Finding the correct tyre

Tyre Advice from Tyre Express Hove Ltd

• What are asymmetric tyres?

  An asymmetric tyre has different tread patterns on its inner and outer sides, giving it a distinct performance advantage over a tyre with a symmetrical or directional tread.

  • The tread pattern on the inner side of a typical asymmetric tyre has more grooves giving better grip on wet roads
  • The outer side has a higher plain surface area, the benefit of which becomes apparent when cornering at speed on a dry road

  Unlike a directional tyre, an asymmetric tyre can be used in any position on the vehicle. It is very important however, to follow such wording as ‘inside’ and ‘outside’ on the tyre’s sidewalls to ensure that the tyre is fitted ‘the right way round’. As with directional tyres, incorrect fitment can seriously impair performance. And, again if you do use a wheel with an incorrectly fitted asymmetric tyre, it should be regarded strictly as a ‘Temporary spare.

  • A ‘Temporary spare’ is limited to 50mph (80kph)
  • A car whose tyres are not fitted in accordance with the instructions on the sidewall will fail its MOT
    Some tyres are both asymmetric and directional. These tyres carry markings on the sidewall which a) indicate the correct direction of rotation and b) identify the inner and outward-facing sides. Failure to observe these instructions will impair tyre performance. The above recommendations will also apply.

• What are directional tyres?

  Designing a tyre tread which will operate equally well rotating in either direction inevitably involves compromises which limit ultimate performance in some respects.

  Directional tyres, on the other hand, are designed to rotate in one direction only (indicated by a conspicuous arrow pointing in the direction of rotation on the sidewall along with the word “rotation”); so performance – wet grip, directional stability and handling – far from being compromised, is optimised.

  Obviously, a directional tyre can deliver this improved performance only if it is correctly fitted to ensure that it rotates in the direction indicated on the sidewall.

  What happens if you are unfortunate enough to have a puncture and your spare wheel is fitted with a directional tyre designed to rotate in the opposite direction to the tyre which it replaces? You are recommended to treat the spare wheel as a ‘Temporary spare’, drive at reduced speed and have the tyre correctly refitted as soon as possible.

• What does the XL stand for in some tyre size designations?

  XL is the “Extra Load” load index. It means that the tyre is developed for a higher maximum load.

  RunOnFlat

  A tyre designed with reinforced sidewalls inside the tyre to keep the car on the move for approx. 50 miles at a reduced max speed of 50mph.

  The law and tyres

  It is illegal to have a tyre on the road if:

  • Tyre is unsuitable for the use to which the vehicle or trailer is being put or not suitable relative to the other type of tyre fitted.
  • It is not properly inflated.
  • Any tyre fitted to the vehicle has a cut or break in the rubber in excess of 25mm or 10% of the section width in length which exposes the casing cords.
  • Any tyre fitted to the vehicle has a lump or bulge caused by separation or partial failure of its structure.
  • Any tyre fitted to the vehicle has any proportion of the ply or cord exposed.
  • Tyres of different structure (i.e. diagonal (cross) ply or radial ply or belted bias) are fitted on the same axle.
  • Radial tyres are fitted on the front axle with diagonal (cross) ply tyres on the rear axle.
  • The grooves of the tread pattern of every tyre fitted to the wheels of the vehicle do not have a depth of at least 1.6mm throughout a continuous band comprising the central three quarters of the breadth of the tread and around the entire outer circumference of the tyre.
  • A temporary spare tyre is being used outside its stated limitations.

• Q: What does the coding and numbering found on the side of tyres mean?

  A: The letters and numbers on the sidewall of a tyre look complicated. However, they explain the exact specification of the tyre you have.

  For example:

  205/50 R15 91V E4
  205 – width of the tyre in millimetres
  50 – known as the aspect ratio, this is the height of the tyre sidewall as a percentage of the width. In this case 50% of 205mm

  R – radial construction
  15 – diameter of the tyre’s inner rim in inches
  91 – this is the load capacity of the tyre. The number is part of an index of corresponding weights, in this case 615kg

  V – all tyres are given a speed rating. This indicates the maximum speed for this tyre at full load. In this case it is 149.1mph. Speed ratings range from S (max 111.8mph) to ZR (150mph+)
  E4 – this is the ECE type approval mark, which demonstrates the tyre has been tested as high quality by the European Regulatory Authorities.