How to choose the right pressure for my tractor tyres?
Authored by: Tractor tyre expert | 02 February 2021
You probably already know that adjusting the pressure in your tractor tyres can reduce wear and help them last longer. But it can also help to protect the soil in your fields and therefore your productivity, while improving your security on road.
However, it is difficult to always have the right tyre pressure to suit every situation. Here are some rules and solutions which will help you work with the right tyre pressure.
1. What is the relationship between the load and the tyre pressure?
The only connection between your tractor and the soil is your tyre. But these tyres can only carry the weight of your machine and its implements, for a specific forward speed, if they are inflated to the correct pressure. You must also consider the tyre’s capacity and the maximum load recommended by the manufacturer.
The volume of air in the tyres is the most important element
The weight that your tractor tyres can carry is a matter of tyre technology, tyre design and components defined by manufacturer, and it’s fundamentally a question of well-defined inflation pressure. In other words, the tyres themselves couldn't properly carried the weight on each tractor axle, if the correct quantity of air held in each tyre.
To improve the efficiency of your vehicle, it is important to adjust your tyre pressure according to several parameters:
Tyre manufacturer information: authorised pressure based on load and speed, provided in load and pressure tables
The weight on each of your machine’s axles
The optimal slip ratio
Protecting the soil in your fields to avoid reducing your yield
The forward speed
How can you increase the load that your tyres can carry?
To carry a heavier load (that of the tractor and its tools), the air contained in your tractor tyres must be proportionate to the load carried. There are two ways to increase this:
Increase your tyre pressure
You can increase the pressure in your farming tyres by injecting more compressible air to allow them to sustain a heavier load. You have to respect load and pressure tables provided by tyre manufacturer. You maybe need to adapt your inflation pressure and your speed to reach heavier load.
This is because the higher tyre pressure can reduce the tyre footprint, so the vehicle pressure is spread over a smaller area and has a greater impact on the ground. To balance the vehicle pressure and the soil pressure, with this smaller footprint, the tyre will sink and therefore compact the soil until the balance point of these two pressures (vehicle/soil) is reached.
On loose soil, overly high pressure will encourage:
Excessive deep soil compaction
Increased slip ratio
Creation of ruts
Increased rolling resistance
So, increasing too much the tyre pressure has a negative impact on the soil, crop development and mechanical efficiency, and consequently on your productivity.
Increase the tyre size and the volume of air
You can increase your machine’s load capacity by choosing tyres with a greater volume of air or greater technology. To increase load capacity by increasing air capacity, a tyre size has equivalent sizes based on SRI (Speed Radius Index) which gather all tyre sizes with same diameter. So you could switch from 85 series to XL series, winning additional air and load capacity. You have to look at Correspondence table to switch tyre size. Be aware that all tyre changes need to be made by an expert with lead-lag calculation.
When working with heavy vehicle on loose or heavy soil, whatever the weather, you must increase your tyre footprint to limit the impact on soil.
Another option is to choose a greater tyre technology as IF or VF. For past generation of VF tyres, you must reduce the rim diameter to a minimum to increase the volume of air in the tyre. Now, with the new VF tyre generation with NRO option, you could obtain the same load benefit without changing the rim. The tyre construction itself, allow higher load capacity.
2. How do you calculate tyre pressure?
To determine the right pressure for your tractor tyres, you must start by identifying the load to be carried by each axle, using one of the following methods. Whichever method you choose, you must consider the forward speed which will automatically vary depending on how it is used on the road or in your fields.
STEP 1: Have correct weight distribution
Optimizing the weight distribution results in better fuel consumption, less wear on the powertrain components & tyres, less soil compaction and more productivity.
Correct weight distribution is 60% rear and 40% front, with or without implement. If you add implement for soil preparation, for example, it will increase weight on the rear and you have to adjust the correct tractor weight distribution by adding weight on the front.
The most precise method is to use at weighing tool, that enables you to determine the actual load carried for each axle.
