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What are run-flat tyres and how do they work?

By Dan BezerTyre Advice11th September 2019

Run flat tyres should be considered as the driver’s first line of defence when faced with a blowout. These tyres keep their shape and strength, so the vehicle remains stable.

Modern cars have lots of electronic safety aids which help prevent a skid or will apply the brakes if it’s heading for a collision. However, these can only work if the tyres are in full and proper contact with the road – otherwise, they are useless.

Run flat tyres guarantee that contact.

 

Why fit run flat tyres?

They provide two main safety benefits; the most obvious and important is that they give the driver full steering and braking control after a puncture instead of veering off either into oncoming traffic or to the side of the road. The second is that they remove the need to change the tyre at what might be a dangerous place such as a motorway hard shoulder or on a poorly lit road at night.

On our potholed roads, punctures are by far the most common type of car failure likely to cause a crash. The RAC recently reported that a third of all recorded vehicle damage is from potholes before adding, “In more severe cases, it could lead to drivers losing control of their vehicles and being involved in an accident”. Run flat tyres largely remove that risk.

 

How do run flat tyres work?

Tyres perform a few key functions without which the vehicle can’t run and one of them is to support its weight. With a conventional tread, if you remove the pressurised air inside it the tyre will collapse under the load of the vehicle and has to be replaced. The chances are that it will also come off the wheel too.

Run flats have reinforced and much stronger sidewalls so they can take that weight and they also have specially designed beading in the wheel rim so the tyre remains attached to it even if there is no air inside.

As well as supporting the vehicle’s weight there is also the matter of controlling the temperature. Bridgestone, for example, has patented a high-tech cooling fin design for its run flats, called the DriveGuard series, which redistribute heat and friction after a puncture.

 

So how do you know if you have a puncture?

Since 2014, every new passenger vehicle sold in the EU has been fitted with a TPMS, a tyre pressure monitoring system, and run flats must ONLY be used with a vehicle which has it.

Many drivers may not feel any difference in the steering or general handling so the only way they can know is through an alarm or warning light on the dashboard. TPMS are fitted inside the wheel and work either through what is called the direct method, where it measures the pressure and temperature inside the wheel, or the indirect method where sensors in the anti-lock braking system spot differences in the rate of wheel rotation because a tyre with less air pressure has a slightly smaller circumference and turns faster than a fully inflated one.

At a pre-set point, normally around 20 to 25% below the recommended pressure, the TMPS will trigger an alert for the driver which is a red light on the dashboard in the shape of a tyre’s cross section with a warning sign.

 

What happens then?

If the warning alert does come on, the run flat will let the driver continue but at reduced speed and for a limited distance, normally at no more than 50 mph and for up to 50 miles. That is enough to either get home or to work and call for a mobile tyre replacement service or to get to a garage. These two limits cannot be ignored or broken because, among other things, it would invalidate your car insurance in the event of a crash later.

 

Do all manufacturer’s have the same system?

Manufacturers have different naming systems to signify a run flat tyre.

These are as follows:

 

Which manufacturers fit run flats?

Several car manufacturers, such as BMW, MINI and Mercedes-Benz, already fit them as standard and the list of cars with them is growing.

 

Do run flat tyres cost more?

They are more expensive to buy, typically around 20 to 25%, but that must be set against the increased safety.

 

Can I fit run flat tyres to an older car or one not originally fitted with them?

Take advice from a tyre fitting specialist but remember the vehicles MUST have a TPMS fitted. Also, tyres are part of the suspension, steering and braking systems and are tuned to work in harmony during the vehicle’s development before it goes on sale. Because of their much firmer sidewalls and construction a run flat tyre could have a huge impact on those functions.

To find out more about the run flat tyres we offer, input your car registration and postcode into our search tool. Alternatively, call our team of friendly advisers today on 0333 016 6550.

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