Top tips on selecting tyres for your machines

Tyres on modern machines are much larger and more costly than they would have been even a decade ago. It would be easy to assume that they are as big as we need – but are they? Machines have gotten bigger. Selecting the right tyres for an application is not an easy task. But it is critical, as tyre selection will affect the performance of the machine and particularly the impact it has on the soil.

The role of tyres

Tyres have two main functions on agricultural machines:

Tyres also affect machine stability, steering and suspension.

Power transmission

To transmit power efficiently, tyres must have a large enough contact area with the soil to ensure the necessary pull can be generated. Also, there is a need for weight on the wheel to prevent wheel slip. Power and pull are not the same. If we try to transmit a lot of power at a slow forward speed, then we need to generate a huge traction or pulling force. This is the situation with ploughing where the working speed might be 8km/h or less. In these cases, heavy or ballasted tractors will tend to pull better. However, if the same power is used to pull a stubble cultivator at 14km/h, for example, then much less weight is needed on the drive wheels. The simple formula is outlined below.

For example, a 110kw (150hp) tractor pulling a 5F plough would benefit from 8t of weight on the driving wheels, whereas the same tractor pulling a stubble cultivator at 14km/h would only require 4.5t weight on the driving wheels to pull efficiently.

But while weight can be beneficial in heavy draught situations, to avoid damaging the soil, this weight must be carried by large tyres to spread the load on the soil. These large tyres will reduce rolling resistance in the field also and pull more efficiently.

Supporting the load

With today’s machine weights, the risk of compaction and consequently the selection of tyres to prevent soil damage, is by far the most important aspect of tyre selection.

There are two factors which affect the risk of damaging the soil – wheel load and ground pressure. Lighter machines pose less of a threat to the soil and their ground pressure can be easily reduced by changing tyre size. Bigger loads and particularly axle loads in excess of 6t pose a greater threat and are much more difficult to resolve from a tyre perspective.

Bigger tyres have a larger contact area between the tyre and the soil and consequently they reduce the ground pressure compared with smaller tyres on the same machine. In simple terms, it’s the air pressure within tyres which supports the load. If a large tyre is fitted, it will be able to carry the load at a lower inflation pressure and exert a lower ground pressure. This forms the basis for tyre selection – select tyres that can carry loads at low inflation pressures.

What ground pressure do we need?

The ground pressure that’s required depends totally on the soil conditions. Soils that are wet and/or cultivated are weaker and require a lower ground pressure to avoid damage. Broad guidelines are given in the table.

To determine the correct size tyres for a particular situation, the following four steps are useful:.

1. Determine the target ground pressure / inflation pressure.

2. Measure/estimate your axle load.

3. Consider tyre diameter limitations (4WD ratio or mudguard clearance).

4. Search for tyres that will carry that load using manufacturers’ load and inflation tables.

As tyres and particularly wheel changes are expensive, it’s usually best to consider tyre changes when a machine is being purchased. An example of this tyre selection approach is outlined below.

110kW (150hp) tractor and 5F mounted reversible plough

1. Determine target pressures:

For working on stubble <1.0 bar.

2. Estimate loads.

3. Other limitations

Transport speed = 30kmh

Rolling circumference of tyre: 5.3m to 5.8m

Options and pressures for 4.15t wheel load

In the example of a 150hp tractor and five-furrow mounted plough, three of the tyre options selected are not capable of carrying the load at any pressure. The 600/65R38 tyre has the capacity to carry the axle load, but only at an excessive inflation pressure and consequently ground pressure. The commonly used 650/65R38, which would be considered a large tyre by many, is too small to carry this load without risking soil damage. The only options that meet the low pressure requirements are tyres over 700mm (27in) wide.

A 110kw (150hp) tractor pulling a 5F plough would benefit from 8t of weight on the driving wheels, whereas the same tractor pulling a stubble cultivator at 14km/h would only require 4.5t weight on the driving wheels to pull efficiently

Many would consider these tyres too wide for ploughing, but perhaps it is better to suffer some visible surface marking rather than compacting the soil beneath. Once the rear tyre size is selected, a suitable front tyre/wheel combination must be selected using the 4WD drive ratio to ensure the sizes match correctly.

Tyres must match machinery weights

This modest example of a 150hp tractor and five-furrow plough clearly shows that our move to bigger machines requires much more attention to tyre size. Today 200hp+ tractors and matching implements are becoming increasingly common. The base specification tyres on these tractors and trailed machines are frequently totally inadequate considering the weight and power of the equipment. The same applies to wheels on trailers, slurry tankers and other machines. We need to be particularly careful in Ireland, where our soils are often moist. Tyre options must be selected to protect the soil.

In the last two decades the availability of more aspect ratios such as 70 and 65 series tyres has increased the number of options available to users, many of which gave lower ground pressure opportunities. However, until quite recently, most tyres were limited to the same level of tyre deflection, meaning that for a given size tyre, the contact patch and ground pressure achieved was fixed and common across most tyre manufacturers.

But a new development pioneered by Michelin over a decade ago is delivering greater scope for reducing ground pressure. The basis is simple – run the tyres at lower pressures. However, the tyres must be designed for that. Tyre carcases that are capable of running at a greater level of deflection allow a given size tyre to have a lower inflation pressure. These tyres are denoted by the letters IF or VF preceding the tyre size figures. This can have a significant impact on the contact patch and ground pressure that these tyres exert. Michelin now uses this technology in a number of tyre ranges for tractors and combines (Xeobib, Axiobib, Cereabib) and other manufacturers have adopted this technology over the last few years. This increased flattening of the carcase allows a very real decrease in ground pressure. Some examples are shown below.

IF and VF tyres allow lower pressures

Pressure (bar) at set wheel load

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Top tyre tech to carry the weight on your farm