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  In the market for a new set of tyres? Understanding what goes into a tyre and what the sidewall markings mean will help you make an informed decision on which tyre is right for you and your vehicle.

  Tyres are made up of several different components;

• The bead - the bead is a high-strength steel cable coated with rubber. It’s job is to ensure that the tyre stays seated on the wheel rim and handle the forces of the tyre-fitting machines when the tyre is installed.
• The body - the body of the tyre is made up of several layers of different fabric called ‘plies’. The most common fabric being polyester cord. The cords in a radial tyre run perpendicular to the tread and are coated with rubber to help them bond with the other components and seal in the air. A tyres strength is often decribed by the number of ‘plies’ it has, with most car tyres having two plies.
• Steel Belts - steel belted radials have belts made from steel that are used to reinforce the area under the tread. These belts help to increase puncture resistance and help the tyre maintain a flat contact patch with the road.
• Cap Plies - an extra couple of layers of polyester fabric used to hold everything in place. Mostly used on tyres with a high speed rating.
• The Sidewall - the sidewall provides lateral stability for the tyre, protects the body plies and helps keep the air from escaping.
• The tread - the tread area is made from a mixture of many different kinds of natural and synthetic rubber. The tread area and sidewall is extruded and cut to length. The tyre is then put into a curing machine and the tread pattern and sidewall markings are molded in.

 The sidewall markings cover specific information regarding;

• Tyre Type - ‘P’ for passenger car, ‘LT’ for light truck and ‘T’ for temporary or spare tyre are some of the markings that are used.
• Tyre Width - measured in millimetres from one sidewall to the other when fitted on the intended rim size.
• Aspect Ratio - measurement of the tyre height from the bead to the top of the tread, expressed as a percentage of the tyre width. e.g. 205/75 indicates a width of 205mm and a height of 75% of 205mm or 153.75mm. The lower the aspect ratio the wider the tyre is in relation to it’s height. High performance tyres generally have a lower aspect ratio, providing increased lateral stability. Tyres with shorter, stiffer sidewalls resist cornering forces better.
• Tyre Construction - the most common tyre these days is the radial, indicated by an ‘R’ usually after the size designation.
• Rim Diameter - the size of the rim, in inches, that the tyre is designed for.

  Another system used for grading a tyre is the Uniform Tyre Quality Grading (UTQG) system. Three ‘ratings’ are included in this system;

• Tread Wear - the higher the number the longer the tread should last. The tyres are tested in a controlled enviroment on a government test track.
• Traction - a rating based on the tyre ability to stop a car on wet concrete and ashpalt. The ratings go from AA being the best, down to C being the tyre with the least grip levels in these conditions.
• Temperature - this is a measure of how well the tyre handles heat build-up and how well it dissipates heat. Once again A,B and C ratings are used.

  The Service description of the tyre covers two criteria;

• Load Rating - a number that correlates to the maximum rated load for that tyre. A higher number indicates a higher load carrying capacity. The rating ‘105′ for instance, corresponds to a load carrying capacity of 2039 pounds. A seperate note on the tyre indicates the load rating at a given inflation pressure.
• Speed Rating - a letter indicating the maximum allowable speed for that tyre as long as the load is within specifications. An example is the letter ‘S’ that indicates the tyre is suitable for speeds up to 180 km/hr.

  Hopefully this information will help you decipher the tyre ‘jargon’ next time you are on the hunt for new rubber. For specific information on what tyre size, speed rating etc your vehicle requires, be sure to check the tyre placade or owners handbook. In the next post we will look at tyre problems and their possible causes.

Until then take care and be safe on the roads.

Craig