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Car Tyres and Your Safety |
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Types of Tyres | Don't Mix Tyres | Keep up The Pressure | Inspection and
Maintenance |
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| TYPES OF TYRES
In general there are 2 types of tyres. 1. Radial, 2. Diagonal (cross-) ply. Radial tyres may either be steel or textile braced, and are identified by the letter R in the size marking and usually the word "Radial". Except in the case of temporary use spares it is illegal in the United Kingdom, and dangerous, to mix radial and cross-ply tyres on the same axle, or to have radial tyres on the front axle and cross-ply on the rear. Recommended tyre inflation pressures for your vehicle can be found either in the vehicle or tyre manufactures handbooks. Correct pressures are related to loads, speeds and vehicle handling and are vital for even breaking, maximum grip and good tyre life. Prolonged under-inflation causes excessive flexing, deterioration of the casing, and rapid wear of the tread edges. Over-inflation results in an uncomfortable ride, a reduced tyre area of contact with the road surface and accelerated wear on the centre of the tread. Inflation must be checked at least once a fortnight and should only be checked when the tyre is cold, since there is an increase in pressure when the tyre has warmed up after being run. A reliable pressure gauge should be used. Tyres should be examined regularly, removing stones and other objects embedded in the tread. If the tyre has lumps or bulges it must be examined by a tyre specialist since they could indicate internal damage. Wipe away any oil or grease with a suitable diluted detergent. Tyre treads are designed to give good wet grip but the road surface condition plays a major part in tyre to road adhesion. In general, wet grip decreases as tyre tread patterns wear down or as the depth of surface water increases. Motorists should take this into consideration and reduce speed when it is wet. Most car tyres have tread wear indicators in the tread grooves which show when the tyre is worn to 1.6mm remaining tread. The appearance of these indicators, level with the tread surface, should be taken as a sign that the tyre is ready for replacement. The legal limit of tread depth for car tyres in the United Kingdom is at least 1.6mm throughout a continuos band comprising the central three quarters of the breadth of tread and round the entire outer circumference of the tyre. Both radial and diagonal (cross-) ply tyres have markings to indicate their load and speed capabilities. It is important that tyres are suitable for the maximum speed at which the vehicle will be driven. In some overseas countries tyres are required by law to have a speed capability to match the vehicles maximum speed.
With certain exceptions, regulations in the United Kingdom require tyre speeds to be shown in accordance with an alphabetical list of symbols indicating speeds at 10km/h intervals. The relevant speed symbol is positioned near the size designation in conjunction with a load index number, e.g. 155R12 76S, where ‘76’ is the load index and S the speed symbol. Details of speed and symbols are shown in tables 1 and 2. It is important that the tyre load capacity is adequate. Under revised UK regulations introduced in 1988, retreaded tyres will be speed rated in the same way as new tyres. Prior to the introduction of these regulations, retreaded tyres are required to perform a minimum of 70mph. Tyres branded ‘Regraded Quality D.A.’ are capable of the same speeds as new tyres of the same type and size. TYRE SPEED MARKING TABLE
TYRE LOAD INDICES AND RELATED MAXIMUM LOADS
Inner tubes of the correct size and type may be used if correctly fitted. It must, however, be emphasised that the result of fitting tubes in tubeless tyres is that, in the event of a puncture, the assembly will behave in exactly the same way as a tube tyre and tube assembly – i.e. it is likely to lose pressure more rapidly than a tubeless tyre assembly. Where tubes of certain low profile tubeless tyres are not available, an alternative tube size should not be fitted. Tyres which are marked ‘Tube Type’ or are not marked ‘Tubeless’ must be fitted with an inner tube. Some vehicle manufactures recommend that only tubed type tyres are fitted to their vehicles. It is important that the advice in the owner’s handbook is followed. A new tube should be fitted when the tyre is being replaced. There are severe operating restrictions on temporary use spares. Failure to observe the advice given in the vehicle owner’s handbook could have the most serious consequences. A new valve should be fitted when replacing tubeless tyres. When checking or adjusting inflation pressure always ensure that the valve is not leaking. A valve cap, preferably of the sealing type, should be fitted. Caravan and car trailer tyres may be either radial or diagonal (cross-) ply irrespective of which type is fitted to the towing vehicle. Both tyres on a caravan or trailer axle must be of the same type. Caravan and trailer tyres may require higher inflation pressures than are required for the same tyres on a car. It is therefore essential that the correct pressure be determined and maintained. When towing it is recommended that the pressures in the rear tyres on the towing car are increased by about 4-6 psi to give improved stability. Tyres can deteriorate with age which shows as cracking of the tread and sidewall rubber, sometimes accompanied by carcass deformation. Cracking is usually an indication that tyres have been in service a long time, but rubber deterioration may be brought about by poor storage conditions. If cracking is severe, the tyre must be replaced. Tyres fitted to vehicles such as caravans and boat trailers which are parked for long periods, particularly in coastal regions, will tend to age and crack more quickly than those which are used and run frequently. In such circumstances it is important to jack the weight off the tyres and to cover them so that they are protected from direct light. Repairs to car tyres and tubes must be carried out in accordance to the current British standard AU 159 and should, therefore, be entrusted to a specialist. All punctured or damaged tyres must be removed from the wheel for internal and external examination to ensure there is no hidden damage which could cause a later failure. In order to avoid such a hazard, neither externally applied plug type repairs nor liquid sealants are recommended and tyre manufactures cannot be responsible for problems resulting from their use. If studded winter tyres are used, they must be fitted in full sets to maintain good handling. Speeds should not exceed 130km/h (80mph). The tyres should be removed as soon as conditions permit, because in ice-and snow-free conditions road-holding can be reduced. It is essential that the wheel size is and approved fittment for the tyre and vehicle concerned. Tyres must not be used on damaged, distorted or modified wheels since this could result in tyre damage and deflation and possible loss of control of the vehicle. These operations must be entrusted only to a tyre specialist who has the necessary equipment and expertise. Inexpert fitting can result in injury or damaged tyres and wheels. Wheels should be balanced after tyres are fitted or replaced. Driving over pot holes, kerbs or other obstructions, even at low speeds, can result in the weakening or fracture of the casing structure. It is dangerous to re-inflate a tyre which has been run flat or seriously under-inflated and such tyres must be removed for complete examination by a tyre specialist. Tyre manufactures cannot accept responsibility for problems arising from modifications to their products, or from the use of sealants which they have not approved. |
