Draft buffers

[Tim Harrigan]

I have been a little harsh on the nylon tow rope as a draft buffer. The idea of a buffer that can absorb energy in high end pulls and give it back when the required forces diminish has merit. The big challenge that I see is designing a buffer that will break over and extend or compress at the correct time. So if you had work that was very predictable you could design a buffer that could do exactly that. But, the tasks that most folks do with their horse are too variable in their pulling forces to make it practical. Either the springs will extend when they should not or not extend when they perhaps should. The other problem is the speed of response. Will it actually extend and retract in synchrony with the demands of the load? It it this lack of practicality for most farming and logging uses that makes me consider them as shock load protection rather than a practical and useful tool for other methods of conveyance.

The nylon traces have been promoted as a draft reduction tool for horse drawn wagons. We have have compared the nylon trace with standard traces on horse harness with both pneumatic and steel tires, with oxen compared to a steel tow chain with pneumatic and steel tires, compared the nylon traces to standard traces with on a sled or scoot on snow, pasture ground and two-tracks. The one application where we have seen a draft reduction is with a pneumatic-tired wagon. We loaded a wagon to 6100 lbs with logs and made multiple runs over a surface that was part hard packed two-track and hard pasture/hay ground. The graph of the pulling forces with horses with standard traces and nylon traces (buffer) is shown below.

The average draft with the standard trace was 273 lbf (4.5% of GVW), the average draft with the nylon trace was 225 lbf (3.6% of GVW). You will notice more frequent draft values on the low end with the nylon buffer and more on the high end with the standard trace. We did not see this advantage with the steel tires that create considerably more resistance. Because the motion resistance with the pneumatic tires is relatively low the expansion and contraction of the nylon buffer appears to be in synchrony with the movement of the horse and the transmission of forces. In this case the nylon traces do what they are supposed to do. There was about an 18% reduction in the average draft with the nylon traces.

[mitchmaine]

hey tim, you say there is a greater drag with steel wheels? didn’t realize that. we were actually considering going to steel on our hay carts cause we must have 50 tires here holding up some kind of farm machine and for some reason, hay wagons always have one flat tire. what are your numbers?

[Tim Harrigan]

Here is the graph I used in the logging draft bulletin. Flat tires are a pain for sure, but you will be asking a lot more from your team with steel tires. Draft with the pneumatic tires ranged from 4% to 9% of GVW, the steel tire wagon draft ranged from 9% to 16% of GVW. Steel tires do not deflect under a load so they sink in until the carrying capacity of the soil equals the downward pressure of the tire. Rubber tires deflect and increase their surface area under load so sinkage is less. Less sinkage means less motion resistance. You can see here that on the hard surface of the gravel road (mostly hard-packed sand) the average draft for the steel tires increased 115% compared to the pneumatic tires. On the hay ground, more irregualar surface, some sinkage but barely noticeable, 79% increase, and on the firm soil that was soybean stubble ground from the previous year early in the spring, the steel tire average draft increased 56% compared to the rubber tire. The background picture shows how the steel tires were cutting into the ground. It takes energy to compress that soil to move the wagon over the surface.

[J-L]

Very interesting Tim. Those figures validate what my dad always told me in regards to the steel wheels. Pretty significant.
Just at a glance it doesn’t seem too great a difference between nylon and leather tugs though.

[Tim Harrigan]

No, it is not a big difference but it is fairly consistent. It also occurs at the relatively low draft but there could be some benefit for young or small teams over the course of hours of pulling. It does demonstrate that the concept of a draft buffer is sound, the problem is that it is not practical unless you are doing repetitive and predictable work, perhaps like carriage driving. In general, for farming and logging, I really see the benefit as shock load protection. So if you have springs you rarely want to see them doing anything. My opinion.

Another interesting thing about these nylon traces, when we switched to the wagon with the steel tires the draft reduction advantage disappeared. Actually, the draft with the nylon traces was even a little higher.

