How to adapt your combine harvester tyres to meet your needs

2 - Determining the weight of the header

The weight of the header will also be taken into account when calculating the load.
Example of header weight: 3,550 kg.

As the header is at the front of the machine, this will generate an weight transfer from the rear axle to the front axle using the see-saw principle with the front axle being the pivot point of the see-saw. It will therefore be necessary to measure the impact in kilograms on the axle, taking into account the distance between the headers centre of gravity and the distance to the centre line of the front axle between the tool's centre of gravity and the centre of the axle hubs.

3 - Determining the combine harvester's offset and load transfer

As there is an offset with the cutterbar, we will calculate the load transfer. To do this, we first need to estimate the value of the offset.

To calculate the offset, we need to consider the attachment point of the picker and estimate the tool's centre of gravity. Taking as a reference a spot approximately 1 metre in front of the attachment point, we need to measure the distance between the middle of the front hub and this spot.
In our example, we measure an offset of 3.05 metres.

You also need to know the wheelbase (the distance between the centre of the rear hub and the centre of the front hub). It's usually given in the technical documentation, but you can also measure it easily.
In our example, we measure a wheelbase of 3m55.

The load offset coefficient is calculated as follows: Distance between the centre of gravity and the centre of the front hub divided by the wheelbase.
In our example: 3m05 / 3m55 = 0.86 is our load offset.

This coefficient is used to estimate the additional weight on the front axle.

We will now multiply the weight of the front tool by this coefficient:
In our example: 3,550 kg x 0.86 = 3053 kg (rounded up to 3050 kg). This means that we have a load transfer of 3050 kg.

Calculate your combine harvester's front/rear weight distribution

For the front/rear, a safe 70/30 ratio can be used, which applies to most machines: 70% front load and 30% rear load. This information can be checked with your dealer.

So to calculate the weight on the front axle, we apply a percentage of 70% to the empty weight. And for the rear axle, we apply a percentage of 30%.

Our result is

• 18,900 kg x 70% = 13,230 kg at the front
• 18,900 kg x 30% = 5,670 kg at the rear

You can now start transferring the data you have collected into the table, like this:

4 - Calculating the weight of the grain in the grain tank

As for the combine harvester's grain tank, you normally know its volume. The idea is to take its capacity in litres and convert it into kilos. To do this, we recommend using a generous coefficient of 0.8.

In our example, the hopper has a capacity of 10,500 litres, so applying this coefficient would mean :

10,500 litres x 0.8 = 8,400 kg of hopper weight.

As a large part of the combine harvester's hopper is located at the front, we'll apply the same 70/30 ratio to calculate the front/rear weight distribution.

Our result is:

8,400 kg x 70% = 5,880 kg at the front.
8,400 kg x 30% = 2,520 kg at the rear.

You can add these values to the table.

In our example, the total weight of the front axle is 25,710 kg.
To find out the weight per tyre, we simply divided this number by 2: 25,710 kg / 2 = 12,855 kg per front tyre.

Same calculation for the rear:
Total rear axle: 8,190 kg, 8,190 kg / 2 = 4,095 kg per rear tyre.

The special case of sloping ground

If there is a slope in your fields, your combine harvester will not be completely horizontal and this will mean a transfer of load from one side to the other. In this case, we will increase the pressures slightly to protect the tyre. The method of increasing pressure is shown at the end of the Michelin technical documentation

In other cases, stick to the recommended pressures