discussion that I hadn’t noticed on the first read. In thier introduction, the authors mention that tendons and ligaments can indeed store energy to produce a “bounce.” Later in the paper, they measure the large amounts of energy stored and released by the verticle displacement of the entire animal. This was substantial at the trot and gallop, but negligable at the walk. This is interesting in the context of a horse raising and lowering it’s head a couple inches during a heavily loaded walk. When a horse in the study was producing alot of power (as in a gallop) a verticle displacement of 10-15 cm (or 4-6 inches) was needed to store all that energy. If only half of the body was raised, the body would have to be rising and falling a full 8-12 inches… Different horses, different loads, and very different speeds, but it seems unlikely that raising and lowering the head slightly would fully recover the energy.

[Carl Russell]

Countymouse;17660 wrote:

….. it seems unlikely that raising and lowering the head slightly would fully recover the energy.

Yes, but it is representative of the lifting motion that the horse is using to get “under” the load, even if it is not entirely “lifting” the body off of the ground.

Carl

[near horse]

Hi Andy,

From the paper (actually in the abstract)

The estimated total mechanical work
(WTOT=WINT+WEXT) increased with speed, while metabolic
work (C) remained rather constant. As a consequence, the
‘apparent efficiency’ (effAPP=WTOT/C) increased from
10 % (walking) to over 100 % (galloping)

AND LATER ON (in the discussion)

…. the
value of C during walking is lower than that reported by Hoyt
and Taylor (1981). This is because the subtraction of basal
(standing) metabolic rate from the metabolic measurements, as
applied here, has a greater effect at low speeds than at higher
speeds (for a review of different techniques for calculating C,
see Full, 1991).

My point is that some of these numbers can be misleading.

Another interesting point is when isolationg the vertical, horizontal and lateral displacements as speed increased (Fig 5), it looks like there is an increase in vertical displacement (likely the change of gait) but no change in the horizontal displacement (stride length). That doesn’t make sense to me so I must be reading it wrong. Help me out here. OOPS – it’s displacement of the “center of body mass” – now I get it.

[Andy Carson]

I did a quick math check and have come to a very different interpretion of the radical verticle movement during galloping which seems to be efficient at storing energy and how that relates to energy storage with a horse pulling a heavy load. From Tim’s force data, the draft pulling a heavy load produces high draft peaks of about 200 lbf (890 newtons) if this force acts on a 1760 lb horse (800 kg), then the accelleration would be 1.11 M/s/s (F=MA: 890=800*A). As the acceleration of gravity is 9.81 m/s/s, this force wouldn’t even get the horse off the ground, much less elevate it 4-6 inches. This means that elevating a smaller portion of the body (such as the head) could indeed store energy efficiently. I think that in the case of the galloping horse, there is a rythmic up and down motion that is efficiently returned and added to slightly with every thrust. I am thinking of jumping on a trampoline, where each strong trust from your legs is extended in action by the springs of the trampoline and returned efficiently upon landing. Just like the trampoline (or a pendulum), there seems to be a small range of rythyms and speeds with which these different gaits can return energy efficiently. Perhaps this is why the animals in Tim’s experiments (and perhaps in real life) tend to top out at a specific speed when under heavy draft. Perhaps this is where the energy return is most efficient. On the surface, it seems that if the work is twice as heavy than the horse could expend the same amount of energy but do the work twice as slow. Maybe it’s just me, but my horse tends to gravitate towards a few different apparent speeds where she can get into a rhythum of raising and lowering her head in time with her foot falls. She seems much more happy at these specific speeds than if I speed her up or slow her down 20%…

[near horse]

I know this paper might be getting away from the mechanics of draft but is worth a look. It kind of addresses what I was saying earlier about how human selection for specific traits may have created a wide range of physiological attributes in differing horse breeds. Plus, I like their treadmill/draft setup! They are attached to a “load” in this paper.

