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Introduction to Tyre Safety:
The failure to attend to tyre safety is a vital factor in thousands of road accidents every year. Research by the CSIR indicates that nearly 20% of accidents involving minibuses have tyre failure as a contributing factor. It won't help if you have the best brakes on the market, but your tyres are worn. When braking, the idea is to have sufficient friction between the road surface and tyre to bring the vehicle to a standstill. If the tyres are worn there will not be enough friction and the tyre will slide over the road surface, not stopping the vehicle. This is also true for handling and steer ability.
Basic rules about tyre safety:
Always spend time to ensure you have correct, safe tyres on your vehicle:
- Tyres should always be replaced with the same size designation as recommended by the vehicle or tyre manufacturer.
- Tyre brand, size and tread pattern must be the same on each axle.
- All four tyres should be of the same size, speed rating and construction (radial or cross ply).
- Guard against used tyre imports, many of which are beyond retreading but are retreaded and sold illicitly. Similarly watch out for counterfeit tyres - illicit copies of respected brands. The advice is to always look for the SABS stamp of approval.
- When two radial tyres are used with two cross ply, put the radials on the rear axle. In some cases (especially commercial vehicles) the manufacturer might recommend different-sized tyres for the front and rear axles.
- Never assume that the tyres on your vehicle are correct, even if you have newly purchased it. Unless you bought new from an authorised dealer your vehicle may already be fitted with potentially lethal tyres.
Over inflation/ Under Inflation
- Over and under-inflation reduces tyre to road contact and shortens tyre life.
- The tyre responds in the same manner to under inflation as to overloading.
- The same applies to over inflation / under loading.
Over inflation / under loading: Reduces cushioning power of tyre. Tyre is more susceptible to impact, penetrations and abrasion. Reduced road-tyre contact. Negatively affects the handling characteristics of the vehicle. (Excessive centre wear.)
Under inflation / Overloading: The biggest single cause of "burst" tyres. Under inflation cases excessive flexing of the tyre sidewall which leads to overheating and ultimately, casing break-up and tread separation. Reduced tyre-road contact leads to poor handling and faster wear. (Excessive shoulder wear.)
Tyre Pressure
The importance of the correct pressure cannot be over-emphasised.
- Check tyre pressure, including the spare, once a week or before undertaking a long journey – or before 10 km have been driven.
- Check tyre pressure early morning (low ambient temperature)
- Always use a reliable pressure gauge
- Only use tyre sizes recommended by the manufacturers at the recommended inflation pressure
- If the pressure is too low it affects the tyre in a number of ways.
As speed rises, excessive contact patch deformation leads to a wave being formed in the tread, which generates excessive heat, causing structural damage or even tyre failure. The extra flexibility will affect steering behaviour, directional stability, durability and rolling resistance. Even occasional low pressure driving, or periodic vehicle overloading, may cause damage that only shows up much later as a blowout.
Inspecting the tyres / Driver Checklist for tyres
Tyres should be inspected often, and the following list highlights some of the faults to focus on:
- Check tyres regularly for punctures, penetrations, cuts and bulges.
- Cuts in the tyre could permit damp to reach the casing plies. This is harmful for both textile and steel casings and will affect safety and tyre life.
- Wheel alignment - misaligned wheels will lead to excessive tyre wear.
- A worn steering mechanism, ball joints and wheel bearings will also reduce tyre life.
- Consult your vehicle manual for the proper size and speed rating. Some tyres are now marked with letters to indicate their speed ratings. Tyre speed ratings do not imply that vehicles can be driven safely at the maximum speed for which the tyre is rated, particularly under adverse road and weather conditions, or if the vehicle has unusual characteristics.
- One-sided wear. This takes various forms. A regular smooth band of wear all around the tyre on the inside or the outside of the tread is a sign of incorrect camber. Too much toe-in causes irregular one-sided wear.
- Tread-centre wear. Regular wear of this kind is normally a sign of high tyre pressure. Driving fast for long distances may cause this on low-profile tyres, because traveling at 120 km/h wears a tyre out twice as fast as travelling at 70 km/h.
- Inner- and outer-edge wear. If both inner and outer edges are worn, it usually implies that the tyres have been run at too low a pressure at normal speeds.
- Irregular bald spotting. Known as cupping, this is usually caused by worn shock absorbers, worn suspension bushes, or even loose wheel bearings.
- Missing valve caps should be replaced, since they are there to prevent dirt from clogging the valves, which could cause a loss of air pressure.
- Torque wheel studs /nuts to correct setting when mounting new tyres - check for loose or missing wheel nuts.
- Check tread depth on all tyres and replace well before they reach regulatory minimum depth of 1.00 mm to reduce the risk of aquaplaning on wet roads.
General Advice on Tyre Maintenance
- Tyres, sizes speed / load ratings must conform to the specifications in the Vehicle Owner’s Manual.
- Check tyres and rims for any accidental damage after impacting with potholes/curb stones or other obstacles in the road.
- Tyres, brakes, shock absorbers and rims should always be checked when servicing the vehicle.
- Vehicle wheel alignment should be tested at regular intervals to obtain even tread wear and maximum service life.
- No matter how good your car, or its tyres, this is not justification for breaking speed limits. If a tyre burst, do not apply the brakes; rather use the momentum and gears to slow down the vehicle. A sudden change in direction or braking will result in loss of control over the vehicle. Rather lose one rim than your car and probably your life!
- Tyre rotation is a very contentious area, with some companies recommending it, other companies saying nothing, and BMW for one advising against it. Rotating the tyres regularly evens out the wear, but eventually all the tyres will have to be replaced at the same time, which is expensive.
Conclusion:
Always refer to the vehicle owner’s manual for all tyre pressure specifications and other related information. Never forget that four tyres are the most important components between you and the road. If you suspect any damage to the tyres - consult a tyre expert!
Also view the following sections:
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