[Andy Carson]

Tim, again fascinating work! It is interesting and encouraging to see that a buffer can be effective in some applications. And, in my mind, an 18% reduction in draft accompanied by a virtual eliminiation of any draft forces over 500 lbf is a big deal! It does make sense that the spring constant would need to be tailered to the draft forces for each individual job. Perhaps the nylon traces are too “stretchy” to be effective in the case of the 1900 lb log or the wagon with steel wheels. Looking roughly at the pneumatic wheel example, it looks like the traces would be extended only about 10% of the time (I added up the frequencies of the high draft areas where the nylon traces are significantly less common than when using standard traces). Replacing springs for different jobs would be a pain in the butt, but it would be pretty easy to adjust the preload on one spring even while in the field. It might be similar to the system used to adjust the preload on the springs on a motorcycle (necessary for different sized riders). This could provide a quick and easy way to use a buffer that will only be in action about 10% of the time. Based on Tim’s pneumatic wheel study, I think a spring that is in use only about 10% of the time seems to have potential for reducing draft. Also, using a steel spring would probably address any concerns over the speed of response. Just thoughts…

[Tim Harrigan]

Yes, it is very interesting. My interest and motivation is to understand the nature of the pulling forces and how they are transmitted to the team. An understanding of those issues can reveal opportunities to improve animal comfort and productivity. I have to think the animals would appreciate damping out those 1200 to 1600 lbf sulky plow forces measured with the ox team.

[Andy Carson]

I might make a mock-up of the design I have in my head and see what happens. I think I can test it out on my spring tooth, as I have some work to do with it anyway. Unfortunately, unless anyone can think of anything better, I will only be able to make a very subjective judgement of it’s effectiveness… Maybe if it looks like it has potential and the “bugs” are worked out, I’ll take some pictures and others can give it a try too. It would definately be useful to me if it works.

[Carl Russell]

Tim Harrigan;17273 wrote:

Yes, it is very interesting. My interest and motivation is to understand the nature of the pulling forces and how they are transmitted to the team. An understanding of those issues can reveal opportunities to improve animal comfort and productivity. I have to think the animals would appreciate damping out those 1200 to 1600 lbf sulky plow forces measured with the ox team.

This is what has been going through my mind.

First of all is the difference in pneumatic vs. steel a matter of elasticity of the tire, or a matter of the steel wheel getting “trigged” against the berm of soil formed in front of the depression?

I am also interested in knowing the dynamics associated with changing nature of the required draft power. My sense is that the animals already are the buffer. After-all they are not gear operated.

The mental requirements for moving weights must take into consideration the varied nature of the working situation. This is why conditioning is so important for working animals. They need to have a built-in elasticity to the demands of the load, which has to be related to strength, agility, and stamina.

I can understand the interest in placing buffers between the animal and the load, but I wonder if it is un-necessary, and in some ways may counter the development of the energy management that is required of a working animal.

Carl

I came back, because it occurred to me that in the case of a logging arch, advantage is afforded not only by lifting the weight, but by allowing for an elastic forward movement as the angle of the chain changes to lift the log before it actually moves the log forward. So it may not be out of the question to add some such mechanism. I just want us to keep in mind how the horses or cattle work physically to overcome the draft requirements.

[near horse]

I think that Carl makes a good point about the steel vs pneumatic wheel issue – for example, isn’t the friction force that must be overcome to start a load, greater for a pneumatic tire than for a steel wheel – on a solid surface with essentially no deflection, like pavement? That’s why we choose them for use on highways because we can use that friction to stop us more effectively when we apply our brakes. That said, it then seems that the “downside” of steel stems from the effect it has on the soil/substrate that you’re trying to roll on – since it can’t “deflect” or compress in response to a load, the soil ends up compressing and results in higher draft numbers –

One additional advantage of pneumatic tires is the flexibility of adding or removing air to increase flotation or decrease friction depending on the substrate we’re moving on.

Sorry if I ended up repeating some earlier comments – I think that I did. But with regard to the buffering of the load with nylon tugs it seems to me that the E required to pull a 12″ share say 200′ through soil that has spots of higher and lower draft is independent of what type of tug you’re using – the stretch and return of the nylon tug all add up to the same amount of E as the standard tug (it has to doesn’t it?). If you graphed the draft over the whole 200′ pull wouldn’t you see fewer highs and lows with the nylon tugs but in the end, the same total E expended? I guess that’s why I see the benefits more as shock load absorbers but I’m no physicist.

[Tim Harrigan]

Carl Russell;17278 wrote:

This is what has been going through my mind.

First of all is the difference in pneumatic vs. steel a matter of elasticity of the tire, or a matter of the steel wheel getting “trigged” against the berm of soil formed in front of the depression?

In most cases it is a combination of both those things. I will comment on the animal-as-buffer later.

