An evaluation of the motions of competition seesaws
For a long time Monica Percival, editor of Clean Run magazine, has been interested - and concerned - about the construction of various seesaws used in American agility and their possible effect on their dogs. More and more it seemed like they might be sacrificing safety and function in an effort to produce equipment that's easier for us to lift, move, and store. So she asked I. Martin Levy, MD and Peter A. Torzilli, PhD to do a small research study to evaluate the normal motions of the seesaw, factors that influence those motions, and to delineate the undesirable events that occur. The resulting paper was too large to publish here but they have allowed us to preview this summary of their findings.
The seesaw is unlike any other obstacle on a dog agility course because the performance on the obstacle depends on the performance of the obstacle. Variations in plank, fulcrum, and base construction directly influence the motion characteristics of each seesaw design. In an effort to insure consistency of performance, the various organisations for dog agility have been quite specific about plank dimensions and pivot height. Still, they have been less precise when defining a seesaw’s response to varying conditions of load. Because of this, various seesaw solutions have been designed and constructed, each with its own set of performance characteristics. The rate of descent, support-base movement, plank vibration, and noise are all influenced by the design solution and the materials chosen to execute that design.
effect of design on performance
Materials and Methods
We performed three sets of tests. In the first test, we placed sandbags of known weights (5, 10, 20, 30, and 50 pounds) on the descending arm of the seesaw, at known distances from the end of that arm. Initially, we held the descending arm, with a sandbag in place, in the starting position. We then released it and measured the time from starting position to impact with the floor to the hundredth of a second. We performed a second set of tests to determine the stiffness of the boards. In the third set of tests, we evaluated the amount of ascending-arm-induced bending ('board whip') by tracing the travel of that arm during normal seesaw motion.
We plotted the results of the drop tests for a known load for each seesaw as 'distance from tip' versus 'time.' We compared the drop tests at 5 and 30 pounds for the three seesaws and illustrated them on a single plot shown in the graph. We determined the stiffness of each board: The Premier board had the greatest stiffness and the Action K-9 board the least. The Max 200 board was only slightly stiffer than the Action K-9 board.
The 'board whip' measured at 30 pounds for all three boards. The averaged K-9 board-whip value was 14 cm., the averaged Max 200 board-whip value was 10.4cm and for the Premier board, 9.3 cm.
Board whip appeared to be a function of the stiffness of the board. In this study, the more flexible boards (Action K-9, Max 200) were associated with greater amounts of board whip, resulting in catapulting of their loads—the sandbag bounced off the end of the board. All three boards still had to dissipate the energy developed in the ascending arm. This energy was transferred to the base, which resulted in two of the seesaws 'hopping.' In the case of the Premier seesaw, however, the linkage in the base dissipated the upward force, and whipping, catapulting, and hopping were minimal.
Note: The information included in this article is a brief summary of their findings. For a free copy of the complete research report, please go to www.cleanrun.com/surveys.cfm.
Or you can read the full article in the August issue of Clean Run magazine.