Draft Horse Mechanics

by Edward N. Wentworth
re-printed from The Country Gentleman, May 10, 1913
re-re-printed from Small Farmer’s Journal, Fall 1977
drawing by LRM

We present this by special request of lifelong Draft Horse Champion and SFJ friend John Erskine.

In the minds of many purchasers the mentality of the horse, his temperament, traits and habits overshadow his actual build, and as a result some of the organs and parts most prone to weakness are absolutely neglected. This is particularly likely to be true in the case of brood mares. The old saying that a chain is as strong as its weakest link finds admirable illustration in this species. The working portion of the animal comprises a large number of “links” in the bones, joints, muscles, tendons and ligaments, and these through the sheer force of their number cannot always be equally strong and secure. The market teaches a severe lesson when offered horses that are even slightly defective, for cuts of $5 for a small bog spavin and from $75 to $100 for a cocked ankle are ample evidence that some one appreciates the lack of efficiency in those particular links. While such blows may teach a man what not to do, they resemble too much the familiar “don’ts for children,” in that they fail to educate him for what is needed. It is not through the eye of the artist, with his concept of beauty, that the following discussion is planned, but rather from the viewpoint of the artisan with his knowledge of mechanics.

Other things being equal, draft power is in direct proportion to weight, while endurance at work requires in addition what is commonly known as substance or the size of the muscular and bony framework of the animal. On the American market and in the French and Belgian breeds, weight is the all-embracing requirement. The average of the Chicago draft markets for the last three years shows that the first two hundred pounds over 1600 pounds are worth $37.50 each, while the second two hundred average about $75 each. If a 1600-pound horse sells for $225 the 1800-pound horse is worth $300 and the ton horse $450.

The Englishman and the Scotchman set their standard by another means, using height and bone for their gauge of draft value. As to which of the two standards is the more desirable it is difficult to say, each having its advantages and disadvantages.

A simple law of physics tells us that a body in motion tends to remain in motion, and another states that the energy of a moving body is the product of its mass by its velocity. This being the case the heavy draft horse should have advantage not only in the ease with which he draws the load when traveling at a rate of speed equal to his lighter rival’s but also in the ease with which he keeps it going, since gravity can overcome the movement of heavy bodies less rapidly than that of lighter. However, on the American market weight too often means simply fat; in fact, the advertising posters of the average horse buyer clearly state that anything fat will be bought, whether sound or unsound, at the same time issuing a warning against bringing thin horses with the expectation of sale.

The Strength of Muscles

Fat is simply a solace to the eye in the case of the majority of horses, as it is soon lost at hard work, and can only in an incidental manner assist in draft service. The height and bone standard on the other hand guarantee a powerful frame, but fail to assure the buyer of the economy of food consumption. The horse needs his food for the same purpose that an engine needs fuel, but in either case if fuel capacity is limited the efficiency and usefulness are restricted. The ideal would be a combination of the two, but each market needs much education before such a standard will be adopted.

A muscle is composed of peculiarly shaped cells, that join nearly end to end to form what are popularly known as muscle fibers. These muscle fibers run parallel to each other, each fiber being an independent unit of strength. A muscle is strong in proportion to the size of its cross section, or rather in proportion to the number of muscle fibers that compose it. Speed in contraction depends upon an entirely different thing. A muscle cell can contract one-fourth of its entire length. Since the nerve stimulus reaches each cell at practically the same moment, contraction takes place in about the same length of time, whether the muscle is long or short. In other words a 24-inch muscle will contract its 6 inches in the same time that a 16-inch muscle will contract its 4 inches. Since the muscular power is applied to the joints at approximately the same place in the two animals, the one with the 6-inch contraction cannot help but develop more speed. Length has no relation to strength except in so far as the greater number of cells offers a larger number of chances for a weak cell to be present and thus may actually contribute to weakness. The above discussion shows very plainly why the racehorse man asks for the long, rangy type of horse and why the draft-horse breeder looks only to thickness of muscle and compactness of build.

The bony skeleton has two functions: first to serve as a framework to support the body, and second to give mechanical advantage to the horse in motion. The first function is so self-evident that it need not be discussed. The second is perhaps an unfamiliar conception and will require a little elaboration.

The simplest means of gaining mechanical advantage is by the use of a lever. A lever has three working points: the point where the power is applied, the point where the weight is applied and the point over which the lever works as a base, or the fulcrum. There are three classes of levers. A lever of the first class has the power and weight at the ends and the fulcrum in the middle, as in the seesaw. This may be illustrated in the horse when kicking, the foot and the lower limb being the weight, the power being in the hips above and the fulcrum being the hock. A lever of the second class is where the fulcrum is at one end, power at the other and weight in the middle, as in the wheelbarrow. This is illustrated in the horse when moving forward by the limb that is applied to the ground. The foot is the fulcrum, the strain or weight comes on the joints and the power is in the thighs. This lever is particularly emphasized in the hind limbs when the horse is pulling, the hocks commonly being the point at which the strain is shown. The third class of lever is where the power is in the middle and the weight and fulcrum at the ends, as in the sheep shears or fire tongs. The first two levers give mechanical advantage through increased power, the third gives it through increased speed but with a loss of power. The third class is very common in the horse, the jaw being a particularly apt illustration. Here the weight is at the teeth when chewing, the fulcrum at the juncture of the jaw with the head, and the power at the heavy muscles of the jaw. Another example would be the limb swung forward in walking. Here the lower limb is the weight, the power is applied at the hock and the fulcrum is at the hip joint. Other illustrations of the same lever are found when the horse rises from a recumbent position or when he jumps. These various conformations give the horse the big advantages in motility that he has over other animals of his size and smaller.

