Abstract
Recent years have seen the development of high mobility, high speed tractors incorporating front axle suspension or all wheel suspension. This paper reports the initial results of experiments investigating the effect of wheel vibration on traction using a 495/70 R24 XM47 radial tyre and coil spring and damper suspension as used on high mobility agricultural tractors. A single wheel tester has been designed and built to allow the measurement of drawbar pull and vertical wheel movement. The Harper Adams single wheel tester represents a quarter of a high mobility agricultural tractor - a suspension was fitted between the wheel and the supported mass. The suspension could be set to be active or locked out of use.
The aim of the experiments reported was to investigate the effect of a suspension on off-road traction. The main objectives were to:
1 determine the drawbar pull variation developed by the tyre and the vertical vibration associated with this force while driving the tyre at two tyre pressures, a range of input vibration frequencies, two wheel loads and with the suspension active or locked out of use, ie. to investigate the effect of tyre input frequency, tyre pressure, tyre load and a suspension;
2 evaluate the effect of radial tyre stiffness, damping and suspension characteristics.
Traction and vibration data were collected using a single wheel tester operating on a compacted sandy loam soil in a ground level soil tank. The general approach to this investigation has been to consider the tyre and load (suspension locked out) as a one degree of freedom vibration system and the tyre, unsuspended mass, suspension and suspended mass as a two degree of freedom vibration system. These vibration systems have been subjected to a ground displacement sine wave input and the resulting vibration output and drawbar pull measured. A sine wave was cut into the soil surface. A track vibration input amplitude of b 5 mm to b7 mm and a wavelength of 0.76 m were used. Tyre pressures of 1 bar and 2 bar were used. The range of forward speeds included input vibration at the tyre¡¦s natural frequency. A wheel static load of 2060 kg was compared with the control load of 1560 kg. The tyre characteristics of dynamic radial stiffness, damping, and vibration amplitude transmissibility were determined. Drawbar pull data were collected with the suspension active and locked out of use.
For the sandy loam soil and tyre used, the main conclusions were:
1 The greatest variation of drawbar pull was found at the tyre¡¦s natural frequency and at the higher tyre pressure of 2 bar at a wheel load of 1560 kg with the suspension locked out of use. Increasing the wheel load to 2060 kg had a large effect in reducing the draw bar pull variation. Reducing the tyre pressure to 1 bar had a greater effect in reducing the draw bar pull variation. Reducing tyre pressure increased tyre damping and increasing the tyre load also increased tyre damping.
2 The suspension had a large effect in reducing the variation in drawbar pull at the higher tyre pressure of 2 bar and the wheel load of 1560 kg but had little effect at a tyre pressure of 1 bar. As the input frequency reduced towards the natural frequency of the spring and sprung mass the draw bar pull variation increased to be comparable with or greater than with the suspension locked out of use.
3 The drawbar pull variation with the suspension active was found to be very similar over the input frequency range for all the combinations of tyre pressure and load used.
4 The effects of longitudinal tyre stiffness and damping and longitudinal soil effects require further investigation. Wheel and vehicle inertia effects also require consideration. The effect of reducing the tyre load needs investigating further with the range of tyre loads fully representing the range that can be found in normal tractor operations for this tyre.