Tyre Size Designation

Tyre Dimensions – Definitions



Overall Diameter (OD):
Inflated diameter of the tyre under reference tyre pressure, mounted on its recommended rim.

Overall Width (OW):
Inflated width of the tyre under reference tyre pressure – including elevations due to lettering, decorations and protective ribs.

Section Width (SW):
Inflated width of the tyre under reference tyre pressure – excluding elevations due to lettering, decorations and protective ribs.

Section Height (SH):
The distance from the bead seat to the outer tread contour of the inflated tyre at center line.
Tread Width:
The distance between the extreme edges of the tread, as measured at the surface of the tread.

Aspect Ratio:
The ratio of a tyre’s section height (H) to its section width (W) in percentage.

Aspect Ratio (AR)  =	Section Height ( SH ) X 100
	Section Width ( SW )

Rim Width:
The liner distance between the flanges of the rim.

Rim Diameter:
Nominal outer diameter of bead seat area of the rim. (It is a size code figure for reference purpose only)

Static Loaded Radius (SLR):
It is the minimum radius acquired by the tyre under reference load and pressure at static condition.

Rolling Circumference:
The distance covered by one revolution of the tyre ( in mm ). The rolling circumference of a tyre is important for establishing the correct tyre slippage.

Tyre Type Nomenclature

• Conventional: The cross – section shape of the tyre has an aspect ratio ( ratio of section height to section width) in the rage of 0.95 Conventional tyres are also referred to as narrow base tyres. Tyres of size designation such as 18.00-25 , 24.00-35 ; 24.00-49 etc belong to this category.
• Wide Base: The cross – section shape of this tyre has an aspect ratio in the range of 0.83. Tyre size designations such as 20.5-25; 23.5-25; 26.5-29 etc belongs to this category.

Design Tread – Depth