What are run-flat tyres?

They'll leave you deflated - but not stranded. How run-flat tyres work

BuyaCar team
May 18, 2018

A puncture doesn't have to leave you stranded at the roadside: run-flat tyres are designed to keep you going, even when the air has escaped.

It means that you can continue driving until you reach a tyre fitter who can replace the damaged part, rather than having to stop and wait fo a covery vehicle.

Run-flat tyres can also help you to maintain control of your car if the tyre has a blowout and suddenly deflates. Reinforced sections make them more likely to hold together in one piece.

However, they do involve some compromise because the most common type of run-flat tyres are heavier and more expensive than normal. In general, they lead to a bumpier ride over rough roads because they are stiffer and less able to absorb the impact of dips and bumps, although they are improving in this respect.

Finally, a run-flat tyre can't run for long once it has been punctured. You're limited to a reduced speed and a set distance, beyond which the tyre may be unsafe.

      

How do run-flat tyres work?

Conventional tyres only work when they are pumped up. They require a cushion of air to inflate and to keep their shape. So if they suffer a puncture and the air leaks out, they go flat. Trying to drive on could mean the tyre becoming detached from the wheel or shredding.

Most run-flat tyres don’t just rely on air to keep their shape. They have thicker sides that are reinforced. These tyres still deflate if they get a puncture, but not completely. The strong sides maintain enough of their shape that they can still grip the road. These are also known as self-supporting run-flat tyres.

There’s another, less common type of run-flat tyre that’s made with a sticky sealant layer inside. After a small puncture, the sealant oozes in to plug the gap and prevent the tyre from completely deflating. These tyres don’t affect the ride comfort of the car in the same way as self-supporting ones do, but neither do they offer the same level of security: the system only works if the puncture hole is 5mm in diameter or less. Continental Tyres says that this accounts for 80% of all punctures.

  

Can I drive on a flat tyre?

Many tyre companies, including Continental and Pirelli, advise driving at a maximum of 50mph for a distance of 50 miles on a self-supporting run-flat tyre. This is because you damage these tyres when you drive them and they aren’t inflated. Travel too fast or too far and they can start to break up.

You’ll need to find a tyre workshop quickly, which could leave you in the lurch late on a Saturday evening in the rain.

Few tyre fitters will contemplate repairing a self-supporting run-flat tyre because of the damage that may have been caused when driving deflated, so you’ll almost certainly have to buy a replacement.

Self-seal tyres don’t have the same restrictions, but you’re advised to get the tyre examined soon after a puncture to ensure that it’s safe and allow it to be repaired if possible.

   

Run flat tyres: the good

 Continue your journey after most punctures
 More control after a sudden deflation (blow-out)
 No need for a spare tyre or tyre repair kit

Self-seal run-flat tyres: downsides

 Only seals puncture holes 5mm wide or less
 Tyre must be examined after puncture
 Expensive

Self-supporting run-flat tyres: downsides

 Expensive and can’t be repaired
 Can result in a bumpier ride
 Limited speed and range after puncture

     

How do I know when a run-flat tyre has had a puncture?

Every car with these tyres has a pressure sensor that should alert you, with a light on the dashboard (shown above), if air escapes from the tyre - even if it’s only a small amount from a self-sealing one. Without these sensors, you may not notice and continue driving at full speed.

These systems regularly issue irritating false alarms. Driving too quickly over a pothole can squeeze the tyre and activate a low pressure warning.

     

How can I tell if my car has run-flat tyres

Some tyres clearly state "run-flat" on their side. The letters RFL make it easy to identify many others. However, some acronyms are a little less obvious.

  • RSC This is used on tyres that are made for BMWs and stands for Run Flat System Component
  • ROF Goodyear uses these letters to indicate a Run on Flat tyre
  • ZP Standing for Zero Pressure, this is Michelin's label for run-flat tyres
  • SSR Continental use the term Self-Supporting run-flat
  • DSST These letters stand for Dunlop Self Supporting Technology
  • DriveGuard This is Bridgestone's run-flat branding

    

Do cars fitted with run-flat tyres come with a spare tyre?

Part of the point of a run-flat is to avoid having to carry a spare tyre. Where they are fitted as standard equipment there's not always a spare tyre or wheel-changing tools such as a brace and jack. As a side-benefit, the resulting weight reduction beings a small improvement to the car’s fuel economy.

    

How long do run-flat tyres last?

Since they’re built from the same materials as normal tyres, run-flat tyres should last as long as standard tyres.

    

Can I fit ordinary tyres to a car with run-flats?

Yes. But if your car has been designed to run on stiffer self-supporting run-flat tyres, then it may not steer with the same precision on softer, conventional tyres. You’ll probably also want a backup in case of a puncture. This could be a tyre repair kit or a spare tyre, which will take up boot space.

   

Can I mix run-flat tyres with normal tyres?

You shouldn't because self-supporting run-flat tyres affect a car’s steering response and its bump absorption in a different way to conventional ones.

Also, you’ll be stuck if one of the conventional tyres has a puncture unless you have a spare or repair kit.

    

Can I fit run-flat tyres to my car if it hasn’t had them before?

You can but it’s not advisable. You’ll notice the car’s ride and steering response changes because self-supporting run-flats are much stiffer. To improve things, you may need to change the car’s suspension settings.

However, more crucially, a car fitted with run-flat tyres must have a tyre pressure monitoring system. This will alert you if the tyre has been punctured and is losing air, requiring you to slow down. If your car hasn’t got one of these, then fitting these tyres is a risk.

   

Which cars are fitted with run-flat tyres?

Tyre pressure monitors were made mandatory on new cars in 2012. Since then, manufacturers have increasingly been fitting run-flat tyres to their vehicles.

You’ll need to check whether a particular car has them, because many manufacturers fit them on some cars and not on others.

Companies that use self-supporting run-flat tyres include Mini, BMW, Mercedes and Audi.

Volkswagen offers self-sealing run-flat tyres on its Arteon and Tiguan which are usually come with a spare wheel too. self-sealing tyres have also been fitted to the VW CC and Sharan in the past.

     

How much do run-flat tyres cost?

Run-flat tyres are around twice as expensive as conventional tyres but the mandatory fitment since 2014 of tyre pressure sensors on new cars means more vehicles are being fitted with them, helping to drive down prices. As always, shop around.

    

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