Draft is the pulling force (pounds-force, lbf) measured as tension in the towing chain needed to move an implement in the direction of travel. Draft for wagons and carts on level ground is largely the force needed to overcome the motion resistance of transport wheels. Motion resistance is the force needed to keep an implement moving at a constant speed while compressing or moving soil and overcoming wheel and axle bearing friction. Motion resistance is largely a function of the road surface. A hard surface offers little motion resistance while a soft surface offers considerable resistance.

Important design considerations affecting wagon draft are tire width, tire height, and whether the tires are pneumatic or steel. Pneumatic tires cushion the impact of stones and other obstructions and are particularly helpful on a gravel road. Pneumatic tires also deflect under a load. As the load increases, tire deflection increases the tire/soil contact area. This provides a larger bearing surface, improves flotation, reduces tire sinkage, and reduces motion resistance and draft. When a steel tire contacts an obstruction such as a stone on a hard surface it cannot deflect and easily roll over it. Instead, it acts as it would going up a steep incline, if only for the few inches it takes to get over the stone. Draft forces spike up in that short distance as it climbs over.

A wheel will sink into the ground until the resistance offered by the soil equals the pressure applied by the tire. Wheel sinkage increases a tire’s contact area and bearing surface. Tire sinkage increases the tire/soil contact area when steel tires are used because steel tires do not deflect under a load. Tire sinkage increases wagon motion resistance and draft.

A large tire bearing surface generally improves flotation, reduces sinkage and reduces motion resistance and draft. Tire bearing surface is directly related to wheel height. In other words, doubling wheel height doubles the tires contact area. In a uniform soil and under similar loads a 48-inch wheel will sink to only one-half the depth, but will have the same contact area as a 24-inch wheel.

In many cases, wagon design limits our ability to use taller wheels to improve flotation. And, taller wheels may be a hindrance when loading or unloading a wagon by hand because a higher reach is needed. Perhaps a better way to increase tire contact area is to increase tire width. Tire bearing area increases in direct proportion to tire width. In a uniform soil we expect a four-inch tire to sink to one-half the depth of a two-inch tire. Wider tires typically create less motion resistance and draft than narrower tires

[Tim Harrigan]

Carl Russell;17278 wrote:

I am also interested in knowing the dynamics associated with changing nature of the required draft power. My sense is that the animals already are the buffer. After-all they are not gear operated.

In this work measuring wagon draft we loaded a steel-tired wagon and a rubber-tired wagon to 6100 lbs. Pulling forces were measured with horses using standard traces and a nylon trace. We also used an ox team with a standard North American-style neck yoke with a tow chain or a nylon tow rope.

Because an average draft represents a composite of pulling forces, it does not reveal much about the nature of the forces transmitted to the team as it pulls the load. Is the load steady and predictable, or does it bounce between a wide range of high and low values? It seems that a steady and predictable pull would be more suitable and comfortable for the team. The graph below shows how much and how rapidly the pulling forces fluctuate. These were the pulling forces for the horse-drawn, rubber and steel-tired wagons with standard traces. The average draft for the rubber tired wagon was 282 lbf, 490 lbf for the steel tires. The forces were measured five times per second and bounced between 350 lbf to more than 1000 lbf for the steel tires and 350 lbf to 700 lbf for the rubber tires. This is largely the pulsing of forces from speeding up and slowing down as the team steps into resistance of the load.

There were big differences in draft between the steel and pneumatic tires. There were small differences between the standard traces or steel chain and the nylon traces. Probably the most interesting result of this work was the effect that the type of hitch had on the wagon draft. Hitch in this case refers to the team (horses or oxen) and the harnessing method (collar, standard tugs and evener for horses; neck yoke and steel chain for oxen). Compared to the horse hitch and rubber-tired wagon (average draft of 260 lbf), draft increased about 19% (to 309 lbf) when using an ox hitch. When using the steel-tired wagons the ox draft increased about 17% from 490 lbf with the horse hitch to 574 lbf with the ox hitch.

At least two things could have contributed to the lower drafts of the horse hitch. One is the greater angle of draft with the horse hitch. The hitch angle with the horse-drawn wagon was 16 degrees but only 8 degrees with the ox-drawn wagon. A low angle of draft provides a horizontal pull but little lift for clearing bumps and small obstructions. A greater angle of draft provides more lift which lessens draft on uneven ground.

