Also the popular model of Wismer and Luth [2.29] addresses bias ply rating tires: K=0.75-(1-e-0.3 CN-i)-(12+0.04) (2.13) Gross traction μ Rolling res. p with slip i and "wheel numeric" CN: b.d.CI CN= FW (2.14) Fw= Fw,f+Fw.r - Fp sina B • (2.25) Fp sin α and Fp cos α increase the rear axle load and decrease the front axle load. This is usually favourable in the case of rear 2WD, but front ballast can be neces- sary for adequate steering safety. Rear axle torque can be expressed by T= (K₁+Pr) Fw,r Fr . . (2.26) b [m] = tire width = D [m] tire diameter. Based on [3.17, 3.18], the projection b.D and the tire load can be used defining a tire load Contact pressure index = (3.2) b.D A limit of 0.25 bar (3.6 psi, 2.5 N/cm²) can be recommended for soft soils.