Tyres

Details

Category: Asides

Last Updated on Thursday, 15 September 2011 19:19

Written by Site Admin

Hits: 2051

Tyre sidewall markings explained
Understanding the markings on the sidewalls of tyres (tires) will help you to choose the right tyres for your car. Below is a guide to those sidewall with further explanation after the graphic.

Tyre sidewall markings

Tyre Width

Width of the tyre measured in millimetres from sidewall to sidewall. The tyre in the graphic above is 165 millimeters wide.

Aspect Ratio

Ratio of the height of the tyre's cross-section to its width. 70 means that the height of the sidewall is equal to 70% of the tyre's section width.

Wheel Diameter

The width of the wheel from one end to the other. The diameter of the wheel in the graphic above is 17 inches.

Construction

This tells you how the tyre was put together. The "R" stands for radial, which means that the body ply cords. These cords are layers of fabric that make up the body of the tyre, and run radially across the tyre from bead to bead. A "B" indicates the tyre is of bias construction, meaning that the body ply cords run diagonally across the tyre from bead to bead, with the ply layers alternating in direction to reinforce one another.

DOT

Means the tyre is compliant with all applicable safety standards established by the U.S. Department of Transportation (DOT). Adjacent to this is a tyre identification or serial number; a combination of numbers and letters with up to 12 digits.

Tyre Type

American markings define the proper use of the tyre. "P" means this is a passenger car tyre. "LT" means it is for a light truck.

UTQG

Stands for Uniform Tyre Quality Grading, a quality rating system developed by the US Department of Transportation (DOT).

Load Index and Speed Ratings

The majority of tyres carry coded markings on them, which correspond to their load carrying and maximum speed capabilities.

For example: 79T

79= Index of maximum load carrying capacity per tyre, in this case equates to 437kg.

T= Symbol which equates to a speed rating of 190km/h (approximately 118mph)

The load index is a numerical code, which corresponds to the maximum load a tyre can carry at the speed indicated by its speed symbol, under specific service conditions. For specific load index details see below.

The speed rating symbol indicates the speed at which the tyre can carry the load corresponding with its load index. For specific speed rating details see below.

Tyre age - Important safety tips

There is some evidence to suggest that aged tyres have an increased likelihood of failure due to exposure to the environment

How do I tell the age of a tyre?

All tyres are produced with a serial Tyre Identification Number (or serial TIN) that shows the date of manufacture of a tyre. The last three digits (for tyres made pre 2000) or four digits (for post 2000 tyres) of the serial TIN indicate the week and year that the tyre was made.  Also a tyre made in the 1990's can be distinguished from a tyre made in the 1980's due to a triangular indentation after the last number which is not present on 1980's tyres. 

This is a subject of much debate within the tyre industry and no tyre expert can tell exactly how long a tyre will last. However, many tyre companies warrant their tyres against manufacturing and material defects for five years from the date of manufacture. Based on their understanding a number of vehicle manufacturers are now advising against the use of tyres that are more than six years old due to the effects of ageing.

There are three main mechanisms of tyre ageing. The first involves rubber becoming more brittle. Sulphur is used to link rubber molecules together during vulcanisation with the application of heat and pressure, giving the rubber its useful elastic properties and strength. As the tyre absorbs energy in the form of light, heat or movement the tyre continues to vulcanise. This ongoing vulcanisation causes the rubber to become stiffer and more brittle. 

The second mechanism of tyre ageing is oxidation involving oxygen and ozone from the air compromising the strength and elasticity of the rubber and the integrity of the rubber to steel bond. Basically heat and oxygen cause cross linking between polymer chains (causing the rubber to harden) and scission of polymer chains (leading to reduced elasticity).

Thirdly, breakdown of the rubber to steel-belt bond will occur due to water permeating through a tyre and bonding with the brass plate coating on steel belts.  This causes the steel to rubber bond to weaken leading to reduced tyre strength and reduced heat resistance. If compressed air used for inflation is not completely dry, tyre strength will be affected over time. Even unused tyres will become more brittle, weaker and less elastic with exposure to water, air, heat and sunlight. 

Warning signs

Regardless of their age tyres should be replaced if they show significant crazing or cracking in the tread grooves or sidewall and or bulging of the tread face or sidewall.  All tyres, especially unused spare tyres, should be inspected periodically to determine their suitability for service. If there is any question about a tyre's suitability please consult your local tyre centre expert.

Other Markings

M+S - Identifies Mud and Snow tyres

E-Marks - Tyres for sale in the European Community must carry an E -Mark in accordance with ECE Reg 30 - eg E4 027550

Air Pressure

The main function of a tyre is to carry the load of the car. Therefore, it is vital that correct air pressure is maintained. Over-inflation will cause rapid and irregular wear. Under-inflation will damage the tyre structure leading to possible accidents. Improper air pressure will not allow the tyre to function within its design. For best performance maintain the air pressure recommended by the vehicle manufacturer and/or the tyre manufacturer. Keeping the tyre at the correct pressure will also keep money in your wallet you'd otherwise be spending on fuel and new tyres. Of course if you're into drifting or doing doughnuts in the Co-op car park this may not apply to you

Wheel Alignment and Balancing

A major cause of rapid tread wear is improper wheel alignment and balancing. Abnormal noise, vibration and harshness of steering is a signal that alignment and balancing may be improper.

Tyre Law (UK)

1. Tyres on cars, light vans (not exceeding gross weight) and light trailers must have a tread depth of at least 1.6mm across the central three-quarters of the breadth and in a continuous band around the entire circumference of the tyre.
2. Tyres (including the spare tyre) must be suitable (ie of the correct specification and size) for the use to which the vehicle is being put and must be inflated to the manufacturers' recommended pressures.
3. Tyres of different types must not be fitted to opposite wheels of the vehicle (for example, radial-ply tyres must not be fitted to a wheel on the same axle as wheels already fitted with cross-ply tyres and vice versa, and a two-axle vehicle must not have radial ply tyres on the front axle if cross ply tyres are fitted to the rear axle)
4. No tyre must have a break in its fabric or a cut deep enough to reach the body cords. No cut must be more than 25mm or 10% of the tye's section width in length, whichever is greater.
5. There must be no lump, bulge or tear caused by separation or partial fracture of its structure, neither must any portion of the ply or cord structure be exposed.

Anything to add? email This email address is being protected from spambots. You need JavaScript enabled to view it.

Share this post

• < Prev