HOW TO READ A TYRE?

There are essentially  two types of size marking used for motorcycle tyres:

1.Imperial
2.Millimetric

All of these marking systems describe the width (W), height (H) and rim diameter (dr) of a tyre size. The tyre aspect ratio drawing (on the Wheels and Rims) shows where these dimensions are measured from and the overall diameter (D).

The height of the tyre and hence its overall diameter (D) is controlled by the Aspect ratio (Ar).

Tyre sizes for all motorcycle tyres are expressed as:

Width/Aspect Ratio – W/R Construction or Speed Symbol Rim diameter – dr

For older Imperial sizes the Aspect ratio is 100% and not shown.

For Example:

3.50 x 18 = 3.50″ sectional width, 3.5″ sectional height — 18″ rim diameter

Perhaps the most easily understood size markings are the modern millimetric type.

For Example:

110 mm sectional width/110 x 90% = 99 mm sectional height — 18″ rim diameter

The effect of changing the aspect ratio is to change the overall diameter of the tyre:

120/110 – 18 D = 120 X 100% X 2 + (25.4 X 18) = 697.2mm

120/90 – 18 D = 120 X 90% X 2 + (24.4 X 18) = 673.2mm

120/80 – 18D = 120 X 80% X 2 + (25.4 X 18) = 649.2mm

Each of these rims is 120mm wide and fitted on an 18 ″ rim diameter, but as you can see there is a significant difference in overall diameters.