The size and weight of machinery operating on tillage and grassland farms continues to increase. Tractors, trailers, slurry tankers, ploughs, cultivators, harvesters and more have all increased and Ireland’s farmers and contractors continue to purchase some of the largest machines available.
The previously established threshold for subsoil damage risk, 6t per axle, is frequently exceeded, with many field operations going over 10t per axle.
Tyres or tracks fitted to these machines are all that can protect the soil. Larger sizes are available, but the wider application of more flexible tyre carcases (VF and IF rated tyres) offer additional benefits that are worth exploring.
Flexible carcases: IF and VF tyres
The development of tyre carcases with increased flexibility was pioneered by Michelin over 15 years ago with the introduction of the Xeobib range, which targeted combined field and road use.
The company has since expanded this technology to a range of tyre types and today manufacturers such as Bridgestone, Firestone, Mitas, Trelleborg and BKT all produce tyres with similar technology.
Ireland’s farmers and contractors continue to purchase some of the largest machines available
While the IF and VF technology that underpins these tyres was originally restricted to unique tyre sizes that required different rims, in recent years they have become available in more common sizes that can replace standard tyres on original rims. The tyre manufacturer’s guidelines must be followed regarding rims.
It is only tyres with the letters VF or IF in front of the tyre size that have these characteristics. So, instead of a standard 650/65R38, the size marking would be IF650/65R38 or VF650/65R38. But what is the technology and what does it deliver?
Ground pressure, inflation pressure and soil damage
For any tyre, the pressure it exerts on the soil is determined by the load it carries and the size of the contact patch between it and the soil. Larger tyres have a bigger contact patch and can carry a given load at a lower inflation pressure compared to a small tyre with the same load. When correctly inflated, the air pressure in a tyre is closely related to the ground pressure that the tyre exerts.
Larger tyres have a bigger contact patch and can carry a given load at a lower inflation pressure
But of course, we can only use low pressures if the tyre is large enough. If we used inflation pressure that was lower than allowed, we would reduce the ground pressure further, but by increasing carcase deflection, we would quickly damage the tyre. The tyre manufacturer’s load and inflation pressure tables must always be followed.
Deflection, contact area and ground pressure
Standard tyres are limited to a deflection value of about 20%, where the distance between the rim and the ground of a loaded tyre can reduce to about 80% of the height of an unloaded tyre.
This deflection level varies with speed – with low working speeds, greater deflection is allowed and bigger loads can be carried.
If these low pressures were used at higher speeds with higher loads, the tyre carcase would overheat, causing it to fail
The tyre’s load and inflation tables reflect this, with higher loads being allowed at lower speeds and even greater loads allowed in cyclical loading operations, such as combining where the grain tank load could vary from zero to 8t of grain as it harvests. If these low pressures were used at higher speeds with higher loads, the tyre carcase would overheat, causing it to fail.
The IF and VF technology allows greater tyre deflection and a bigger contact patch between the tyre and the ground. Consequently, greater loads can be carried at a given inflation pressure or, for a given sized load, inflation pressure and consequently ground pressure can be reduced.
How much better are they?
IF-designated tyres can carry approximately 20% more load at the same inflation pressure compared to a standard tyre of the same dimensions. In addition, they can carry the same load as a standard tyre but at about 17% lower inflation pressure. The VF-rated tyres allow even more deflection and can either carry up to 40% more load or can work at 30% lower pressure with the same load.
IF-designated tyres can carry approximately 20% more load at the same inflation pressure compared to a standard tyre
These are significant improvements that result in very real reductions in ground pressure. It is achieved by the tyre contact patch widening, but particularly lengthening, when the tyre is allowed to deflect more.
This means more contact area without changing the tyre width much, which is useful when ploughing or keeping wide machines like combines manageable on the road. The impact this could have in a couple of real-life situations are as follows.
Tractor with 8t rear-axle load
An 8t axle load would be typical of a 120kW (160hp) tractor carrying a five-furrow mounted plough or a heavy one-pass or this tractor fully ballasted hauling a very large trailer or tanker imposing a 4t hitch load. Some tyre options and their required inflation pressure, which indicates their ground pressure, are presented in Table 1. The benefits of the IF and particularly the VF options are clear.