The Walking Point

The efficiency of a lever depends upon the relative length of power arm and weight arm. In a lever of the first or second class, the longer the power arm and the shorter the weight arm, the greater the efficiency, but one will not gain mechanical advantage unless the power arm is longer than the other. In the third class of lever the power arm is never so long as the weight arm, since the power is applied between weight and fulcrum, but it may be of approximately the same length. These principles particularly apply in draft work to the question of lowsetness. The weight of the load is a direct force that corresponds to something pulling back on the animal. As stated above, the longer the power arm the greater the advantage, hence the longer the limb of the horse the greater the advantage that the load will have over the animal. The teamster consequently requires that his draft horse be lowset, in order to gain a greater efficiency.

There is a second consideration, however, which indicates that too short-legged a horse may be actually at a disadvantage, for in addition to mechanical power the average driver requires a certain amount of speed. Snap in action is impossible for a horse when he draws a load beyond a certain limit. In other words there is a point at which any draft horse will be held to a drudging walk if sufficiently loaded. Carefully planned tests have shown that this point varies only slightly with horses of equal weight. If this is true, then the horse with the longer swing to his stride will make much more rapid progress than the other fellow and will consequently be a more desirable market animal. On the average the short-limbed horse will not have the free swing in moving that the long-limbed animal will. It must consequently be realized that there is a happy medium at which the horse will be reasonably efficient for both propulsion and progression.

The race-horse man requires that his horses be long above the knees or hocks in proportion to their length below. This gives a longer stride and a greater control of the limbs. The only part of what is commonly termed the limb of the horse that bears muscle and is able to generate power of itself is the part above the knee and hock. In a partial sense the cannons correspond to stilts, and the shorter their proportion to those parts above, the more manageable the limb will be for the animal. Thus the proportions of the limb have a very definite bearing on the efficiency of the animal in motion. In regard to length of stride the best comparison might be to two sticks of equal length but jointed at different places. If a person is held in a fixed position and tries with two sticks each 36 inches long to touch a certain object 5 feet away he will find that one jointed at 30 inches will give much more accuracy and precision than one jointed at 20 inches. The reach will be farther and more easily controlled. Limb proportion then has two very separate values.

A foreign investigator has recently completed a series of chemical analyses to determine whether there is a difference between what is popularly called bone of good quality and bone of coarse quality. He finds that there is some difference, the bone from race horses containing a higher percentage of bone salts than the draft breeds. He states, however, that the difference is very slight and that the variations in strength depend, not on chemical distinctions but rather on physical variances.

Other investigators have attacked the physical side of the problem and have found a certain degree of foundation for the belief in flinty bone and spongy bone. These distinctions, however, exist more as a result of actual breaking tests conducted on the bone than on the microscopic study of the structure itself. The breaking tests have indicated that the strength does not vary proportionally to the size of the cross section of the shaft, and thus permit one to infer a qualitative difference.

The external indications of quality are many, but are best described by the oft-used statement that the limb of a horse should appear as though the skin had been removed, the bones and tendons scraped and then the skin returned to its former position. Meaty tissue covering the bone is considered a sign of coarseness and of bone of a spongy nature. Habitually “stocked-up” legs are indicative of a lymphatic temperament. This does not necessarily indicate a spongy bone, but does usually imply a “logy” disposition.

Where the Horse Beats the Engine

The importance of quality may best be realized by an extreme comparison. At each step the average weight on the fore limb of a horse in traveling is about two-thirds of the entire weight of the body. The jar on the limb of a horse weighing a ton would be about 1350 pounds, while that of a 1000-pound horse would be about 666 pounds. The light horse for its average work will maintain a speed of about eight miles an hour; the heavy horse is exceptionally good if it does four. The strain borne by the limb of either one is the product of the weight times the speed at which the animal travels. This results, in the case quoted, in an equal strain being borne by the two animals.

A story related by an early agricultural writer to illustrate nerve power perhaps explains better than any detailed discussion just what this functions. A stranger was going through a rural district and happened to be impressed by the physique of some rustic he met by chance. He inquired the countryman’s weight in a rather superior tone and was startled by the drawling reply: “Wal, stranger, ordinarily I weigh about sixteen stone, but when I’m mad I weigh a ton.”

Nerve power is just the difference between the horse and the engine, the living and the nonliving, in regard to staying power and ability to overcome extreme stresses. It is the reserve energy that will take a load of coal up a steep embankment or an overtaxed gravel wagon out of the soft and yielding pit. It is the force that holds the race horse to the track or that keeps the city draft horse going during the hot summer days until he drops under the fierce rays of an overhead sun. It is the quality that has made the horse the best loved of the domestic animals, and the trait that will preserve a constant demand in the future, no matter how many motor-trucks and tractors threaten.

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A serviceable farm team of grade draft geldings.

Big mares will more than pay their board and raise colts that are in demand.