Alex Keen Harper Adams University College, The Effect of a Suspension on the Tractive Performance of Agricultural Tractors

Suspensions are becoming more available on agricultural tractors but there is little published information on their effect on traction. This paper reports part of an investigation into the effect of a suspension on traction for a 495/70 R24 XM47 tyre and a coil spring and damper suspension as used on high mobility agricultural tractors. A single wheel tester was built to represent a quarter of a high mobility agricultural tractor fitted with a suspension. This allowed the suspension to be locked out of use and also restricted movement to the vertical direction so as to represent one and two degree of freedom vibration systems. A sine wave surface, cut into a compacted sandy loam soil, was used to excite the driven wheel.

The tyre stiffness and damping coefficient were determined for a range of tyre pressures and tyre loads. The effect of ground induced vibration on wheel pull was determined for a range of input frequencies, tyre pressures and tyre loads both with the suspension active and locked out of use. The Janosi and Hanamoto thrust prediction equation was modified to model dynamic wheel load using standard vibration equations for one degree of freedom vibration when the suspension was not active and using computer multi-body simulation, to determine the dynamic wheel load, for two degrees of freedom when the suspension was active. This paper reports the results for the suspension active and compares and evaluates them with the results for the suspension locked out of use.

It was found that the variation in wheel pull, with no suspension, is very dependent on the tyre characteristics and wheel load and is greatest at the tyre’s natural frequency. The suspension reduced wheel pull variation to within ± 20% of the average wheel pull, a similar level to operating the tyre at a low tyre inflation pressure of 1 bar, over the range of frequency input used. For the compacted soil surface used, the direct effect of dynamic wheel load in the traction prediction equations gave an indication of traction variation both with and without the suspension active. The values and effects of tyre and soil damping need more investigation.