Force, speed, and oxygen consumption
in Thoroughbred and draft horses
U. SILKE BIRLENBACH POTARD,1 DAVID E. LEITH,2 AND M. ROGER FEDDE1

[Andy Carson]

Another report from the spring draft buffer… In an attempt to find some more dramatic differances to display, I set the preset on the spring to 5 inches (595 lbs), loaded up the sled to 1130 lbs, and pulled some hills. My thought process was that by increasing the draft forces and efforts the point that they are much more difficult, differances in the buffered versus unbuffered system might be more apparent. I made a number of interesting observations, some seem obvious now but might be interesting to discuss nonetheloess…

1. When pulling a heavier load, the horse is more likely to bob it’s head, even if the system is buffered
2. The buffering capacity of this spring is not high enough for this particular load, especially on hills, where it bottoms out every now and then
3. Starting the load using the spring buffer appears very easy and the horse simply walks off without any double leg thrusts, even up hills
4. The buffer maintains a steady forward speed even when the horse is moving very slowly up a hill, and as the load never completely stops, the horse is more comfortable pulling at a very slow speed
5. Some of these hills make my horse very tired and I am used to seeing a little shaking in the shoulders when I give her a break at the top of one particular hill. The use of the buffer completely eliminated this, maybe because the horse didn’t have to rely on a strong (though brief) front limb push to keep the momentum going and allowed the rear legs to produce a larger percentage of the power…
6. I saw no obvious improvement in overall efficiency, in other words, a the end of the run, my horse seemed just as tired with the buffer as without. There are several explanations for this, but it still remains that the buffer is not clearly demonstrating an advantage in overall efficiency, at least by a very subjective measure…

After this set of observations, I switched back to the 4 inch preset (476 lb) and 920 lb sled to verify if what I had observed previously was reproducible. The reduction in vertical head movement is reproducible when the spring is in action. When the spring is not compressing at all (as in a downhill), the horse moves normally, likewise, if the spring does not return to the preset point (as in an uphill) the horse moved normally too. This different action on the part of the horse occures when the spring is rythmically compressed at a specific range of loads. It’s kinda cool to watch, almost like a dancer finding the beat, and there were several parts of the pull when this rythum could be maintained for a while, but I am beginning to think that this type of fine tuning might not be the best way to go. Another couple cinder blocks of load, for example, and the “beat” could never be found again…

I think I am going to focus on determining if and demonstrating that the buffer
1. Eases the starting of a load
2. Smoothes the forward speed and prevents stopping (and the energy expenditude to start again)
3. Reduces maximum draft forces
4. Eases the strain on the front quarters and allows the hind legs to contribute a higher percentage of the draft power

[Tim Harrigan]

Good report, Andy. It is really hard to assess these draft buffers unless you have some quantitative measures to compare. I like your attention to the signals your horse is sending, if it turns out we can do some actual measurements that confirm what you suspect is happening you will have more confidence in your interpretation.

It looks like at 4 inches the buffer is set to act at just about the average pulling force. You must be seeing a lot of action at that point. Does that seem favorable?

[Andy Carson]

With a 4 inch preset and a 920 lb load, I see action when the terraign is moderate to difficult and the horse is in the “powerstroke.” This action produces a audible, steady beat for most of the time we are going. It sounds like “ka-chunk, pause, ka-chunk, pause, ka-chunk, etc.” The “Ka” is the rubbing of the threads on the metal sprign holder as the spring is compressed, and the “chunk” is the sound of the set nut coming to rest on the holder when the pull falls below the preset. So, in one way, the spring is in action most of the time (as it only fails to periodically compress if I am going downhill). In another way, it is not in action very often because when it compresses during the normal stride, it only does so for a fraction of a second and quickly falls back to the preset point until the next stride. I think this is line with the periodic peaks in draft force that Tim has measured. I had thought the average draft on this load might be 368 pounds (920*0.4), which would only decompress the spring when the load is exceeded by 108 pounds ((4*119)-368), a nearly 30% increase. Perhaps because the trail is muddy in parts and full of ruts the draft is not best estimated by simply multiplying the weight by 0.4 it seems substantially higher… My thought was that reguardless of the terraign, I can vary to load, observe the behavior of the spring and the horse, and later “zoom in” on load and spring ranges that seem to warrant further observation. Using this stratagee, I can see “interesting behaviors” at all spring/load combinations where the spring is compressed briefly during the “powerstroke” and returns to the preset point during the rest of the gait. In other words, “Ka-chunk, pause, Ka-chunk, pause” seems interesting. “Kaaaaaa-Ka-Kaaaaa” does not seem interesting, and no sound does not seem interesting either. It is a good that the “Ka-chunk, pause” type of setting is pretty flexible and can be achieved over a wider range of spring/load combinations. Also, this observation gives the user an easy way to adjust the preload on the spring without requiring precise calibration. Interestingly, it seems that the lower preload settings (Kaaaaaa-chunk) seems more comortable than the higher preload settings (K-Chunk). What seems most uncomfortable is if there is no “Ka” or no “chunk.”

