Manufacturing

An analysis of technical enquiries received by BTMA members has confirmed the belief that many customers - dealers and users alike - are of the opinion that, when only two tyres are replaced on a car, they should be fitted to the front "axle". This was undoubtely the case a number of years ago when it was felt that the retention of vehicle control was more critical with front wheel deflations and, therefore, that the new, and thus less vulnerable tyres should be fitted to the front.

However, the current recommendation on the part of the tyre industry is, as a general rule: NEW TYRES TO THE REAR. This applies to both front and rear wheel drive cars.

Primarily, the justification is increased safety, particularly in the wet, where it has been demonstrated that, with the partly worn tyres fitted to the rear, their diminished water dispersal capability leads to a greater tendency towards oversteer and thus loss of control. Similarly, straight-line brakings in the wet can be adversely affected. It is also arguable that used tyres are more prone to punctures and since it has now been established that rear deflations are more likely to cause loss of control, this is another reason for fitting new tyres to the rear.

As a secondary benefit for the owners of low annual mileage front wheel drive cars, the switching of the partly worn tyres to the front, enabling the new tyres to be fitted too the rear, creates a cycle which helps to prevent their deterioration due to ageing/prolonged exposure. The rear tyres wear relatively slowly and leaving them in their original position for a prolonged period can result in the need to replace them before they are significantly worn.

Note: There are certain circumstances where the principle of NEW TYRES TO THE REAR does not or may not apply. For example:

• Where the front and rear tyre sizes are different.
• Where the two new tyres are of a lower speed rating.
• Where the characteristic rear concave wear pattern has developed to an extent that moving rear tyres to the front would affect the stability of the car.
• Where the "system" concept of directional front and asymmetric rear tyres is applicable. 
• With certain combinations of winter and summer tyres.
• On certain front wheel drive vehicles where it would result in significant differences in tread depth.

If in doubt please contact the relevant tyre manufacturer.

BTMA continues to be concerned both about advertisements that advocate the use of liquid sealants in tyres as a means of effecting a permanent seal or repair and the potential misuse of the type of sealants which are intended merely as a temporary "get you home" measure.

The appropriate British Standard, BS AU 159, specifies permanent, vulcanised repairs and the tyre must be removed from the rim and thoroughly examined for secondary damage not visible from an external inspection. Liquid sealants do not meet this requirement.

Since the preparation of the previous statement in December 1994, the industry recognises that liquid sealant development has taken place and that products available generally fall into two categories, pre-puncture sealants and post-puncture sealants.

Pre-puncture sealants are applied to the tyre, through the valve, at the time of fitting and are considered by the Industry to be potentially the most hazardous because in many cases they provide an instantaneous, reasonably effective seal following a penetration and thus the driver is unaware that the tyre has been punctured, possibly more than once. In these circumstances, there is an inherent risk of internal secondary damage and continued use of the tyre in such a condition may lead to serious tyre failure. BTMA does not approve of this type of sealant.

Post-puncture sealants are applied, through the valve, following a puncture and partially reflate the tyre, thereby addressing one of the concerns expressed with pre-puncture sealants. Whilst there is some danger in using a tyre which will have run in a underinflated or deflated condition and may also have sustained secondary damage, it is recognised that there are circumstances when such a sealant may be of value - for instance, to a lone driver physically unable to change a wheel. Nevertheless, the industry is concerned that drivers may disregard the sealant manufacturer's instructions or falsely believe that a permanent repair has been effected. In such circumstances a post-puncture sealant would effectively become a pre-puncture sealant.

Points to Note

• Some sealants use an inflammable gas as the propellant/pressurising agent. This is known to have caused incidents when accidentally ignited.
• The chemical composition of some sealants has been found to react with the material of the wheels, tyres or valves with which it comes into contact, causing degradation of these components.
• The physical composition of some if not all sealants, may impair the efficiency of the valve causing blockage or leakage as well as inhibiting routine pressure maintenance.
• All sealants are more or less difficult to remove from a tyre to enable inspection and possible repair. In some cases the tyre may be rendered economically unrepairable due to their presence.
• Liquid sealants may permeate the tyre casing, either through the liner or via the site of the damage, causing long-term effects on the integrity of the structure.
• Sealants are seen by the Industry as being ineffective in tube type tyres.

