BRMA Statement on Self Supporting Run Flat Tyres
Self Supporting Run Flat Tyres (SSTs) SST tyres are designed to provide a run on period following a deflation. The range of SST tyres when running flat is dependent on the conditions of use (vehicle load, vehicle speeds and driving conditions). The tyre or vehicle manufacturer should be contacted for details about the potential run flat range. Whilst at present a universal standard has yet to be defined regarding the minimum range of run flat tyres in run flat conditions, the industry feeling is that this is likely to be 80km (50 miles) at speeds of up to 80 km/h (50mph) for motor cars in normal use. Repairing of SST tyres SST tyres have specially reinforced sidewalls to enable them to perform, when deflated, for a limited distance and at a restricted speed. The quoted distance and speed permitted under these conditions may vary according to the tyre or vehicle manufacturer concerned. In the course of running in a totally deflated or significantly under-inflated condition the tyre structure is subjected to high stresses and hence may become weakened and permanently damaged rendering the tyre both unsuitable and unsafe to repair. When a standard tyre is run deflated (or significantly under-inflated) there are visible symptoms, which indicate that damage to the tyre’s structure has occurred. However because of the reinforced sidewall structure of an SST tyre these symptoms associated with use in a deflated or partially inflated condition are likely to be masked and not be visible even if they are present. Even a fully qualified tyre repairer may not be able to detect run flat damage on SST tyres.
For these reasons alone, and in the interests of safety, BRMA does not recommend the repairing of SST tyres. Individual tyre manufacturers’ views may, however, differ from the above regarding their own brand of SST tyres and specific enquiries should be directed to the relevant manufacturer. Retreading of SST tyres The responsibility for retreading any tyre lies with the Retread manufacturer and not with the original tyre manufacturer. To date there is very limited experience of retreading SST tyres but in principle there is no reason why a sound SST tyre may not be retreaded. Given the unique structure of an SST tyre it must never be mixed on a vehicle with a standard tyre. To avoid this risk, a retreaded SST tyre must be marketed and identified by the retreader as a SST tyre and not as a conventional tyre. Retro fitting of SST tyres Consult the vehicle or tyre manufacturer regarding which vehicles are suitable for SST fitment. SST tyres may only be fitted to vehicles equipped with a Tyre Pressure Monitoring System (TPMS). Replacement tyre fitment The retro fitting of conventional tyres to vehicles originally fitted with SST tyres will remove the vehicle’s run flat capability, potentially leaving the driver immobile in the case of a deflation, and could compromise vehicle handling. It is therefore recommended to consult with the vehicle manufacturer or tyre manufacturer before replacing SST tyres with conventional tyres on such vehicles. Mixing SST tyres with standard tyres The handling and performance characteristics of SST tyres are different to that of conventional tyres, and so they must not be intermixed on a vehicle.
aMixing of different brands of SST tyres As with standard tyres there may be characteristic differences between different makes of SST tyres. It is therefore advisable to consult the tyre manufacturer regarding mixing of different makes of SST tyres. It is recommended that the same make and type of SST tyre is fitted across the axle. Service life of SST tyres v standard tyres The tread rubber compound used in SST tyres is generally the same or very similar to that used in conventional tyres and hence tyre wear rates between the two tyre types should also be similar. It is important to note that best tyre performance is achieved by maintaining correct inflation pressures for the loads and vehicle use. Use of SST tyres on towing vehicles Regarding SST tyres fitted to towing vehicles different issues come into play. In their inflated condition SST tyres may be used in the same way as a standard pneumatic tyre. Any experienced touring caravanner knows, stability of the car / caravan combination is of primary importance to road safety. Whilst a deflated SST is designed to cope with the demands of the solo vehicle, the additional mass of a caravan will subject the tyre to higher stress levels and impact on the tyre’s inbuilt torsional and lateral stability. The consequence of this is that even travelling at a restricted speed of 80 km/h (50 mph), the additional flexibility of the deflated SST tyre may negatively affect the stability of the car/caravan combination and reducing road safety. To avoid this potential hazard, it is recommended that the distance travelled and maximum speed on the deflated SST tyre should be reduced. As the vehicle itself has an influence on the tyre’s performance, refer to the vehicle manufacturer’s handbook for further information. Use of SST tyres on caravans and trailers SST tyres must only be fitted to vehicles which have a Tyre Pressure Monitoring System (TPMS) as a visual/audible warning to drivers of a deflating tyre. As yet no caravans or trailers are equipped with TPMS systems and hence should not be fitted with SST tyres.