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 an extra year. But it can also help to protect the soil in your fields and therefore your productivity, while reducing the risk of road accidents for heavy loads.
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 tools, 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 isn’t a matter of brand, production year or any other basic parameter, but solely a question of the quantity of air in each tyre. In other words, it’s not so much the tyres that carry the weight on each tractor axle, but the quantity of air held in each tyre.
To improve the efficiency of your machinery and tools, it is important to adjust your tyre pressure according to several parameters:
Manufacturer information: authorised pressure limits (maximum and minimum)
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.
Beware of the consequences, as the higher the pressure the higher the soil compaction. This is because the tyre pressure is nearly the same as the pressure exerted on the soil.
On loose soil, overly high pressure will encourage:
Excessive deep soil compaction
Increased slip ratio
Creation of ruts
Increased rolling resistance
So, increasing 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. By increasing the size of your tyres or by swapping your tyres for wheels and tyres with a greater air capacity, you will improve your tractor’s efficiency: reduced vertical soil compaction due to a wider contact of the tyre on the soil. This will also reduce the slip rate and fuel consumption.
When working with heavy machinery on loose or heavy soil, whatever the weather, you must increase your tread contact length without impacting the soil . To do so, be sure to reduce the rim diameter to a minimum to increase the volume of air in the tyre, while complying with the preponderance norms determined by the manufacturer.
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.
METHOD 1: calculate manually
If you don’t have access to weighing materials, you can calculate the load on each axle manually before adjusting the pressure of your agricultural tyres.
To do so, use the following formula:
The following information is provided in the manufacturer’s documents:
M1 : the weight of front ballast (or front tool)
M2 : the weight of the rear tool
FRONT axle Weight: weight on the front axle when empty
REAR axle Weight: weight on the rear axle when empty
You must then measure the distances:
D1: distance between the centre of the front axle and the centre of the front ballasting weights/or the front tool
D2: distance between the centre of the rear axle and the centre of the rear carried tool
E: distance between the centre of the rear axle and the centre of the front axle
METHOD 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.
As with the first method, 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
METHOD 3: use weighing equipment
This method is the most precise as the weighing tool enables you to determine the actual load carried for each axle.
All you need to do is weigh your tractor and your tools, then divide the result in two to find the weight born by each tyre/axle. Then refer to the tyre lookup 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.
3. The best way to always have the right tyre pressure: the central tyre inflation system (CTIS)
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.
What is a central tyre inflation system (CTIS)?
This system is installed directly on your farming tyres to quickly adjust their pressure while in use, depending on your needs.
The system is controlled from the tractor cabin using an electronic control module which displays the pressure levels and their configuration.
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 to install a central tyre inflation system (CTIS)
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 instal 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
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 by 4% and your productivity by up to 7%.
Installing a CTIS should be seen as a worthwhile investment in the long term, starting in the second year.
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With Firestone, you have the guarantee of tractor tyres that deliver a real advantage, and that in choosing our brand, you can work worry-free.