Run-flat tyres are becoming increasingly commonplace on the roads of the UK as more car manufacturers fit these advanced tyres as standard equipment or as an optional extra. However, there are still some misconceptions surrounding the fitment and use of this new type of tyre.
Run-flat tyres, or self supporting run-flat tyres to give them their correct title, should not be fitted to a vehicle not designed to accept them or without a pressure monitoring system. Equally they should not be mixed with conventional tyres as the handling and performance characteristics of the two types of tyres are quite different. Mixing of different brands of SST tyres may also not be advisable and the motorist should check with the manufacturer of the tyre or vehicle for advice. It is recommended that the same make and type of SST tyre be fitted across the same axle.
Distance and speed
SST tyres are designed to provide a run-flat capability following deflation. The range when running flat is dependent on the conditions of use, vehicle load, speeds and driving conditions. Each manufacturer of SST tyres may well have a slightly different recommendation related to speed and distance travelled in a deflated condition. However, whilst a universal standard is yet to be defined, the industry feeling is that this is likely to be 80 km (50 miles) distance at speeds of up 80 kph (50mph) for cars in normal use.
Repairing run-flat tyres
Run-flat tyres have specially reinforced sidewalls enabling them to perform whilst deflated for a limited distance and speed as already mentioned. In the course of running in a deflated or significantly under inflated condition, the tyre structure is subjected to high stresses and therefore may become weakened and permanently damaged rendering the tyre both unsuitable and unsafe to repair.
When a standard tyre is run in a deflated condition there are visible signs which indicate that damage to the tyre’s structure has occurred. Because of the reinforced sidewall structure of a run-flat tyre these telltale signs are likely to be masked and not visible even if they are present. Even a fully qualified tyre repairer may not be able to detect run flat damage to a tyre. For these reasons alone and in the interests of safety the British Rubber Manufacturers’ Association, the body representing the major tyre manufacturing companies, does not recommend repairs to a run flat tyre. The advice from individual manufacturers of run flat tyres may differ however and motorists are advised to check with the tyre company for any different advice.
Retreading of run flat tyres
The responsibility for retreading any tyre lies with the retreading company not the tyre manufacturer. To date there is very limited experience of retreading this new type of tyre but in principle there is no reason why a sound run flat tyre may not be retreaded. Given the unique structure of the run flat tyre, as already mentioned, they should not be mixed with traditional tyres and so to avoid this risk a retreaded run flat tyre must be marked and identified by the retreader as a run flat tyre.