Draft buffers

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.

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.

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.

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

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.