Tire Combinations

The size ratio of the front wheels to the rear ones is precisely determined in order to produce a positive front wheel lead of between 1.5 and 4%, otherwise the tires will become deformed or suffer undue wear. To ascertain the correct ratio when changing tires, proceed as follows:

Ascertain tractor data:

1. Transmission ratio of the differential gear pair. This information is displayed on the transmission type plate. The following pair is possible:

  Gear pair Transmission ratio
  51/9 5.667

 

2. Transmission ratio of front axle. This figure is displayed on the front axle type plate. The following ratio is possible:

  • 15.692

3. Transmission ratio of gear pair for FWD axle (output). This figure can be found on the transmission serial number plate. The following ratios are possible:

  • 1.446
  • 1.465
  • 1.497
  • 1.532
  • 1.564
  • 1.597
  • 1.625
  • 1.658
  • 1.692
  • 1.725
  • 1.760
  • 1.795
  • 1.833
  • 1.870
  • 1.907
  • 1.943
  • 1.990
  • 2.028

Ascertain tire data:

This information must be ascertained from the tire manufacturer's manual.

1. Select tires with suitable load-bearing capability.

2. Select tires appropriate to the tractor's top speed.

3. From the manual, ascertain the rolling circumference of the tire desired for the rear wheel.

4. From the manual, ascertain the rolling circumference of the tire desired for the front wheel.

Ascertain the following data:

1. Calculate the difference between the rolling circumferences of the tires on the front and rear wheels.

IMPORTANT: The largest front tires on 6520 and 6620 SE-tractors must not exceed an index radius of 640 mm (25.2 in.).

The difference in rolling circumferences between the front and rear tires must be between 1035 mm (40.7 in.) and 1756 mm (69.1 in.).

2. Calculate the overall transmission ratio using the following formula:

    A1 * I1 * I3 * 100  
  I =
 
    A2 * I2 * 102.75  
       
  A1 = Rolling circumference of front tires
  A2 = Rolling circumference of rear tires
  I1 = Rear axle ratio (7.0714)
  I2 = Front axle ratio
  I3 = Transmission ratio of the differential gear pair

 

Ascertain the gear pair that is appropriate for the overall transmission ratio.

OU12401,001045F -19-01MAR03-1/2

Calculated transmission ratio Transmission ratio of the required gear pair
(corresponds to a pos. front wheel lead of 1.5 - 4.0%) (corresponds to desired average value of pos. front wheel lead)
1.428 - 1.446 1.446
1.447 - 1.478 1.465
1.479 - 1.512 1.497
1.513 - 1.544 1.532
1.545 - 1.576 1.564
1.577 - 1.617 1.597
1.618 - 1.637 1.625
1.638 - 1.669 1.658
1.670 - 1.703 1.692
1.704 - 1.737 1.725
1.738 - 1.772 1.760
1.773 - 1.810 1.795
1.811 - 1.846 1.833
1.847 - 1.882 1.870
1.883 - 1.918 1.907
1.919 - 1.964 1.943
1.965 - 2.002 1.990
2.003 - 2.062 2.028

 

Sample calculation

The intention is to fit a 6520 SE tractor with 540/65R38 and 480/65R24 tires, made by a certain manufacturer.

1. The transmission ratio of the differential gear pair is 5.667.

2. The transmission ratio of the front axle is 15.692.

3. The transmission ratio of the gear pair for the front wheel drive axle is 1.833.

4. The rolling circumference of the tire on the rear wheel is 4995 mm (196.7 in.).

5. The rolling circumference of the tire on the front wheel is 3707 mm (145.9 in.).

6. The difference between the rolling circumferences of the tires on the rear/front wheels is 1288 mm (50.7 in.), i.e. within the acceptable range.

7. Calculate the overall transmission ratio.

3707 * 7.0714 * 5.667 * 100  
= 1.845
4995 * 15.692 * 102.75  
   

 

The gear pair that is required must have a ratio of 1.833.

OU12401,001045F -19-01MAR03-2/2