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What if we were to change the tire size of our tractor, shrink it or enlarge it. Can we fit a tire we like and want?

Double-wheel drive, ie 4 × 4 tractors, the proportions of the front tire and the rear tire must be compatible. For this, it is necessary to know the meaning of the numbers on the tire.

There is a phrase on the tires containing 3 numbers to indicate the dimensions. Like 320 70 24. If we know the meanings of these numbers exactly, the problem will be solved, which most people know this is missing or wrong.


1-) The first number, 320, is the tire base width in mm. So the base width of our sample tire is 32 cm. Everyone knows that.

2-) The last number, 24, refers to the rim diameter and the figure is always given in inches. Everyone knows that too.

3-) The most important and unknown number is the number in the middle. Sidewall width of the tire. Here is important.

This figure is neither millimeters, centimeters, nor inches. This number is not just any unit of length but a "percentile".

So what percentage?

Answer: The percentage of the sidewall to the tread!

It is very simple for those who know the cheek width calculation. All we have to do is find the sidewall sizes of the tires we want to change. If this sidewall height difference is less than 3 cm, we can replace these two tires without any problem or use them interchangeably.


Let's give an example:

280 85 24 (base 280 mm, up to 85% of cheek base)

280 x 0.85 = 238 mm i.e. 23.8 cm. (this is the cheek width)

Other tire:

320 70 24 (base 320 mm, 70% of cheek base)

320 x 0.70 = 224 mm i.e. 22.4 cm. This is our second cheek width.

Let's look at the difference:

23.8 - 22.4 = 1.4 this is the difference in one cheek. 1.4 x 2 = 2.8 cm, as there are 2 cheeks both lower and upper.

Since the difference is less than 3 cm, we can easily use these two tires interchangeably.

If you ask why 3 cm difference, this is the ideal figure for the difference that gearboxes can tolerate. Otherwise, there may be contractions in the tractor.

At the simplest, consider the original factory tires that came on the tractor. Even they wear out over time and finish their teeth, and when the 5 cm teeth are finished, a full 10 cm diameter difference can occur between the rear tire and the front tire. It is interesting that the transmission can still work without problems.

To sum up:

The numbers 320, 70, 24, respectively:

Base width (in mm), percentage of sidewall to base (in%), rim diameter (in inches)

Note 1: 1 inch: 2.54cm 25.4mm

Note 2: Unless the rim is changed, there is no need to add the last figure to the calculation.

Note 3: If you are going to change the rim size, the last digit, which is the rim diameter, is also added to the calculation. It is slightly different on his account.

Now let's get to this work. We do that with the "newspaper test":


Newspaper test:

1-) Pull the tractor on a flat and hard ground.

2-) Lay a newspaper on the floor 1 meter ahead of the front wheel.

3-) Without connecting the front (4 × 2 shape), just go over the newspaper with the front wheels.

4-) Put this newspaper aside

5-) Do the same operation with another newspaper, only this time by 'tying the fronts' (4 × 4 shape).

Then compare these two newspapers. According to the results:

A-) If there is no difference between them: your front-rear tire fit is super

B-) If the newspaper that was passed over in the second trial (in the form of 4 × 4) is more wrinkled than the first newspaper: there is a mismatch between the front and rear tires. The warmer the newspaper is, the more incoherent it is. This means there is an increase in fuel, a drop in traction, plus the risk of malfunction. It is used, but taking these risks.

C-) If the newspaper passed through is torn or torn in the second attempt (in the form of 4X4): the incompatibility is dangerous. There is a high probability of tripping a fault. Unless there is a very urgent situation on the soil, it is not necessary to tie the front in any way on the asphalt ground. So size change is mandatory.

Another problem that arises when changing the wheel dimensions is that the size statements on agricultural tools and work machines are given in the old-fashioned inch format. Although the new metric system has started to be used, many companies still continue to give the dimensions in inches (). Then:

What are the meanings of wheel sizes defined in inches? In addition, how are they converted to meric systems?

The reading of size phrases in inches is slightly different. Here again, 3 numbers are given, but since the inch is a large unit, there are fractions and decimals where necessary. So four in the number 12.4 just represents a fraction in decimal. It is not a separate unit.

In addition, the middle number, which always causes malfunction, shows itself again and this time begins to express the length of the cheek in inches, not as a percentage.

As an example, consider the front wheel size 12.4 / 11 24 used in most tractors.

12.4: the width of the base in inches. We have a base width of 12.4 x 2.54 = 315mm (31.5 cm). (Most manufacturers round this to 320)

11: height of cheek in inches (no longer percentiles). We have 11 x 2.54 = 279.4mm (27.94 cm) cheek height (most manufacturers use this to 280 '

e rounds)

24: this is the rim diameter that never changes.


12.4 / 11 24 dimensions can be named as 315 280 24 or 320 280 24 as merik. The reason for the millimeter roundings between those is that the 1-2 mm width and sidewall differences should not have any significance in heavy duty machines such as tractors. When a tractor's rear tire with a tread length of 7 cm becomes rough, it loses exactly 140 mm in diameter compared to the first day and can still match the front-rear transmission ratios. Differences of 1-2 or even 5 mm are nothing.

Another important point is that some old tires only have 2 numbers. In these cases, the sidewall height of the tire is the same as the base width and no further specification is required. This is also true in cases where the middle number is '00'.

For example, a tire with the size of 6.50 / 00 16 actually refers to the size of 6.50 / 6.50 16, where '00' is actually put because the ratio of the cheek to the base is 100%.


Tires, in which all figures are given in inches, are mostly British or British companies, and they make this annoyance because they easily abandon traditions.

With the above method, we can convert the tire size in inches you want to metric and compare.

Note: Tires specified with expressions such as 6 × 13 express only the base width and the total diameter of the wheel. Doesn't care about cheek width. It is used only in some heavy duty and special industrial machines.

Additional note: Tire base widths actually always end in 5. Like 195, 245, 315. The reason is just an old habit. It has just begun to change.

Tractor & Farm Machinery
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