Masters Theses

Joe R. Sarten

Date of Award

5-1990

Degree Type

Thesis

Degree Name

Master of Science

Major

Biosystems Engineering

Major Professor

Fred D. Tompkins

Committee Members

Luther R. Wilhelm, James A. Euler

Abstract

Increasing use of riding mowers by homeowners and the rapid growth of professional lawn care services has placed new demands on tires for such equipment. Little research has been done in this area. Therefore, the purpose of this study was to assess the performance of commercially-available lawn and garden tractor tires. A tractor-mounted instrumentation package was designed, installed, and used to collect field data on longitudinal tire slip on level and sloping ground. Tests were also conducted to determine if tire design affected static overturn angle.

To measure tire performance, a microcomputer-based instrumentation package that was partially transferable between two tractors was used. This instrumentation package monitored axle rotation, true ground speed, draft, and vehicle pitch and roll. Optical encoders, gear driven from each axle, monitored axle angular displacement. An ultrasonic speed sensor, mounted on the right side of the tractor, gave true ground speed. A load beam draft sensor, hitched between the tractor and a specially built load cart used to provide the towed load, monitored draft. Capacitance clinometers, mounted to the tractor frame over the rear axle, indicated vehicle pitch and roll. Sensor outputs passed through circuits designed and built in-house to a single-board computer that collected data. The single-board computer was controlled using a BASIC language program. The tractor battery provided power for the system. Data were temporarily stored in RAM on a laptop computer mounted on the tractor hood and were later downloaded to disk files for permanent storage.

A tilt table was also constructed and setup in the laboratory to provide a variable slope surface for static roll-over angle determination, A hydraulic cylinder and hand pump provided a continuously variable surface angle.

For each series of test, data were collected with enough replications to detect any significant differences. Tests were conducted on asphalt to determine the rolling radius of each tire and to establish the zero slip condition. The level-ground traction tests included operating each tractor on dry and wet turf under no-load and loaded draft conditions for each of the four sets of tires. Data were collected once each second and averaged to a single value for each tire over the measured 196.8-foot (60-m) course for each test run. These tests indicated the following: (1) tractive performance was the same for the front tires between tractors for both turf conditions; (2) addition of a towed load caused front wheel slip to become more negative; (3) operation of the tractors with no load on a turf surface produced negative slip on the rear wheels, based on a zero slip condition as established on an asphalt surface; (4) operation on a wet turf surface produced less rear wheel slip than operation on a dry turf surface; (5) as expected, addition of a towed load increased rear wheel slip, and the lighter tractor had a higher average slip on the rear wheels; (6) there was a difference in tractive performance between tire sets on both front and rear, but all tire sets were not different from all other tire sets in performance; and, (7) tractive performance was not the same from side to side on the tractors.

Sloping-ground traction tests involved driving the tractor in reverse up a continuously increasing slope until spinout occurred and recording this slope angle. These tests indicated no significant differences among tires or between tractors.

Static roll-over tests involved placing the tractor on the tilt table, increasing the slope until the uphill wheels lifted off the surface, and recording this platform angle. Values were recorded for each end of the tractor and for both sides. These tests showed that static roll-over angles were different between tractors, the sides of the tractors, and each end of the tractors. These differences were due mainly to differences in weight distribution. Significant differences were also found between tire sets even though the differences in magnitudes were small.

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