Authored by: Tractor tyre expert | 19 November 2021
Tractor wheel slip, if excessive, brings only disadvantages: it increases the number of hours you have to spend on each campaign, it increases your fuel consumption, it causes soil sealing, contributes to soil compaction and, lastly, rapidly increases tyre wear. Not taking account of your slip ratio will therefore have a highly negative influence on the profitability of your campaigns. To control it better, you must bear in mind that a minimum amount of slippage is essential to ensure traction but only up to a certain limit, which you must absolutely control.
In this article, we explain the points to check and the adjustments which will help you obtain an optimal slip ratio:
1. Slip is directly linked to the type of soil and the weather conditions
Different factors may cause tractor wheel slip, starting with the soil structure of your land at the time you begin work. This structure will be the result of the type of soil combined with the moisture level and weather conditions.
To move forward, your tyres crush the earth to the point of soil resistance in order to transmit torque to the rim.
This soil resistance point will be different each day depending on bad weather and soil composition. This resistance corresponds to the wheel slip ratio, given that the loss of power linked to slippage may reach 30% or more depending on the weather conditions and the tyre specificities:
It is therefore better to take account of these factors before you start work and adapt the equipment so as to avoid an excessive slip ratio.
What type of soil do you have on your land?
The type of soil has a direct impact on tractor wheel slip.
To work in the best conditions while preserving your soil, it is recommended that you first analyse the type of soil and the condition of your land.
A saturated clay soil increases the risk of getting stuck
Hard, dry earth:
When the ground is dry and quite compact, you can drive any type of vehicle without the risk of slippage, the earth offers a good level of resistance, the tyres don’t sink in very deep and are easily supported by the ground. In this case, the slip ratio is very low. However, beware of large amounts of slippery residue which may impact slippage even on hard ground.
Dry but sandy earth:
If the soil is dry but not very compact, the lugs do not face much resistance, if the soil has a sandy texture, it will have trouble bonding together. Some tractive force will be lost, because the lugs cannot get a good grip. The tyre will tend to sink into the ground until it comes into contact with more compact soil deeper down. This will cause a high level of slip, fuel consumption will increase and the tyres will wear more rapidly.
In this situation, the only possibility is to work with low pressure tyres to increase the tyre’s contact patch with the ground and increase the lugs’ grip capacity.
Soft, light earth:
The earth is crumbly and very slightly wet when you squeeze a clod of earth in your hand, the lumps of earth can be easily squashed between your fingers.
You can work without any risk of slip and traction should be optimal.
Wet, sticky earth:
With a wet clay soil it will be preferable to postpone the campaign (if possible) in the hope of being able to work on soil that has dried fully to avoid slippage and the risk of compaction under the slightest tyre pressure.
Very wet soil causes wheel slip, which inevitably accelerates soil damage, fuel consumption and rapid wear to the tyres.
Are the weather conditions and the season adapted to tillage?
The weather conditions and the seasons are decisive factors if you are looking to optimise quality of work and productivity.
Slip: Beware of weather conditions
If you have to work in wet conditions, for tilling at the beginning of the season or corn or beet harvesting at the end of the season, the risk of wheel slip is very high.
It will be important and necessary to make calculations and pressure adjustments at the start of each campaign. The quality of the tyres will be decisive in this case, the flexibility of the casing, the type of lug profile and the level of tyre wear will all have an impact on the tyre’s capacity to evacuate the earth which accumulates between the lugs.
2. Slip is linked to wheel size
There are several ways of reducing wheel slip. One efficient solution, if you work in a generally wet region with clay soils, is to increase the tyres’ soil footprint to improve grip and traction by opting for larger tyres when you replace the old set: series 75, 70, 65 or even 60 if necessary.
Increasing the width of the tyre effectively increases the contact patch with the ground but also allows you to work with a heavier load while limiting the extent to which the tyre sinks into the ground, meaning that wheel slip is still kept under control in wet conditions.
3. Adopting the right inflation pressure based on the load
When working in the fields with heavy implements, we often tend to increase pressure to avoid excessive crushing of the tyres. Yet overinflation is not always a good solution and is even unadvisable in certain circumstances.
Slip linked to overinflation on heavy soil
Overinflated tyres increase slip, rolling resistance and soil compaction.
For a simple reference, we can consider that the level of pressure within the tyre corresponds to the push force exerted on the ground.
In wet conditions, it is highly unadvisable to overinflate your tyres because by increasing the pressure to compensate for the load, the tyre will tend to sink into the ground to find resistance at the risk of increasing the slip ratio considerably. The higher the slip ratio, the more your fuel consumption increases, given that a slip ratio of 30% can lead to up to 20% more fuel consumption.
Tyres that sink further into the ground also result in an increase in rolling resistance. The quantity of earth to be found in front of the tyres is much higher. The tyres therefore need more power to keep going forward at the same speed, given that an additional depth of 1 cm will generate an increase in fuel consumption by 10%.
To compensate for these problems with slip and rolling resistance, it is therefore recommended that you work at a very low inflation pressure of around 0.8 bar if you have sufficiently high-tech tyres.
4. How to calculate your slip ratio at the beginning of a campaign
If your tractor cab is not equipped with an on-board slip control system, it is important, before beginning work, to define your machine’s slip ratio with reference to the tools used, the type of soil and the weather conditions. This slip ratio will allow you to adapt inflation pressure based on the result and obtain better transmission of tractive force.
Here is how to calculate the slip ratio of tractor tyres
To establish your slip ratio simply and precisely when you arrive in the field, you must follow the steps below:
STEP 1. Make a mark on the sidewall of the rear tractor tyre: make the mark at the spot where the wheel is in contact with the ground.
STEP 2. Drive for 10 rotations of the rear wheel while ploughing, then mark the arrival point on the ground.
STEP 3. Go back to the starting point: once you are back at the starting point, drive the same distance between the two points with the plough raised, then count the number of wheel rotations.
Imagine for example that you counted 9.1 wheel rotations during the second passage with the plough raised. The calculation would be as follows:
The slip ratio in this example is therefore = [(10 – 9.1) / 10] x 100 = 9%, which is perfect for good transmission of engine power to the ground.
If the slip ratio is above 15% it is excessive; you can then reduce pressure to increase the tyre’s contact patch with the ground. If the slip ratio is more than 20 to 25%, it is definitely worth tyring to postpone the operation until the soil is dryer.
5. The advantage of IF tyres in reducing slip
Among the solutions for better control of agricultural tyre slip, the use of IF (Improved Flexion) technology tyres allows you to work at very low pressure to reduce slip while increasing traction capacity.
Thanks to their low inflation pressure, these tyres have a larger soil footprint, which increases the lug contact area.
IF tyres have a greater flexion capacity in their sidewalls linked to the use of more robust, technological materials in the production of their casing. In practice, these new generation tyres allow you to drive at a very low inflation pressure while increasing the load capacity by 20% compared to standard agricultural tyres.
This type of IF casing also has other major advantages, in particular in terms of saving time on tyre pressure adjustments. These tyres are effectively designed for use in the fields and on the road without adjusting pressure based on speed. You no longer have to get out of the tractor to change the pressure settings each time you go from one type of ground to another.
They offer exceptional driving comfort and limit the risk of damage to the casing following incorrect pressure adjustments for example. They are also a good alternative to the more costly solution of using duals when working in wet conditions.
For more information on tractor tyres
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.
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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.