The most likely cause for the improved efficiency of the horse hitch is in the nature of the hitch itself. There no reason to think that the wagons pull harder with the oxen. The difference is in the transmission of pulling forces to the team. The horse hitch reduced the high-end drafts compared to the ox hitch. The horse hitch redistributed the load through four traces and the two-horse evener and shared the load between two collars. The ox yoke transferred all the pulling force through a single towing chain. There was no redistribution or sharing of high-end drafts, and every surge and shock was transferred directly to the yoke beam and bows. The multi-component horse harness was an effective draft buffer.

So it is difficult to separate the animal from the hitching system from the load. Of course, the teams are responding to the resistance of the load so they perceive the magnitude of the fluctuating forces. They absorb the pulling forces transmitted from the load and through the hitch. This is why proper fit and maintenance of equipment is important. Conditioning is important so they maintain a working reserve of power to respond to the spikes in pulling forces, so they are not exhausted and uncertain from working near the edge of their ability.

[Tim Harrigan]

Here is a force frequency graph slicing the distribution of pulling forces into smaller 50 lbf increments comparing the horse hitch and the ox hitch drawing the steel-tired wagon. As I mentioned, there could be some small effects from the lower hitch angle with ox team, but the surface was quite hard and fairly even so it seems any advantage from a higher hitch angle would be small. But the buffering effects of the horse hitch are very clear. Look at the greater frequency of high-end forces on the right side of the graph for the oxen.

So what about draft buffers? Well designed and proper fitting horse harnesses are an advancement in draft animal technology compared to the ox hitch. The beauty of the North American ox hitch is in it’s simplicity and attention to animal comfort. I think there is value in draft buffers as equipment and animal protection from shock loading for both horses and oxen. From a practical standpoint, it does not seem necessary as a draft reduction tool for horses if their harness fits and they are conditioned to work. But that does not seem to be the case in many parts of the world.

As we look around the world we can find many cases of draft animals that are poorly fed and cared for with harness or yoking systems that are not attentive to animal comfort and productivity. Not out of malice, but for lack of resources, education and a cultural understanding that places little value on these things that we understand to be so important. For instance, it would not be uncommon to find folks clearing overgrown land for agricultural production with underfed oxen with poorly designed and fitting (by our standards) yokes with primitive plows catching on roots or obstructions every several feet. Any question why they might not demonstrate willingness? So perhaps buffering systems, including harness and yoke improvements offer a possibility of real improvement in many situations.

Closer to home, some type of buffer may offer more to oxen than horses. Some may promote the 3 pad ox collar but I do not know much about it, I have never used or tested one. Presumably, it could offer some of the same benefits as the horse collar. I would consider a compression spring buffer for some jobs for my team, but those would be higher draft tasks with an eye toward shock load protection. My understanding is the 3 pad collar may not be best for the high draft work. We always need to keep an open mind toward possible improvements in our hitching and harnessing systems.

[Carl Russell]

Tim, great work!

Why don’t you think that the ox yoke acts like an evener?

I would think that draft angle would have more effect than you seem to indicate.

It seems to me like the differences in harness systems (leather with collars vs. wood and chain) and angle of draft are variables that could contribute to the differences in results.

I also wonder about the natural motion of horses. Does their motion dynamics offer superior draft buffer?

Would the more fluid motion of the horses have an affect on reducing the spikes in draft, which would in essence reduce the average?

Carl

[jac]

This thread is realy interesting. Most of my work involves wagon and hitch cart. I try to set up the angle of draft so that an imaginary line passes from the hame hook,thru the tug and on thru the equalisers to end in a point on the ground 2 3rds of the way back on the wheel base. The older men tell me that a lot of modern Clydes at 18 hands are too tall for efficient draft because of the uplifting action of the pull. 16.2 is said to be the best.. I imagine if the draft angle on a wagon is steep and thus the line of draft ends near the front of the wheelbase, it would have the effect of trying to lift the front of the wagon as well as trying to pull forward.. Also if too low and ends with the line out behind the wagon, will that not create a slight downward pressure on the front axle.. not a real problem on tar but what about on soft ground ? I try to spend a bit of time setting this up as im trying to eliminate as much unnesisary draft as I can. When I look at the plans for the old railway delivery wagons the splinter bar was always set higher than the hocks and most ran on 39″ front wheels with a full lock..
John

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