An outdoor soil bin was built in England to measure the tractive performance of full-size wheels under paddy field conditions. The soil conditions found in paddy fields in Thailand were reproduced as accurately as possible on heavy clay site, 100m long by 12 m wide. Steel rails were built along each side to support a gantry spanning the paddy field. A tractor was mounted on the gantry. The test wheel was fitted to one end of the tractor rear axle and propelled the gantry along the rails. A controllable restraining force was supplied by a cable and winch. The thrust produced and the wheel-slip were recorded by transducers and tractive performance parameters were calculated by an on-line computer. Tests were carried out on a conventional tractor driving wheel tyre, a tyre and retractable strake, an opening cage wheel and a low pressure tyre. All wheels and tyres produced satisfactory thrust, except for the low pressure tyre. The rolling resistance was very high in all cases, however, so that none of the wheels or tyres was able to achieve a tractive efficiency of more than 30%. In the case of the tyres, high rolling resistance was due to the drag produced by the puddled layer and, in the case of the cage wheel, by digging into the hardpan.
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