Prediction of the contact area of agricultural traction tyres on firm soil

Equations were developed to enable reliable calculations to be made of the contact area for a wide range of traction tyres from independent easily accessible variables taken on firm ground. Given that when tyre size TS (section width [m] × outer diameter of tyre [m]), increases the specific contact area, As (ratio of the contact area to the tyre volume), decreases and tyre stiffness increases, it is worth taking into account tyre intrinsic properties by studying the impact of external constraints such as load and inflation pressure on the contact area. Based on a wide range of measurements (64, from a total of 28 tyres used on 12 grassland and open ground sites), three classes will be proposed for estimation of the contact area on firm agricultural soil: i) small tyres with TS < 0.6, ii) medium tyres with 0.6 ≤ TS < 1.2, and iii) large tyres with TS ≥ 1.2. When considering not only TS but also load and inflation pressure as explainable variables, an increase in R2 by 5% was observed. In addition, after taking into account intrinsic properties of the tyre, a new tyre classification has been proposed leading to an additional increase of R2 by about 2%. Compatibilities with algorithms developed by other authors are discussed.