All you need to do is weight each axle of your tractor (with implement), then divide the result in two to find the weight supported by each tyre. Then refer to the tyre load and pressure table to determine the optimal pressure for the front and rear axles.
Here are some guidelines for carrying out the weighing process correctly:
To weigh the front axle: weigh with the rear tool down
To weigh the rear axle: weigh with the rear tool raised
10.010 kg represents the mass distributed over the tractor’s rear axle (rear tool raised in road use)
For frequent road use, don’t forget to add 0.4 bar to the indications in the pressure table to reduce premature wear to the tyre rubber caused by the road surface.
STEP 2: use a dedicated app
Many manufacturers such as Firestone provide mobile apps accessible through smartphone and tablet to easily determine the right pressure for your farming tyres.
You need to have all the necessary data to use the online application. This data relates, in particular, to the weight of your tractor and its tools.
In just a few clicks you can have an estimation of the approximate load for the optimal pressure of your tractor tyres.
› Download the free Firestone Agriculture app for Android or iPhone
3. Best ways to improve the right tyre pressure control: TPMS and CTIS systems
You’re well aware that low tyre pressure is suitable for working in the fields as it optimises comfort and limits soil compaction and spinning.
However, a higher pressure is recommended for road use, as it reduces rolling resistance and consequently fuel consumption and wear to the tyres, while increasing the tractor’s stability.
The problem arises when determining the best pressure for mixed use (road and fields). To comply with the pressure recommendations, whatever way you use your tractor, CTIS is the most effective solution.
TMPS: Tyre Monitoring Pressure System to control in real time and warn you when you are far from recommended inflation pressure.
CTIS: Central Tyre Inflation System is to control and adjust in real time pressure.
How work TPMS?
This system is installed directly on your farming tyres to quickly control their pressure and temperature while in use.
The system is controlled from the tractor cabin using an electronic control module which displays the pressure and temperature levels, with warning parameters in case the levels are too far from minimum or maximum limits setup.
This system can represent an average investment of about €400 to €500 excluding VAT. It is available for 4 wheel tractors, but can be adapted for tractor with twin wheels and trailers.
How work central tyre inflation system (CTIS)?
The better models allow you to adjust pressure as you go, to allow you to change from field to road use without having to stop: the pressure is reduced in the field and increased for road use, without leaving the cabin. It is easy to instal and easy to use providing constant information about pressure and detection of slow punctures.
How you install your CTIS depends on your tractor’s braking system:
A Pneumatic braking system: is generally equipped with an integrated compressor which can be connected to the CTIS (be sure to install a cut-off valve to prioritise the flow of air to the brakes).
A Hydraulic braking system: does not generally integrate a tyre compressor, so your supplier will need to install the appropriate compressor so that the installation is adapted to your specific CTIS use (tractor + trailer for example).
In both cases, the air circulates towards the wheels in a rotation thanks to an airproof circuit equipped with rotary joints.
The time required to inflate/deflate depends on several elements: your compressor, the volume of each tyre and the number of wheels.
In general, to increase by 1 pressure bar takes around 3 minutes, and it takes around 1 minute to reduce the pressure by 1 bar.
12 months to amortise your central tyre inflation system (CTIS)
Despite its initial cost, your CTIS can pay for itself within the first year and your costs will be reduced in the second and following years.
As well as the time you will save, this system allows you to spend up to 30% less on fuel in road use with 2 bars of pressure instead of 0.8 bar. This also reduces wear to the tyres and improves your comfort (comfort and stability).
As well as improving the tractor’s traction capacity by up to 35%, CTIS also reduce soil compaction, increases your mechanical efficiency and your productivity.
Installing a CTIS should be seen as a worthwhile investment in the long term, starting generally in the second year.
To find out more and extend the lifespan of your tractor tyres, download our free comprehensive guide: “How to adjust the pressure in my tractor tyres to gain hours more use”.
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This information is intended only to make you aware of the technical and functional aspects of agricultural tires and their use. It does not allow you to make a judgment or a definitive conclusion on a given problem. Only your agricultural tire expert is able to make a technical assessment and take a final decision, case by case.