Low frequency angular vibrations in the roll mode on farm tractors

The dynamic behaviour in the roll mode of a tractor, seat, and driver versus dead mass, is studied on a test track by help of motion picture from behind and linear acceleration components measured on the seat. The 1·6 and 6 in high track obstacles and the driving speed were adjusted to give the worst conditions in the side to side mode using the inflation pressures 8, 11, and 17 lb/in2 in the rear tyres. A comparison of seats and effects of roll vibration on man was carried out on a vibration simulation device using the excitation frequencies 1, 1·7, 2·5, and 4 Hz. The natural frequency in the roll mode was in the range 1·5–1·9 Hz. The rotational point for seat roll displacement was found to be 46–66 in below the centre of gravity of the tractor. Two spring suspended seats had a considerable rotation of their own, creating worse conditions than a rigid seat under exposure to roll vibrations. The human reactions, recorded as heart rate, tracking performance, subjective rating and linear acceleration on hips and shoulders, showed marked differences in the quality of seats concerning roll mode excitation.