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Ag Pressure Blog 2

Agricultural Tyres – Ground Pressure & Point Loading Fractures

Ag Pressure Blog

When you work on the land you need to maximise your profits. Keeping your farm tyres well maintained with correct pressures and knowing the signs of irregular wear & point loading fractures are key to getting you safely from soil to silo.

Point loading fracture

Point loading fracture is the failure of the tyre’s structure due to excessive stress applied at a specific point or in a localised area on the tyre.

Tractor tyres are designed to handle heavy loads and harsh conditions in the fields. But, past cultivating, or heavy rainfall over tracks and paddocks can cause uneven terrain and bring objects buried beneath the soil closer to the surface.

When a tractor tyre rolls over a sharp rock or hits a pothole, it causes a sharp increase in ground pressure. When this pressure is concentrated on a very small surface area, it can exceed the maximum point load needed to break the cords in the tyre’s tread or carcass belts, and a fracture happens. This could be a visible crack, a tear, or even complete break in the tyre’s surface.

Contact patch

A contact patch refers to the area of the tyre’s tread that’s in contact with the surface it is driving on. It’s the part of the tyre that supports the tractor’s weight and transmits the driving, braking, and steering forces between the tyre and the road.

Both the tyre and the surface can deform under the pressure of the vehicle. The size and shape of the contact patch vary depending on factors such as tyre design, tyre pressure, load on the tyre, and road surface conditions.

Ground pressure

Ground pressure refers to the pressure exerted on the ground by of different bodies or vehicles by their weight distributed over their contact area. Ground pressure is typically measured in pounds per square inch (psi) or kilopascals (kPa). Here are some examples of ground pressures for various bodies:

Passenger Car: The ground pressure of a passenger car is influenced by several factors, including the weight of the vehicle, the number of wheels, tyre type, and tyre inflation pressure. Typically, its ground pressure is around 30 psi (pounds per square inch) or approximately 205 Kpa.

A Human: Even human feet exert ground pressure, especially in certain circumstances like walking on soft surfaces like mud or sand.  A 1.8 metre tall, medium build male may exert 55 Kpa (8 psi).

Tractor Tyres: Ground pressure for tractor tyres can vary depending on their size, type, and inflation pressure. A 3152 Case Header LHF Tyre for example can exert 268 Kpa (38 psi).

How to recognise a point loading fracture

Spotting a point loading fracture in a tyre can be tricky depending on the severity of the fracture and the type of tyre. Some common signs include:

Visible cracks or tears: A point loading fracture can cause visible scratches, cracks or tears on the tyre’s sidewall or tread lug.

Bulges or deformations: The fracture may result in bulging or deformation of the tyre’s surface, indicating a structural failure.

Loss of air pressure: A point loading fracture can lead to air leakage from the damaged area, causing the tyre to lose pressure rapidly.

Visible tyre damage: In severe cases, the point loading fracture may cause a visible gap or separation between tyre layers or the appearance of exposed cords.

Unseen belt fracture: One of the hardest symptoms to detect with a visual inspection. A break or separation in one or more of the belts concealed within the tyre’s layers can cause vibrations, uneven wear, or in severe cases, sudden failure.

Inner liner break: A break in the inner liner of the tyre lets air pass into the tyre carcass and separates the body plies. Every rotation can force as much as 12.4T of air between the belts.

Folded Tyre Tread: When a tyre Lug nose rolls over a prominent object, the tread can fold inwards. The uneven flex in the tread creates a “hinging” effect in the crown of the tyre that breaks the cords in the belt.

The difference between separations and point loading fractures

When examining tyre failures, the condition of cord ends, and belt angle breakages are a key to understanding whether they are fractures or separations.

Fractured cord ends look frayed due to prolonged rubbing together. Belt breakages, on the other hand, exhibit a characteristic “Y” pattern, caused by successive failures at different angles and places within the tyre.

Separation failures usually pass between the plies of the tyre without any cord breakages. Large separations are usually noticeable when driving and are less likely to rotate under the vehicle’s guards. In contrast, tyres with broken cords deflect inwards, allowing even tyres with massive separations to pass under the guard and go unnoticed by the driver. These separation failures are commonly found in sidewalls and shoulder areas.

Point loading fractures predominantly appear in the crown of the tyre and extend towards the shoulder.

Plunger testing

Plunger testing is a crucial quality control process that simulates various operating scenarios and load pressures under controlled conditions in a factory.

It basically involves pushing a “plunger” or metal probe into different parts of the tyre to measure its resistance to penetration and deformation at various inflation pressures.

The aim is to ensure that the tyre’s structural integrity, performance, and safety meet the required standards. Plunger testing helps identify potential weaknesses in its construction, such as sidewall and tread durability. It also helps manufacturers to make reliable and durable tractor tyres.

Inflation pressures on tyres

Inflation pressure on tractor tyres is critical for their performance and longevity.

As confirmed by testing, lower inflation means a tyre is less likely to pass the plunger test. Lower inflation also increases the risk that the tyre sidewalls may deform, lose shape, and touch the rim or wheel causes increased wear and reduced traction.

As inflation pressure increases, the tyre’s spring rate also rises, along with the chances of penetration. Higher pressure creates a smaller footprint and increases the ground pressure on large volume or large footprint tyres.

Shockingly, a mere 18% decrease in footprint size can escalate ground pressure by a staggering 30%. Properly managing inflation pressure is thus crucial for optimizing tractor tyre performance and minimizing potential risks.

Over inflation

Over inflating tractor tyres can have several adverse effects on their performance and overall operation.

The excess air pressure can cause the central portion of the tread to bulge outwards, creating a higher “crown” or hump in the middle of the tyre’s contact patch with the ground.

This can decrease the width, length, and overall area of the tyre’s footprint. The result is increased ground pressure and a more rigid footprint, making the tyres more susceptible to point loading fractures.

It also leads to rapid, uneven wear, lug chipping and chunking, putting additional strain on the machine and can even cause operator fatigue.

Under inflation

Under inflation of tractor tyres can have very negative effects on how they perform and how long they last.

When the tyres are under-inflated, the increased flexing of the sidewalls causes the footprint to expand, resulting in decreased ground pressure. While this can be beneficial for reducing soil compaction, it comes at the cost of increased tyre heat build-up.

Under inflation raises the risks of separations happening in the bead, sidewall, and shoulder areas, which can lead to dangerous blowouts. Additionally, it reduces fuel efficiency, accelerates wear, and compromises overall traction and stability.

Correct tyre pressures

Determining the right pressure requires accurate load measurement, and an understanding your machine and its usage on your property.

To find out your load measurement, measure the tyre loads on both sides, both when the tractor is fully loaded and empty, as there can be significant differences that necessitate more sophisticated pressure management.

For modern CFO tyres (Cyclic Field Operation), precise calculations of maximum laden and unladen tyre loads are crucial. For instance, Case Headers tend to be 15% to 18% heavier on the left-hand side, requiring tailored pressure adjustments.


It’s all about managing the details to keep you rolling from soil to silo and maximising your profits on the land. If you have any further questions, our friendly team of tyre experts are only a live chat or phone call away!

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Posted 30/09/2023