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How to read the motorcycle tyre size

The motorcycle tyre size is indicated on its sidewall. But how do you figure out this sequence of numbers and letters? Find out in this article.

Contents of the article:

Motorcycle tyre size explained
What is the difference between radial, bias and bias belted motorcycle tyres?
Motorcycle tyre size for road use
Motorcycle tyre size for off-road use
Enduro and Motocross tyre sizes
Motorcycle tyre size for track use
What about retro motorbike tyre sizes?

Motorcycle tyre size explained

 There are a number of elements that determine the size of a tyre. As you will see, it is not just the ratio of height to width. The size of a tyre also contains other important information that defines its type of construction and the limits of its capabilities.

Let's take a concrete example with the MICHELIN Road 6:

As you can see, the sidewall of the tyre shows the following dimensions:
180/55 ZR 17 then:  M/C (73W)

Here is what these codes mean:

  • 180 : The width of the tyre, expressed in millimetres.
  • 55 : The aspect ratio (the sidewall height as a proportion of the tyre width).
  • ZR : The "R" stands for Radial construction. A "ZR" code refers specifically to radial tyres designed for high speeds.
  • 17 : The bead-seat diameter of the wheel, expressed in inches (1 inch = 2.54cm).
  • M/C : This code simply means that it is a tyre for motorcycles.
  • (73 W) : The number is the load rating of the tyre and the letter following it is the speed rating. Correspondence tables show what maximum load the tyre is able to bear and at what maximum speed it can be driven. In this example, 73 means that the tyre can carry a load of up to 365 kg. The (W) means that it can be driven over 270 km/h.

What is the difference between radial, bias and bias belted motorcycle tyres?

Radial tyres

A radial tyre is identified by the letter R on the tyre

Radial tyres provide greater comfort at high speeds as a result of their soft sidewalls which absorb the impact of imperfections on the road surface. Radial tyres are needed for powerful vehicles with very rigid chassis and sporty purposes.  A radial tyre is identified by the letter R on the tyre. (e.g: 120/70 R 15)

Bias tyres

A bias tyre is identified by a dash

Bias tyres are able to carry a greater weight because their sidewalls are more rigid. They are suitable for vehicles travelling at moderate speeds, with small to medium-sized engines and flexible chassis.

A bias tyre is identified by a dash (e.g: 120/70 - 12).

Bias belted tyres

A bias belted tyre is identified by the letter B on the tyre

Bias-belted tyres are so called because a "belt" strengthens the tyre section. This reinforced version of the bias is more suitable for heavy motorcycles.

A bias belted tyre is identified by the letter B on the tyre (e.g: 180/65 B 16).

Now that you know the generalities, let's review the motorcycle tyre sizes for specific uses.

Motorcycle tyre size for road use

If you ride on the road, you are legally committed to the dimensions recommended by the manufacturer of your motorbike. These dimensions are the ones that are marked on your original tyres if you have never changed them.

But as we shall see, these obligations do not apply if you ride exclusively on other types of terrain.

Motorcycle tyre size for off-road use

Enduro and Motocross tyre sizes

If you only ride off-road, you are not bound by the legal requirements for road use. Therefore, it is possible to choose tyre sizes according to criteria that are important for your use.

For Enduro and motocross, some riders may opt for a narrower tyre for better handling. Others choose a wider tyre for more grip with studs.

Note that the dimensions are not the same for these two off-road disciplines. The width of motocross tyres is measured at the base of the studs, whereas the width of enduro tyres is measured at the widest point, i.e. the top of the studs.

To make sure you choose the right dimensions, refer to this enduro / motocross tyre sizes correspondence table:

ENDURO

CROSS

90/90-21

80/100-21

120/80-19

100/90-19

130/70-19

110/90-19

120/90-18

100/100-18

130/80-18

110/100-18

140/80-18

120/90-18

Motorcycle tyre size for track use

For the track, some riders mount a wider tyre at the rear in order to get more grip. The disadvantage is that this reduces the handling of the bike.

Competition tyres usually carry the acronym NHS (Not for Highway Service). This indicates that these tyres are not approved for road use.

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What about retro motorbike tyre sizes?

If you have an older motorbike, you may be having difficulty finding the right dimensions to replace your tyre, as the nomenclature has changed over time. Originally in inches, the tyre dimensions have now become standardised in millimetres (with the exception of the rim diameter which has remained in inches).

Fortunately, you can find compatible dimensions.

Here's how:

Motorcycle tyre size compatibility

You can find the correspondence between the dimensions in inches and the dimensions in millimetres in this table:

DIMENSIONS IN MM

DIMENSIONS IN INCHES

50/100

2.00

60/100

2.25

70/100

2.50

80/80

2.75

80/90

2.75 - 3.00

90/90

3.00 - 3.25 - 3.60

100/90

3.50 - 4.10

110/90

4.00 - 4.10 - 4.60

120/80

4.25 - 4.50 - 4.60

120/90

4.25 - 4.50

130/80

4.50 - 4.60 - 5.10

130/90 

4.50 - 4.60 - 5.10

140/80

4.50 - 5.10 - 5.50

140/90

5.10 - 5.50

Also note that the alphanumeric system is still used on certain Harley Davidson and other American custom bike tyres.

If you have one, you will find the corresponding dimensions in this table:

ALPHANUMERIC SIZE MARKINGS

METRIC SIZE MARKINGS

MH90

80/90

MJ90

90/90

MM90

100/90

MN90

110/90

MP85

110/90

MR90

120/90

MT90

130/90

MU85/MU90

140/90

MV85

150/80 150/90

You now have the essential information on motorcycle tyre sizes.

With our tool below you can easily find the best motorcycle tyre for your needs.

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