[Tim Harrigan]

Countymouse;17680 wrote:

I had thought the average draft on this load might be 368 pounds (920*0.4), which would only decompress the spring when the load is exceeded by 108 pounds ((4*119)-368), a nearly 30% increase. Perhaps because the trail is muddy in parts and full of ruts the draft is not best estimated by simply multiplying the weight by 0.4 it seems substantially higher… My thought was that reguardless of the terraign, I can vary to load, observe the behavior of the spring and the horse, and later “zoom in” on load and spring ranges that seem to warrant further observation.

I like the fact that you have visual and audio ques to help you assess the action of the buffer. I did not have either with the nylon tow rope. The 0.4 rule-of-thumb applies on firm, grass covered pasture or hay ground, level. It can easily vary from .3 to .5 as you do to other surfaces so if you are on a surface that offers more resistance it could easily .5, maybe more if it is soft and you are tilling ruts. A sled or scoot will be more variable than a flat bottom stoneboat particularly in soft soil or muddy ground. Because the weight of the scoot or sled is more concentrated on the runners it tends to cut in more so you not only have the resistance of the implement but the tillage effect as well. A flat bottom stoneboat will increase also, but not nearly as rapidly as a sled.

[mitchmaine]

tim, found that spring and hooked it on my spring tooth to scratch up some fall plowed ground last night. i was interested to know what it would do, and it seemed to be helping. then i noticed i had subconciously (?) shortened up the tug chains more to compensate for the extra length added. and had altered the outcome of the experiment. then started wondering how much efficiency is added just by hooking short. i’d also hooked in a third horse and (duh) altered the experiment again. there are lots of natural things we do anyway to increase efficiency. last winter we were talking snub chains and how to slow a load of wood down. how much efficiency are we talking about with our buffers? 5%, 10%? 25%? how much is worth chasing and how much is gained and lost by chance outside the control?

[Andy Carson]

You were probably going to do this already, but I would be curious to know what would happen if you compared a spring the spring to the “no spring” with the same number of horses and the same draft angle. In the best case scenario, we are probably chasing a 20% improvement in overall efficiency. This is based on Tim’s draft force and speed vs time chart where there were a total of 13 0.2 second peaks over 8 seconds total measurement time. That means the animal is producing high peaks 30% of the time. According to some math I did previously in the thread, these peaks may contain up to 67% wasted energy. So about 70% of the forces would be uneffected and the left over 30% might be reduced by 2/3. That results in a 20% reduction, but it assumes a lot and I would be suprized if this level is achieved.

[mitchmaine]

andy, i had tried same horses, same hitch length,and tried to keep it equal load and did notice a change in the way they pulled. without the spring the were up on their front ends much harder then without. using the tips of theit fronts.
next question. might the spring harrow be considered a buffer of sorts? in plowed ground, it rides along like a drag smoothing the plowed ground. yesterday, in my fall plowed, hard pack, it stood a few inches up off the ground on its points and walked along on the pan relieving the soil. giving and taking, much like the coil spring i was using in the hitch. it was a jerky motion that gave the horse a chance to get away from the constant load. what do you think?

[near horse]

Hey Mitch,

I think that outside of the whole draft buffer discussion here, one would/should change hitch length of the springtooth based on soil – newly plowed vs fall plowed. I would want to get a little lift (hook short) in that looser ground because dredging the springtooth through the soil is a pretty good load if it’s dragging dirt (seen it and done it).

That said, the erratic or changing draft of the springtooth on the fall ground is where I would imagine the spring buffer to work – levelling those peak drafts. Is that right or am I screwed up again?

Noteworthy – in the TB vs Draft horse comparison while both had similar peak force-speed , the draft achieved it at 2 m/s (~ 4.5 mph) vs 5m/s (~ 11mph) and the experimental treadmill couldn’t be set any slower than 2 m/s. Perhaps the benefit of the draft animal is the force generated at low speeds – much more practical for farm work. Ever run a tractor throught the field at 7 mph or more? Pretty rough stuff.

[mitchmaine]

hey geoff, i think you are right, but the only tool i have is observation. one note on the subject is that i noticed with the buffer spring, i could pull the horses into a walk and they would stay there with hardly any lines. without the buffer, as soon as i let out a little back they would go into a quicker pace. your right too about nosediving in softer ground. the spring harrow is a great tool, until it starts transporting the upper end of your field down to the lower end. imagine the draft on the horses, with a half ton of sod caught up in that thing.

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