Your tyres are very good at giving you an indication of the condition of your front and rear suspension and steering systems. They can also tell you if you’ve been bad and haven’t kept an eye on their pressures as well! Know-all little buggers they are! First we will look at front tyres and what their wear patterns can mean.

Tyres that show wear on both edges indicate either under inflation (low air pressure, anything below 30psi could cause this) or overloading of the vehicle. Front wheel drives in particular suffer from this type of wear, most likely because of the weight of the engine and gearbox being directly over the front tyres. Taking cold tyre pressures to 34psi on a front wheel drive can help to reduce this wear.

Abnormal wear down the centre of the tread area indicates over inflation or too high air pressure in the tyre. This causes the tread to take on a concave shape and reduces the amount of tyre in contact with the road and rapidly wears out the part of the tread that is in contact with the road surface.

Tyres that are worn abnormally on either side of their tread are usually the product of bad wheel alignment however there is a number of other causes to consider. Sagged coil springs on Macpherson Strut suspension can alter the camber angle and cause this type of wear. Worn control arm bushes, excessive play in wheel bearings, overloading on Independant Rear Suspension equipped vehicles (namely VR/VS Commodores – there is a bush kit for the rear arms available to correct the camber angle on these cars) and bent axle housing or stub axle.

If you find you have a tyre (or tyres) with irregular wear similar to the picture then chances are you have worn shock absorbers. As the car is bouncing over bumps in the road the downward pressure on the tyre is changing causing these wear patterns. Wheel balance, play in wheel bearings and worn suspension joints can play a part in this also. Four-wheel-drives with worn shocks seem to be prone to this type of wear and early detection is important if you don’t want to shell out for a new set of tyres and shocks in one hit!

The last one that I have pictures for is feathering of the edges of the tread which is generally caused by excessive toe-in or toe-out and provided the suspension and steering are in good condition, it can be corrected with a wheel alignment.

 One other issue we see a lot of these days is ‘tread seperation’ where a portion of the tread area becomes raised like a ‘bubble’ in the tyre. It will be felt in the car as a vibration and at low speed you can feel when the tyre is lifting as the raised area contacts the road. I believe this can be due to many factors and the age of the tyre and the load it has to carry has a lot to do with it. Tyres fitted to caravans and trailers for instance seem to be prone to tread seperation. One reason I have heard for this is because they are left in the one spot for a lengthy period of time, this causes the steel belts to flatten out to a certain degree and when they are used they are usually carrying a fair load which places even more stress on the tyre. If someone has a better explanation for this, please let me know!

 The most important things to remember with tyres is to (obviously!) check your pressures regularly, have the tyres rotated at no more than 10,000km intervals (I do mine at 5000km’s and I get great mileage out of them) and keep a close eye on how they are wearing so you can identify any problems before you have to replace your tyres prematurely.

Take care and be safe on the roads.

Craig

  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