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Understanding Run Flat Tyres

17/05/2022James Harding

Motoring

Run Flat tyres are a type of tyre capable of running on air pressure that would result in a standard tyre becoming unusable.

First patented back in 1892, ‘reinvented’ in 1978 and offered as an option for the public to buy – on a mass scale - in the early 1990s. Their story is not a new one.

The Advantages of Run Flat Tyres

There are a number of advantages for those who have Run Flat Tyres fitted to their car. The most obvious being what happens when a tyre is punctured by a foreign object or tyre failure.

Standard tyres will very quickly lose their shape and therefore become extremely dangerous when punctured. When they lose their structure, the vehicle will become very difficult to bring under control.

Not a run flat tyre

Since Run Flat Tyres have been designed to retain their shape in the event of a tyre puncture and sudden loss of air pressure, drivers are able to maintain control of the car.

This gives the motorist the required time to take their car to a garage for a replacement tyre without having to worry about whether or not they can get their car to the garage, or about replacing the wheel.

The simple fact that drivers are capable of continuing to drive is an important safety feature of Runflat Tyres. As it is not ideal or safe to stop – for instance, a person driving on their own at night, or driving on the motorway.

One other benefit of Run Flat Tyres is that you do not need to worry about carrying a spare wheel in the boot of your vehicle.

Not only will this increase the amount of space, but also it reduces the overall weight of the car, helping with the fuel economy – albeit marginally.

How Does A Run Flat Tyre Work?

Run Flat Technology

The technology behind Run Flat Tyres allows the tyre’s structure to remain stable if air pressure is lost; therefore they functional and capable of gripping the road.

Once a puncture is experienced, it is advised that drivers do not exceed 50mph, for up to 50 miles. This gives them plenty of time to get to a tyre fitting centre or back home.

Run flat tyres mean you can keep driving when in traffic

The basic construction of a Runflat Tyre does not differ from that of a standard radial tyre and works very much the same way in everyday operating conditions – after all the still contain air.

There are currently two main Run Flat Tyre designs on the market.

Reinforced sidewall

The most common variety of Run Flat Tyre features a reinforced tyre sidewall. This supports the weight of the vehicle and helps to keep the shape of the tyre in the event of a loss of pressure. It may bulge outwards but it will keep the structure and not become removed from the rim of the vehicle.

Internal support ring

Not as widely used, the support ring design works by ensuring that the tyre – when it receives a puncture - rests upon the ring. This in turn supports the car, helping to keep the shape of the tyre.

How to Identify Self Supporting Run Flat Tyres

Each manufacturer that produces SSTs creates them with unique “run flat capabilities”.

What is true for one SST – in terms of load it can carry, speed it can reach and distance it can travel – might not necessarily be the same for another. So, it is very important to check the sidewall of a SST to gain the required information you will need to know.

Different manufacturers use different symbols. These include:

Continental Run Flat Tyres – SSR
Dunlop Run Flat Tyres – DSST or ROF
Goodyear Run Flat Tyres – ROF or EMT
Bridgestone Run Flat Tyres - ROF or RFT or RSC
Michelin Run Flat Tyres – ZP
Hankook Run Flat Tyres – tyres will be marked with HRS or have B at the end of the product number (e.g. K115B)
Pirelli Run Flat Tyres – RSC
Yokohama Run Flat Tyres – ZPS

As well as the above symbol they may have the letter “F” preceding the rim mark on the tyre size.

One other version of SSTs is the Extended Mobility Tyre. This works in almost the exact same way as your standard SST.

The difference is, however that these tyres are unable to be used for as long once a puncture has happened. The average for these tyres is around 20 miles if you are driving below 50mph.

To identify the Extended Mobility Tyre, you will see a code along the lines of “EXT” on the tyre’s sidewall. Again to find out the run flat capabilities of one of these tyres, you will need to contact the manufacturer.

Can My Car Support Run Flat Tyres?

If your car was not designed to have Run Flat Tyres fitted, then you should seek advice before attempting to replace your current tyres with them. The reason being that the car will not be fitted with the required tyre pressure monitoring system – without this you will not be made aware of a puncture.

If your car was designed to use Runflat Tyres the suspension system of the car will have been designed around the use of these tyres. Run flat tyres have a tendency to deliver a harder ride.

If you put on softer conventional tyres then you are potentially losing handling and braking capabilities. Again, it is recommended you seek advice from an expert before making a switch from Run Flat tyres to standard tyres.

Top 10 Most Popular Run Flat Tyre Sizes

Run flat tyres come in a variety of different sizes to meet the needs of a wide ranging number of car models. Here are the top 10 most popular sizes:

Top 10 Cars Fitting Run Flat Tyres

Below are the top 10 cars which fit run flat tyres - spoiler alert, it's very BMW weighted.

If you own any of these models, please double check your own tyres before buying either run flat or standard tyres. To ensure you are replacing like-for-like - or if you are changing tyre types, that you are aware.