Starting draft

[Tim Harrigan]

Some of the comments in the previous thread referring to hitch angle and things like head bobbing that a team may do under a heavy load got me to thinking about some work that I did measuring the pulling forces when starting a good load. The forces for 4 starts are in the attached graphs, one showing only the starting forces, the other showing the starting forces and associated travel speed. Read the travel speed from the right-hand axis.

We used horses with standard traces starting a 2200 lb scoot on a firm grass sod. Each start in the graphs shows a start with 6 seconds of pulling. You will notice that the starting forces spike up very quickly and then drop off to a lower but pulsing range of pulls as the sled gets up to speed. The average peak pulling force for these 4 starts was 1272 lbf. The average pulling force measured from 2 to 6 seconds after starting was 682 lbf. So the ratio of the peak pulling force in the first two seconds of starting the load to the average pulling force once the load was moving was 1.9. That means the peak starting force was almost two times higher than the average pull.

This is important because when a team needs to generate these high forces to start a load the angle of draft is very important in allowing the team to use both strength and the mass of their bodies to get the load moving. Once the load starts moving it builds momentum and it requires less effort to keep it moving. So a load requires much more effort to start than to keep it moving. Carl has described several times how a team needs to get under a load by having a low hitch point such as a direct connection to a log, go-devil, scoot or other load. And, the linkage between the team and the load should be no longer than necessary to prevent the team from clipping the load on its heels. This creates the hitch angle, or angle measured from the line of pull to an imaginary line horizontal to the ground and through the connection at the load. Carl also mentioned that about 60% of the weight of the team is over the front legs of the team. This means the team’s center of gravity is shifted forward toward the point of the shoulder. When a team is pulling the line of draft should run through that center of gravity balance point.

When a team has a high angle of draft it can apply both lift and pull to the load. Increasing the lift decreases friction of the load on the surface and subsequently the pull required to move the load. Basically, the team both carries and pulls the load. Many competitive pullers like tall teams because it allows them to take advantage of a high hitch angle. If the hitch angle is low (such as when the traces are too long) the load will tend to pull the front of the team down into the ground. So they not only have to apply the high forces to start the load, they have to divert effort to resisting the downward force to stay upright. If the hitch is too high it will tend to pull the team over backwards, again not an efficient use of power. If the line of draft is correct the load will momentarily provide some lift and allow them to ‘get under’ the load, but not so much that they can not counterbalance the lift with the mass of their bodies. So they can efficiently use the power in their legs and add to that the forward force from laying their body mass into the harness or yoke.

CM mentioned a smoother start with his experimental draft buffer and the bobbing of the horses head under load. The high starting force has not been discussed in the draft buffer discussion, but that is another situation with potential benefit for some type of draft buffer. A team will always seek to counteract the forces applied by the load so they can remain in balance in work. If we understand the nature of the forces acting on a team, the direction and magnitude of the forces, we will have a better appreciation of the work that they do and how we can help them complete their tasks as comfortably and efficiently as possible.

[jac]

Tim Carl and Andy.. forgive the basic questions I have to ask but Im afraid I left school way too soon it seems.. Do these findings upset the theory that 16.2 is the optimum height for a draft horse in any way ? Are ponies working at a disadvantage pulling full size equipment designed for tall draft horses..even with the increased number used ? I ask because we use 17.2/3 hand Clydes and I have allways been pedantic about the draft angles and hitch points. Reading these posts makes me realise I havent been so far off the mark..at least with my wagon work anyway.. but I wonder if there is a way to make a graph to show the ideal hitch point height for a given height of horse/ox and given weight of load for example ? It would be interesting to see how close to the mark some of the old machines that we in the 21st century assume to be made correctly regards hitch point, when in fact they might not have been. If these findings can be put to use then we can make the horse even more efficient than he already is.. Just some thoughts…
John

[Carl Russell]

Thanks Tim, these numbers are great. I will post in the other thread too, but this is the “elasticity” that I was referring to when talking about the inner ability for animals to buffer the draft.

Because they are not geared machines that move at a given speed, or power setting, these animals can apply rapid and escalating, or de-escalating exertion. In this case the draft is in response to overcoming inertia, so there is a measurable point or exertion required to move the load, but the animals start at zero and increase their exertion until they reach that point. The elasticity is not so much in the tissue, but in the way it is applied by the brain.

But if we consider the accuracy of setting the point of draft in giving the animals an effective and consistent angle, so that they can apply not only their rear-end power, but also utilize the ballast from their body weight, then there is also another energy center where the requirements are not directly linear, as in a geared machine.

These numbers also point out how the measured draft increases precisely because the animals are pushing harder. There is a lot of elasticity in that. Animals are clutch, transmission, and throttle all in one, and when they are in good condition they can quickly modify the exertion that is required.

John, the angle of draft, and draft point on the shoulder are consistent from small to big, which is why most of the wagon-hitch horses have been traditionally “leggy”. If you consider some of Tim’s numbers from the other tread relating to wheel size then a tall horse can have reasonably good advantage over the high hitch point on a wagon that will have less draft resistance due to wheel size. This way these delivery wagons could be loaded pretty heavily. Not to mention how the larger wheels would help on the cobble-stone streets.

