Even though tractor and truck pulling contests are wildly popular these days, I much prefer to watch horse pulls. Vast sums are spent on engines, drive trains and tires. But horses, mules and ponies require good breeding, as well as intensive conditioning and training.
Draft horse owners have always asked a lot from their teams – power, versatility, speed, endurance, beauty of form, courage, intelligence and intuition – but most of those attributes are subjective and difficult to measure. A stopwatch was used to measure a horse’s speed, but there was no consistent way of determining the maximum pulling power of the animals.
Determining pulling power In 1921, Professor E.V. Collins of the Agricultural Experiment Station at Iowa State College began tests to determine the work capacity of horses. Although dynamometers were available, they only measured the force required to move a load. This varied because of the surface upon which the load was pulled and because the force needed to start a load is much greater than that required to keep it moving. In order to accurately measure a horse’s pulling power, Professor Collins had to devise a machine that would require the same tractive pull to start it as to keep it in motion.
Collins started with an early International Harvester Auto Wagon with the engine, transmission and body removed, although the drive train and brakes were retained. A single tree was attached to a cable on each side, running over pulleys to the rear of the wagon, where a weight bucket was suspended on the end of each cable. If a horse was hitched to each of the single trees and the team was driven forward, the weights would rise in their guides until they reached the end of the vertical travel, at which time the wagon would also move forward. To keep the load constant, some resistance to the wagon’s forward motion was needed.
To provide that resistance, a rotary pump was mounted so it was driven by the vehicle’s jack shaft sprocket. The pump inlet was piped to the bottom of a 10-gallon tank. Another pipe ran from the top of the tank to a rotary control valve and then to the pump outlet. This valve, when closed, prevented the fluid from circulating and the pump from turning, effectively locking the wagon’s wheels.
The valve was connected to the weights so that when the weights were at the bottom of their travel, the valve was closed and the wheels were locked, providing maximum resistance to the forward movement of the wagon. As the team pulled, the weights rose, gradually opening the valve, until at the top of the weight’s travel the valve was wide open and the wagon rolled easily. When calibrated correctly, the wagon would move just fast enough to keep the weights suspended while the team was moving.
An early dynamometer
The thing didn’t work very well at first, but after several trials and modifications, the dynamometer was successful. With this machine, the tractive effort required of each horse, as well as a team’s ability to continuously pull a load over an extended period of time, could be tested. It was found that horses could, without undue fatigue, exert a tractive effort of one-tenth to one-eighth of their weight while traveling a total of 20 miles a day.
A second larger and heavier machine was built on the chassis of a Nash Quad four-wheel drive truck in which the engine was replaced by a large rotary pump. The pump was geared to all four wheels through the truck transmission and drive shafts. Concrete weights and a single cable from a double tree were used and the machine could provide tractive resistance of 4,100 pounds.
In a thesis written in 1924, Iowa State student Kenneth J. Maltas describes the second machine: “When a team starts to pull on the dynamometer, the wheels are practically locked so that the first action is to raise the weights, which are attached to the weight beam. When these weights are raised to a certain point, an arm that is connected with the weight beam opens an automatic valve on the discharge side of the pump and (the pump offers) very little resistance to the turning of the wheels.
“When the wheels are unlocked, the machine runs forward automatically just fast enough to keep the weights suspended. The height to which the weight is lifted is of no significance, so long as it does not strike the top or bottom of the guides and (it) furnishes an accurate measure of the pull being exerted by the team. The ... surface over which the dynamometer wagon is drawn does not influence the tractive pull. The starting load is obviously no greater than that required to keep the machine in action.”
Taking it on the road Finally, to allow easy movement of the dynamometer from place to place, a machine was built on the rear of a 1925 Ford flat bed truck. This machine weighed 5,320 pounds and had a maximum capacity of 3,200 pounds tractive pull. The design of this dynamometer is the same as the machines used today by the Michigan Dynamometer Assn., along with several others.
A number of large weights consisting of 250-pound cast iron discs rest on the truck bed inside a vertical cage. A flat bar is positioned vertically through holes in the discs’ centers. Holes in this bar allow a rod to be inserted under each disc, thus attaching the weight to the bar. In this way, as many weights as desired can be attached to the bar, which is connected to the lifting cable.
The lifting cable runs over pulleys to change the direction of pull, before running around a movable pulley that doubles the force needed to lift the weights, making each 250-pound weight the equivalent of 500 pounds. The pulling cable is attached to the movable pulley and runs around another pulley under the rear of the truck bed. At the end of this cable is a hook to which the pulling team’s double tree is connected.
To provide the resistance needed to keep the weights suspended during a pull, the same arrangement is used as on the earlier machines. The pump, valve, tank and connecting pipes are mounted behind the truck cab. The pump shaft is connected by sprockets and a roller chain to the truck drive shaft.
Since the resistance to the tractive pull is achieved by locking the truck’s rear wheels, it is necessary to prevent skidding on the track surface, so skid chains are used on the truck rear wheels and, at times, skids under the front wheels. For very heavy loads or slippery track surfaces, additional weight must be placed on the truck bed. In the early days, this usually meant additional men would climb onto the truck; now concrete weights are used.
Pulling contests were originally started for testing and educational purposes, but they have evolved into popular sporting events that provide entertainment, as well as a chance to see beautiful horses, good horsemanship and competition that is both intense and good natured. FC
Sam Moore grew up on a farm in western Pennsylvania. He now lives in Salem, Ohio, and collects antique tractors, implements and related items. Contact Sam by e-mail at email@example.com.