BTMA members consider there are inherent dangers in the use of liquid sealants which they believe constitute an unapproved modification to their products for which responsibility cannot be accepted. Nevertheless they recognise that post-puncture sealants may well serve a useful function if used correctly. To this end, they would not oppose their use in conjunction with strict legislative controls such as those employed for temporary use spares and also with the proviso that once a tyre has been punctured and post-puncture sealant applied, the tyre must be discarded.

3.Speed Rating of Replacement Car Tyres

In addition to ensuring that the load index is equal to or greater than that of the original tyres, British tyre manufacturers strongly recommend that replacement tyres should be of at least the same speed rating as those fitted when the vehicle was manufactures (see footnotes).

The main reasons for this recommendation are:

The original equipment tyre specification was selected to cater for the car's overall performance, not only in terms of its maximum speed but also, for example, its acceleration and handling.

Furthermore, they form an integral part of the design of its suspension, braking and steering systems. Vehicle handling, safety and refinement may be adversely affected if unsuitable tyres are fitted.

Tyres of an appropriate speed rating are essential if the car is driven to its full potential and, even for everyday UK motoring, they maintain the car's design characteristics in terms of safety and performance. · In some countries it is illegal to drive on tyres which are of a lower speed rating than the maximum speed capability of the vehicle.

The most common tyre speed ratings, referring to the maximum speed capability of the vehicle, are: -

km/h mph

• Speed Symbol 'Q' 160 100
• Speed Symbol 'R' 170 106
• Speed Symbol 'S' 180 113
• Speed Symbol 'T' 190 118
• Speed Symbol 'H' 210 130
• Speed Symbol 'V' 240 150 - see footnote (1)
• Speed Symbol 'W' 270 168 - see footnote (2)
• Speed Symbol 'Y' 300 186 - see footnote (3)
• Tyres marked 'ZR' only over 240 over 150 - see footnote (2)
• Tyres marked with both 'ZR' and speed symbol 'W' 270 168 - see footnote (2)
• Tyres marked with both 'ZR' and speed symbol 'Y' 300 186 - see footnote (3)
• Tyres marked with both 'ZR' and speed symbol 'Y' as part over 300 over 186 - see footnote (3) of the service description in brackets, e.g. '(95y)'
• The British Tyre Manufacturers Association, representing all the UK tyre manufacturers, urges motorists to place safety above all else when choosing replacement tyres.

  Footnotes: -

• Some Winter tyres have a lower speed capability than the car and, in these cases, the tyre maximum speed must be observed.
• mph) }

         (2). For speeds over 240 km/h (150 mph) } 

          consult tyre manufacturer for relevant loads and pressures

         (3). For speeds over 270 km/h (168 mph) }

4. Replacement Tyres

1.Tyre size markings across an axle must be identical. (Standard series tyres are equivalent to 80 series, e.g. 165R13 and 165/80R 13 tyres are compatible).

2. Tyre constructions across an axle must be the same i.e. cross-ply or radial.

3. The load index of replacement tyres should be equal to or greater than the load index of the manufacturer approved tyres for the vehicle.

4. It is recommended that the speed rating of replacement tyres is either equal to or greater than the speed rating of the manufacturer approved tyres for the vehicle. The mixing of tyres with different speed ratings is not recommended. 

5. The rolling circumference of replacement tyres should match those of the manufacturer approved tyres for the vehicle. *

6. Cold weather tyres should be fitted in complete vehicle sets. They should not be mixed with other tyres across an axle. *

7. It is not recommended that a new tyre (even of the same type) be fitted across an axle with a tyre that is well worn. *

8. In general when fitting two new tyres, they should be fitted to the rear irrespective of whether the vehicle is front or rear wheel drive.   

9. Verify that wheel width and diameter are compatible with the tyre size, and that the wheel offset and fixings are compatible with the vehicle.

10. Ensure that there is adequate clearance between the wheel / tyre and the vehicle bodywork, as well as all other mechanical components (brakes, suspension, steering parts etc.) in both static and dynamic vehicle conditions.

There are special requirements for the replacement of tyres on 4WD vehicles. Consult the vehicle manufacturer/handbook for recommendations.

* Cold weather (winter) tyres may be of a lower speed rating provided the driving speed does not exceed the rating relative to their speed symbol. If in doubt as to the suitability of existing or proposed tyre fitments, please contact the tyre manufacturer concerned.