Taking the 650/65R38tyre size, the IF version reduces the required pressure by 0.1 bar to 1.2 bar, but by selecting a VF tyre, either 650/65R38 or 710/60R38, the required inflation pressure (and ground pressure) is reduced to 0.9bar. This can’t be matched by the wider standard 800mm and 900mm tyres, showing the potential of the technology.
Combine with 18t axle load
Combines present an interesting challenge, as their field work is usually at quite modest forward speeds, allowing the relatively high loads to be carried at a low tyre pressure. But the use of new technology tyres can reduce the pressures further. A huge constraint with modern combines is their width, and the fitting of very wide tyres can make road transport difficult.
Consequently, tracks are sometimes fitted to larger machines, as they can offer a very long contact area and reasonably low ground pressures, without making the combine wide for transport.
The IF/VF tyre technology offers a partial compromise. Tyre options for a larger combine with a 18t axle load working at under 10kmh in the field are given in Table 2.
The benefits of the IF technology with combine tyres are clear, with substantial reductions in pressure from 2.2 bar to 1.4 bar for 800/65R32 tyres and from 1.6 bar down to 1.2 bar with the larger volume 800/70R32.
These differences also illustrate the need to consult the tyre and inflation tables carefully, as the pressure changes vary between the tyre types and their application. The IF-rated tyres are able to achieve ground pressures at least as low as standard tyres of 900mm or 1,050mm wide, while avoiding the increase in width – up to 0.5m – that the wider standard tyres would cause.
The new technology tyres allow greater tyre sidewall deflection, and consequently lower pressures, to be used, resulting in reduced ground pressure. They are certainly worth considering, as they can reduce pressure on our soils. To get the benefit of any tyre, however, axle loads and the required inflation pressure must be known. A good tyre gauge and compressor are essential, along with once-off access to a weigh bridge. Choose your tyres carefully, as that contact patch between the tyre and the soil is critical.
The size and weight of machinery operating on tillage and grassland farms continues to increase. Tractors, trailers, slurry tankers, ploughs, cultivators, harvesters and more have all increased and Ireland’s farmers and contractors continue to purchase some of the largest machines available.
The previously established threshold for subsoil damage risk, 6t per axle, is frequently exceeded, with many field operations going over 10t per axle.
Tyres or tracks fitted to these machines are all that can protect the soil. Larger sizes are available, but the wider application of more flexible tyre carcases (VF and IF rated tyres) offer additional benefits that are worth exploring.
Flexible carcases: IF and VF tyres
The development of tyre carcases with increased flexibility was pioneered by Michelin over 15 years ago with the introduction of the Xeobib range, which targeted combined field and road use.
The company has since expanded this technology to a range of tyre types and today manufacturers such as Bridgestone, Firestone, Mitas, Trelleborg and BKT all produce tyres with similar technology.
Ireland’s farmers and contractors continue to purchase some of the largest machines available
While the IF and VF technology that underpins these tyres was originally restricted to unique tyre sizes that required different rims, in recent years they have become available in more common sizes that can replace standard tyres on original rims. The tyre manufacturer’s guidelines must be followed regarding rims.
It is only tyres with the letters VF or IF in front of the tyre size that have these characteristics. So, instead of a standard 650/65R38, the size marking would be IF650/65R38 or VF650/65R38. But what is the technology and what does it deliver?
Ground pressure, inflation pressure and soil damage
For any tyre, the pressure it exerts on the soil is determined by the load it carries and the size of the contact patch between it and the soil. Larger tyres have a bigger contact patch and can carry a given load at a lower inflation pressure compared to a small tyre with the same load. When correctly inflated, the air pressure in a tyre is closely related to the ground pressure that the tyre exerts.
Larger tyres have a bigger contact patch and can carry a given load at a lower inflation pressure
But of course, we can only use low pressures if the tyre is large enough. If we used inflation pressure that was lower than allowed, we would reduce the ground pressure further, but by increasing carcase deflection, we would quickly damage the tyre. The tyre manufacturer’s load and inflation pressure tables must always be followed.
