Abstract
THE TECHNICAL testing of tractors may be carried out by using test rules valid only in a particular country, or rules which are worked out in collaboration with several countries and valid in only these countries, e.g. the O.E.C.D. Standard Code and Test Bulletin for Agricultural Tractors. A common factor to these tests is that they are all laboratory tests, the results of which, especially those of the drawbar tests, are obtained neither under agricultural conditions nor on arable land. Drawbar tests are generally carried out on an artificial track with a surface of concrete or tarmac because on arable land it is not possible to reproduce the measurements, and therefore the results of different tests are not strictly comparable. The gap between the technical test results as they are stated in a report and real tractor performance in the field, has become so great that it is impossible for the farmer to determine a relationship between them. Farmers are, therefore, demanding that the technical reports should be supplemented by performance data in agricultural conditions. However it has been impossible to comply with this demand, since for any comparison to be valid the soil conditions must remain constant, and this has not been possible. To overcome these shortcomings of the technical tests the Tractor Testing Station at Darmstadt has worked out a method for calculating the field performance of tractors. The basic idea of this method is to select a field and measure the soil coefficients which influence the performance of the tractor. These coefficients are the characteristics of the selected test-field and the calculation for each test is based on these coefficients. Hence a constant test-field is established and the method of calculating the performance complies with the demand for strict comparability of the performance of different tractors. The results of research work on agricultural tractors and tractor tyres coupled with the laws of mechanics make it possible to calculate the performance of any tractor, once all the necessary coefficients have been determined. The results are no longer influenced by changes in the soil conditions but only by the dimensions and weights of the corresponding tractor. If the calculated values in technical terms are converted into agricultural terms, such as drawn load or ploughed furrow-depth, this method always allows the tractor performance to be calculated and reported in terms which the farmer can understand. In order to use this method of calculating tractor performance, the following characteristic values of the selected test-site must be measured at the same time: (a) The traction-capacity of all sizes of tractor tyres; the traction-capacity is characterized by the relationship between the tyre-slip and the tractioncoefficient;
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