Transference ratio, rolling circumference and inter-axle ratio

When it comes to tractors with four-wheel drive, there is a close relationship between the dimensions of front and rear tyres.

In order to use four-wheel drive effectively, the rolling circumference of front tyres must be less than that of the rear tyres - this is referred to as the inter-axle ratio. Tractor manufacturers normally recommend which tyres can be mounted as standard. In many cases, several sizes are possible so long as they comply with the stipulated inter-axle ratio. To use four-wheel drive effectively, the inter-axle ratio must amount to between +1% and +5% (rolling circumference front tyres > rolling circumference rear tyres). The permitted inter-axle ratio is between 0% and +6%. An inter-axle ratio less than 0% and greater than +6% can have a negative impact on the tractor’s driving performance and steering behaviour (particularly when cornering), and can cause excessive wear and tear in front and rear tyres, as well as premature wear and tear in the transmission. In four-wheel drive mechanisms, the relationship between the number of revolutions of the front and rear axle is a constant factor (and is determined by the cogs in the gear box). This relationship is referred to as the transference ratio and, depending on the tractor brand and type, is normally between 1.20 and 1.50.

The exact mechanical transference ratio will be mentioned in the tractor’s user manual. The rolling circumference of tyres by our own import brands can be found in this guide.

Once you know the rolling circumference for front and rear tyres, as well as the tractor’s transference ratio, you can use the following formula to calculate the inter-axle ratio:

                            

rolling circumference front tyre
* transference ratio 

            
% inter-axle ratio -----------------------------------------         -1 *100%
  rolling circumference rear tyre