This will be dependent on the properties of the kiln shell steel. Between the riding tyres the shell is subject to longitudinal bending stress which rises to a peak at the mid-point between the riding tyres. A model taking into consideration the properties of the shell must be developed to simulate the bending stresses along the kiln axis. The riding tyres must then be positioned to ensure that the maximum bending stress does not exceed the fatigue cracking stress of the kiln shell steel in the service, i.e. hot, conditions. There is no simple formula.