There are various forms of tyre size marking and these differ in order to differentiate between tyre types. The size markings should be treated the same as a part number on a vehicle, so the motorist should ensure that the tyres on his vehicle carry the precise markings indicated in the vehicle handbook or are an approved alternative fitment. Most size markings indicate the dimensions, the type of structure and the speed capability of a tyre.

SIZE MARKINGS

Typical markings as found on a tyre are illustrated:

1 – Tyre section width in mm.

2 – Section height to section width relation in %.

3 – Tyre construction (R = Radial).

4 – Rim diameter in inches.

5 – Maximum load capacity (load index):

91 = 615kgs per tyre

120/116 = 1400kgs per tyre – single mounted

1250kgs per tyre – dual mounted.

6 – Speed symbol (T = 190 km/h, P = 150 km/h).

7 – TUBELESS tyre.

8 – ECE Type approval mark and number and noise number.

9 – Location of treadwear Indicator.

10 – M+S (Mud & Snow) Winter capabilities.

11 – Tested and qualified for ‘Severe Snow Usage.

12 – Production date (week, year: decade 1990 -1999<).

13 – Department of Transportation compliance symbol.

14 – DOT manufacturer code.

15 – Country of manufacture.

16 – Trade name.

17 – Tyre construction details (DOT).

18 – Load and pressure marking (DOT).

19 – Tyre type (radial).

20 – Mark required by U.S.A. Consumer information regulations (Quality Grade).

For clarification purposes, the following comments may be helpful.

POSITION 1 – The tyre section width is the linear distance in millimetres between the outsides of the sidewalls of an inflated tyre excluding elevations due to labelling (markings), decorations, or protective bands or rims.

The LOAD INDEX denotes the maximum load a given tyre can carry at the maximum speed as indicated by the speed symbol.

Not valid for V, W, Y, Z.

Each tyre is designed to carry a specific load at a specific inflation pressure. For a given tyre size, inflation pressure determines how much load can be carried. Loading a tyre above the specified limit will result in overdeflection. Inflating a tyre above the specified limit will result in undrdeflection.

Only the rim contour is presented. The rim disk design and thickness is dependent on individual manufacturer and has an influence on the offset which is of great importance when it comes to tare conversions for example.

Many people think that deflection describes the bulge at the bottom of a loaded tyre. Actually, this bulge occurs as a result of deflection. Deflection really describes the change in the tyre’s radius when a normal load is applied. The radius is measured from the centre of the axle/hub to the ground.

Unloaded Radius is measured with no weight on the tyre. The tyre is mounted on a rim and inflated to working pressure.

Static Loaded Radius is measured with the weight of the vehicle and payload on the tyre. Static means that the tyre is standing still.

The deflection is the difference between the Unloaded Radius and Static Loaded Radius. This is the same distance that the axle lowers when the vehicle is fully loaded. Deflection is extremely important. Engineers design tyres to operate at a certain percentage of deflection. Operating with too much or too little deflection lowers tyre life.

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 relationship on 4-wheel drive vehicles using smaller tyres on the front axle as well as assuring dimensional compatibility for tyres used in dual mounted applications.

The usual rims for passenger and light truck “C” tyres are called drop centre rims due to the dropped centre needed to facilitate tyre mounting. A rim marking (e.g. 7 J x 15 H2) indicates the following:

At Manufacturer’s discretion

(Under the line)

H = one-sided round hump on outer shoulder (formerly H1)

H2 = double round hump

FH = flat hump on outer shoulder (formerly FHA1)

EH2 = extended hump (with extended hump on both sides)

Hump (H)

The exact measurements in relation to a specific rim, such as dimensions, tolerances and strength should be requested from the rim manufacturer.

Tyre companies throughout the world are members of regional tyre manufacturer’s associations (ETRTO for Europe), which establish tyre dimensions and tolerances, load carrying capacities and inflation pressures for the different tyre categories and sizes. The basic tyre and rim dimension nomenclature is explained below:

Outside Radius (OR) – The radius of an unloaded tyre, mounted on its recommended rim and inflated to recommended pressure.

Section Width (SW) – The width of the inflated tyre section, on the measuring rim, excluding any lettering or decoration.

Section Height (SH) – The distance from the bead seat to the outer tread contour of the inflated tyre – at centreline.

Static Loaded Radius (SLR) – The standing height from the road surface to the axle centre under nominal tyre load/inflation conditions.

Loaded Section Width (LSW) – The width of the loaded cross-section.

Aspect Ratio – The section height (SH) expressed as a percentage of the section width (SW).

A tyre is essentially a pneumatic body which links up the vehicle with the road surface. Its major functions can be summarised as follows:

• Load carrying capacity
• Cushioning ability
• Torque transmission
• Steering response
• Road holding ability

These performance characteristics must remain fairly stable during the whole life of the tyre. The tyre should also deliver a high mileage and remain safe throughout its entire life.

Beyond these basic criteria, there are a number of other, more sophisticated considerations a tyre must cope with in order to respond to today’s vehicle performance, such as:

• Low rolling resistance and fuel economy
• Low noise level
• High speed capacity and comfort.

The commercially available tyre is a composite product made up from rubber compounds and textile, steel or synthetic reinforcements. The major components are described below.

Tread – Provides primarily traction and wear and protects the carcass underneath.

Belt – Multiple, low angle, steel cord layers provide strength to the tyre, stabilise the tread and prevent penetrations into carcass.

Sidewall – Provides protection for the ply and withstands flexing and weathering.

Ply – The radial (90º) ply transmits all load, braking and steering forces between the wheel and the road and withstands the burst loads of the tyre under operating pressure.

Innerliner – A layer of rubber in tubeless tyres specially compounded to prevent loss of air.

Bead bundle – The steel bead bundle properly seats and seals the tyre on the rim and maintains it in position.

Apex – Rubber filler in the bead and lower sidewall area to provide progressive transition from the stiff bead area into the flexible sidewall.

Chafer – A layer of hard rubber that resists erosion of the bead zone by the rim flange.