Deflection, contact area and ground pressure
Standard tyres are limited to a deflection value of about 20%, where the distance between the rim and the ground of a loaded tyre can reduce to about 80% of the height of an unloaded tyre.
This deflection level varies with speed – with low working speeds, greater deflection is allowed and bigger loads can be carried.
If these low pressures were used at higher speeds with higher loads, the tyre carcase would overheat, causing it to fail
The tyre’s load and inflation tables reflect this, with higher loads being allowed at lower speeds and even greater loads allowed in cyclical loading operations, such as combining where the grain tank load could vary from zero to 8t of grain as it harvests. If these low pressures were used at higher speeds with higher loads, the tyre carcase would overheat, causing it to fail.
The IF and VF technology allows greater tyre deflection and a bigger contact patch between the tyre and the ground. Consequently, greater loads can be carried at a given inflation pressure or, for a given sized load, inflation pressure and consequently ground pressure can be reduced.
How much better are they?
IF-designated tyres can carry approximately 20% more load at the same inflation pressure compared to a standard tyre of the same dimensions. In addition, they can carry the same load as a standard tyre but at about 17% lower inflation pressure. The VF-rated tyres allow even more deflection and can either carry up to 40% more load or can work at 30% lower pressure with the same load.
IF-designated tyres can carry approximately 20% more load at the same inflation pressure compared to a standard tyre
These are significant improvements that result in very real reductions in ground pressure. It is achieved by the tyre contact patch widening, but particularly lengthening, when the tyre is allowed to deflect more.
This means more contact area without changing the tyre width much, which is useful when ploughing or keeping wide machines like combines manageable on the road. The impact this could have in a couple of real-life situations are as follows.
Tractor with 8t rear-axle load
An 8t axle load would be typical of a 120kW (160hp) tractor carrying a five-furrow mounted plough or a heavy one-pass or this tractor fully ballasted hauling a very large trailer or tanker imposing a 4t hitch load. Some tyre options and their required inflation pressure, which indicates their ground pressure, are presented in Table 1. The benefits of the IF and particularly the VF options are clear.
Taking the 650/65R38tyre size, the IF version reduces the required pressure by 0.1 bar to 1.2 bar, but by selecting a VF tyre, either 650/65R38 or 710/60R38, the required inflation pressure (and ground pressure) is reduced to 0.9bar. This can’t be matched by the wider standard 800mm and 900mm tyres, showing the potential of the technology.
Combine with 18t axle load
Combines present an interesting challenge, as their field work is usually at quite modest forward speeds, allowing the relatively high loads to be carried at a low tyre pressure. But the use of new technology tyres can reduce the pressures further. A huge constraint with modern combines is their width, and the fitting of very wide tyres can make road transport difficult.
Consequently, tracks are sometimes fitted to larger machines, as they can offer a very long contact area and reasonably low ground pressures, without making the combine wide for transport.
The IF/VF tyre technology offers a partial compromise. Tyre options for a larger combine with a 18t axle load working at under 10kmh in the field are given in Table 2.
The benefits of the IF technology with combine tyres are clear, with substantial reductions in pressure from 2.2 bar to 1.4 bar for 800/65R32 tyres and from 1.6 bar down to 1.2 bar with the larger volume 800/70R32.
These differences also illustrate the need to consult the tyre and inflation tables carefully, as the pressure changes vary between the tyre types and their application. The IF-rated tyres are able to achieve ground pressures at least as low as standard tyres of 900mm or 1,050mm wide, while avoiding the increase in width – up to 0.5m – that the wider standard tyres would cause.
The new technology tyres allow greater tyre sidewall deflection, and consequently lower pressures, to be used, resulting in reduced ground pressure. They are certainly worth considering, as they can reduce pressure on our soils. To get the benefit of any tyre, however, axle loads and the required inflation pressure must be known. A good tyre gauge and compressor are essential, along with once-off access to a weigh bridge. Choose your tyres carefully, as that contact patch between the tyre and the soil is critical. 
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