You have heard about your tractor’s front-wheel lead ratio without really knowing what this is about and as long as you use your tractor’s original tyres this problem does not really exist. So why worry about it? What purpose does it serve? And in the end, what benefit will you obtain from knowing? You may not know it, but the lead ratio becomes very important when you decide to use the full potential of your tractor, to use all its power to save time, improve your yields and productivity. It becomes more and more a technical requirement from tractor manufacturer when you decide to replace your tyres.
Unfortunately, most of the time you don’t really use your tractor’s full capacity… No doubt you paid a great deal to have a more powerful machine than the previous one, so why not use this power to the full.
It’s not a matter of modifying or amplifying the power of your engine but rather of knowing how to well transmit this power to the ground. The lead lag ratio is relevant to choose the relevant tyre set, even if you keep the same tyre sizes. It’s essential when you decide to change tyre series, use a different tyre brand or mix different tyre brands.
1. When do you need to pay attention to the lead ratio?
It’s precisely when you choose a new tyre set for a mechanical front-wheel drive tractor that the front-wheel lead comes into play.
Do you know how tractor’s lead ratio is handled for your new tractor? There are two steps:
1st STEP - tyre homologations
the tyres are part of the equipment that the tractor manufacturer has to buy from the tyre manufacturer, so the original mount is often the result of a financial negotiation to obtain the best purchase price for the tractor manufacturer and rarely to obtain the best technical solution for the machine.
2nd STEP - Tractor configuration
Tyres is one of many options. The tractor manufacturer produces a machine for different uses which may vary from working 100% of the time with field crops to polyculture or use in the farm’s courtyard for livestock farming purposes for example, where the work is shorter but more constrictive (loads handled, high-level manoeuvrability required, more abrasive ground). The tyre option specified or no specified could be therefore the intermediary which must correspond with nearly all the activities carried out without necessarily being optimal for a very specific use. As each farmer has different activities, there is a risk that original tyre work correctly but are not being perfectly adapted to your specific case.
So to improve your tractor’s output and use all its tractive force and its load capacities while preserving your soils, it will generally be necessary to replace the original tyre with a more efficient tyre.
2. What is front-wheel lead?
The drive ratio which is normally supplied by the tractor manufacturer gives you the inter-axle ratio (the number of rotations the small front tyre makes in relation to the larger rear tyre) on four-wheel drive tractors. However, this is not enough to obtain optimal traction.
With this type of machine, the front axle must actually pull the rear axle slightly when the front axle is engaged. It’s the calculation of this difference in traction, where the front axle pulls slightly faster than the rear axle, which we call front-wheel lead.
When you replace your original tyres with higher volume tyres, you must obviously keep the same ratio between the size of the front and rear tyres, to respect the front-wheel lead ratio: this allows both front and rear power transmission to work perfectly together in optimal synchronisation, allowing the front of the tractor to pull slightly more than the rear.
3. How do you calculate the lead ratio?
The lead ratio could be calculated thanks to the inter-axel ratio provided by tractor manufacturer. It could be also calculated by your tyre dealer in real condition with your tractor in a flat surface.
In order to make the calculation, you must first gather the relevant information:
You must find the value which corresponds to the mechanical drive ratio in your tractor manufacturer’s guide.
If you cannot find this drive ratio, you can calculate it directly for your tractor with its current tyres. As the front wheels are smaller, they rotate faster than the rear wheels.
You need to establish the exact number of rotations made by the front wheels compared to the rear wheels. To make the calculation precisely, divide your tyres into twelve equal sections using chalk, like in the picture below.
To calculate the mechanical drive ratio easily, draw vertical lines on your tyres.
You must then drive forward for 10 full rotations of the rear wheels. The aim is to count exactly how many rotations the front wheels make over the same distance:
For 10 rotations of the rear tyre we obtain 13 rotations + 5/12ths of the front tyre.
The drive ratio can then be calculated as follows: 13 + (5/12) = 13.41 13.41 rotations ÷ 10 rotations = 1.341 drive ratio
You need to know the rolling circumference of the front and rear tyres (provided by tyre manufacturers in technical data book or app) that you intend to mount on your tractor. You can obtain the data from your tyre manufacturer or tyre dealer.
Here is the calculation formula to apply to the data collected:
Here is an example of the calculation with real tyre sizes:
Mechanical drive ratio obtained from the tractor manufacturer’s guide or calculated = 1.341
Rear tyre 650/75 R38 with a rolling circumference of 5760 mm
Front tyre 650/65 R28 with a rolling circumference of 4450 mm
The calculation can be performed using these figures:
((4450 × 1.341) − 5760) ÷ 5760) × 100 = 3.60%
In this example the lead ratio is 3.6%, which is a good result, with the tractor’s front axle slightly pulling the rear axle, which will lead to better performance in terms of traction.
To define the exact size of the front tyre when you have chosen your large sized rear tyre, you must have the rolling circumference of your new rear tyre in millimetres. You can then calculate the size of the front tyre as follows:
New rear tyre size ÷ drive ratio = new front tyre size in mm
Then you add the lead ratio: New front size + 3.6% = the size of the tyre circumference which you are looking for (approximately).
When you calculate lead ratio based on weared tyre, you have to define its rolling circumference. Rolling circumference is the distance drive by tyre in one revolution, so you could estimate it by measuring the distance drive by the tyre in one complete tyre rotation. This could be also calculated theoretically be your tyre dealer.
4. Why try to obtain an optimal front-wheel lead ratio?
On mechanical front-wheel drive tractors, when you to move up to larger tyres to improve productivity, you cannot just change the rear tyres without creating an imbalance. You must also increase the size of the front tyres, keeping the same proportionality in size between front and rear tyres.
Before buying new tyres, calculate your front-wheel lead ratio to ensure that your set-up is optimal.
How do you analyse the results obtained if the lead ratio?
What happens if the lead ratio calculated is higher than 4%
This would mean that the rear axle is turning too slowly compared to the front axle, so the rear axle is holding back the front axle, which is likely to cause front wheel slippage. The front axle undergoes too much mechanical pressure which may result in a rattling noise when you engage the front axle as well as excessive mechanical heating, and a rapid tyre wear.
Worn gearhead following pressure or heating linked to an incorrect lead ratio
What happens if the lead ratio calculated is lower than 0%
This would mean that the rear axle turns quicker than the front axle, so the rear axle pushes the front axle, which is likely to cause rear wheel slippage. The front axle looses efficiency and steering is less responsive on loose soil.
Consequence of an incorrect lead ratio
5. What are the signs of an incorrect lead ratio?
Very rapid wear to the front two tyres compared to the rear tyres is a sign that your lead ratio may not be suitable.
Steering is not responsive on heavy soils and the rear wheels have a tendency to slip.
You notice an abnormal rattling each time you put the tractor into drive, or a dry rattling when breaking on the road. Indeed, in the latest generation tractors, when breaking on the road, the front axle is engaged automatically in order to improve safety.
There may obviously be other reasons for these signs or anomalies, but to put your mind at rest you can check the lead ratio using calculation formula provided above.
Replacing your tyres is the opportunity to improve your tractor’s output. By using the right information, you will be able to improve traction power, load capacity and therefore use bigger, heavier, more productive implements, reduce time spent over your land and therefore help saving fuel while avoiding soil compaction. In short, it would really be a pity not to seize this opportunity to turn a simple tyre replacement operation into a positive transformation of your machine to improve your yields and your productivity.
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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.