In terms of 16.2 hh being the best size. It may have more to do with manufacturers picking an average size to aim for, so that farmers could have a known entity.

Carl

[jac]

Carl when I mentioned draft point I ment at the trace end. Surely if a smaller horse is put to a vehicle designed for a taller horse and unless the point of hitch is lowered, then is the line of draft not going to come out somewhere behind the vehicle rather than between the wheel base and thus have the effect of not lifting but rather having a negative lift on the front axle? not a big prob on tar but if the horse has to go onto softer ground I feel the extra drft could be in issue.. By the same token I see a few tourist rigs over here with a very long wheel base on small tyres with a big Shire up front and have to wonder the amount of lift on the front axle which has surely got to be transfering extra work unnesisarily. This was my thinking behind the graf idea…
John

[Carl Russell]

John, we’re saying the same thing. I was thinking of “ideal” draft angle which is relative to the animal. The hitch point will change with the equipment, and yes then the animal can be compromised.

Carl

[mitchmaine]

john, i think i see what you are getting at. the pole on our grain drill is like all drills, higher than the wheel hub. and a pair of horse tires quickly on the drill when the evener is hooked high to the pole. i shifted it around under the pole and they went better longer as the draft angle at their shoulder was closer to square than before. i may even hook to the frame of the drill.

[jac]

I think Mitch has pointed out an important issue here. That is exactly what I was meaning.. Somewhere I have a copy of an 1860 chart used by vehicle makers of the time regards traction line and splinter bar height, wheel height and wheel bases ect. As Carl says the draft or traction line is constant thru the different heights of horses. My point is that perhaps we need a new version of these old charts to suit the fact we pull modern tractor implements. I noticed yesterday when I was out with the slitter I made that the front of my hitch cart was light on the front wheel.. The eveners were already in the top hole so mabey I need to make a new mounting with higher holes ??..
John

[Carl Russell]

Tim asked me to post this photo, that Jason posted, in this thread to show change in hitch angle. As you can see the line of draft is so high that the horses can’t get much lift, but the log arch is not only lifting the log, but there is probably some buffering going there also to assist the horses.

I thought I would also post this one as well. In this picture you can see with the draft angle almost perfect, that Jason’s horses are basically balancing on their front feet and using the hinds in combination with their body weight to lift this load.

Carl

[blue80]

Thought it’d be a good time to post this pic courtesy of Ray Steele. Looks like this single is making some adjustments to get/keep the load moving too!

[Tim Harrigan]

Wow, glad we don’t see that very often.

[Tim Harrigan]

What I like about Jason’s skidding pic is that the 4-up is pulling so well together to move a big load and it also shows in action some of the things we have been discussing. The wheel team has a pretty good draft angle on the arch, not quite as great as a choker on the log, but the arch provides lift and some other benefits that amplify the effort in moving the load. The front team has a higher hitch because of the linkage to the wheelers. You can see how that alters the line of draft to less than perfect (a low draft angle) but you can see that team knows how to work with what they have.

The other picture at a pull is a great example of a perfect hitch angle. The line of draft runs right through the balance point at the center of gravity. The balance is so perfect that it looks like the team could just hang there while they dance across the line on their hind feet. Great stuff, Jason.

[near horse]

Isn’t the line of draft in the picture from blue80 way off? The traces are essentially parallel to the ground = zero lift on what ever the load was. Is that right?

Also, here’s food for thought (or maybe the obvious) – when a team digs out on a heavy load and the front feet come off the ground, could there be some small advantage gained by the rotational motion of their body thru the collar – Hang with me yet. Imagine the hind legs as a stationary end of a straight line (the horse body) and the front end being the moving end (like the hand on a clock). As the rear legs dig in and drop the hips, the front end gets a little elevation and as the front comes down (and the horse extends into the collar) the “rotational” motion of the body around the fixed point (rear legs) might provide a little extra burst to start the load moving.

Or for you physics purists – it’s a torque with the horse’s body being the moment arm.

[mitchmaine]

hey geoff, i think i hear what you are saying. i totally understand the force created from a horses hind quarters in a pull like the one pictured above. but after you’ve been hit in the face with the dirt coming out from under their front feet, you start wondering how much is balancing and how much is digging? beautiful to watch, ain’t it.

[Tim Harrigan]

Don’t take my comments about Jason’s horses being able to hang there and cross on two feet too literally. What I mean is they are able to dig in and push into the collar without having to counteract the downward rotation that there would be if the line of draft was too low. It is more with the efficiency of movement and the ability to balance and channel all the effort to the goal at hand.

[mitchmaine]

hey tim, please excuse my remark. it wasn’t meant that way. actually i liked your description and have seen horses pinned, sometimes with such strain that there forelegs were momentarily free. i was trying to say like us all that a horse is a living breathing machine capable of many many shifts of force, balance, muscle or whatever. one hour after birth they are on their feet and a few hours later able to run, stronger and faster than we will ever be. they already have all the skills we want to “teach” them at their disposal. and they just wait there, ears trained on us, all they want to know is what we want. on a good day anyway. mitch

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