Prediction of drawbar test performance

Standard tests of agricultural tractors include measurement of drawbar performance on a concrete or tarmacadam surface. Because these tests are time-consuming and expensive, the possibility of replacing them with axle dynamometer tests is under consideration. To maintain comparability with conventional drawbar tests it would be desirable to estimate drawbar performance on a hard surface from results of axle dynamometer tests. This requires a method for predicting slip-pull relationships on a hard surface. Ninety-nine drawbar tests carried out in France, Germany, U.K. and U.S.A. have been analysed and equations of varying complexity derived to predict performance. It was found that, whereas drawbar pull at maximum drawbar power could be predicted fairly accurately, the corresponding slip and consequently maximum drawbar power could not be predicted with sufficient accuracy to enable valid comparisons to be made with drawbar tests. It is suggested that the reason for the kack of accuracy is the presence of unquantifiable variables such as differences in rubber compound, tread pattern or track condition. It is suggested that these throw doubt on the validity of the drawbar tests themselves as a means of comparison and suggestions are made of ways in which the test could be modified to make it more suitable for comparing different drawbar tests and for comparing drawbar tests with axle dynamometer tests.