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How Do Run Flat Tyres Work?

By Tegan Lawson

Run flat tyres invoke a lot of debate, yet BMW continues to offer Run Flat Tyres (RFTs) as original equipment across most of its range and has done for decades, while among its European competitors, Mercedes-Benz has developed its own version in recent years and Audi is yet to come to the party.

The overarching theory among the detractors is that the RFT is more suited to Europe (where they also are far more prevalent) because urban centres are closer together. There are also clearly defined pros and cons.

Firstly, what are run flat tyres? Essentially a RFT is a self-supporting tyre with a very different construction to that of a conventional pneumatic tyre.

The sidewall is made of thicker rubber and has been reinforced to make it stiffer. It’s this section that will bear the weight once the air pressure is gone.

The bead area is where the tyre connects to the wheel rim, it’s reinforced with steel wire and is a different construction in a RFT hence a different rim is required. The bead fits between the two sides of the rim to keep air in the tyre.

The RFT needs to be fitted to a specific rim, designed for use with the tyre, though tyre sizing remains the same. You can fit an RFT in place of a standard tyre but you’ll need to change the rim as well, and vice-versa.

The treaded face of the tyre that contacts the road is the same as a normal tyre. The rubber used in the treaded surface is also the same as a normal tyre – the major point of difference lies in the construction of the sidewall.

Though the tread is the most likely location for a puncture, for theatrical purposes we tested the ability of RFT’s by drilling a hole into the sidewall. It’s worth noting that no matter where the puncture, the RFT behaves the same.

The first BMW fitted with RFT technology was the E85 BMW Z4 in 1995. Since then the technology has been adopted by almost every BMW model and run flats are optional on most of the Mini range.

We took a BMW 218d for a 40km drive through an urban area and out to some quieter industrial areas to get a feel for the car.

Upon the return journey, we promptly drilled a hole in the sidewall and embarked on the same 40km drive to feel the difference.

It was obvious right away there was something wrong.

These days cars fitted with RFT’s also have tyre pressure monitoring systems to alert the driver to any loss of pressure. It didn’t take long for the warning to activate, but at the same time, there was no way a driver that payed even a sliver of attention to the car wouldn’t have noticed something was wrong.

This was no slow leak, in which case the tyre pressure monitoring system may have tipped you off before things became desperate — it was obvious. The sound was loud and unpleasant, the car was pulling to the right and favouring the flat tyre, and it just all felt wrong.

One of the concerns with run-flats is the apparent need to rely on an electronic system to notify you when there’s a problem. To clarify, everyone should check their tyre pressure regularly anyway, and if there is a full puncture you need to listen to what the car is telling you. In the case of the 218d the poor thing was screaming out for attention.

In our BMW 218d a notification popped up in the instrument cluster, and we could find out which tyre was affected by going through the notifications in the iDrive system menu.

There’s no need to pull over and change a flat tyre on the side of a busy road, in the rain, if there’s three kids in the car, or if you’re running late. You have 150km of range and can travel up to 80km/h to get to your nearest BMW service centre or tyre retailer.

If you are in a remote area and can’t access a retailer with the correct replacement RFT, a conventional tyre can be fitted in its place. Make sure the tyre is the same size, and has the same speed and load rating though. This temporary tyre should be replaced as soon as possible with the correct RFT.

The way RFT’s let you maintain control and confidence in the vehicle is impressive. There was no freak out while driving that the tyre could shatter and cover the road like confetti, endangering your life and the lives of those travelling around you.

It was simply a case of recognising there was a major problem and knowing that you had the range and stability to get to a safe place to sort it out.

In my opinion, RFT’s are worth it. The benefits are that RFTs don’t suffer from sidewall blowouts (unless you drill a hole) that impact steering and stability at high speeds, and therefore can help prevent accidents.

You have the time to drive to a safe place to repair or replace it – in fact, you might just be able to make it home and sort it out tomorrow. A BMW RFT can travel flat for 150km at up to 80km/h.

You also score more cargo space because there’s no need to carry a spare or temporary space saver. This also means the vehicle is lighter and often more efficient.

On the other hand, if an RFT is punctured, more often than not it means purchasing a new tyre. A number of RFT tyre manufacturers don’t support the repairing of a damaged RFT.

There is also a self-perpetuating myth still circulating, that RFTs are expensive and hard to find. While they may be slightly more expensive, these days they are readily available in urban and regional areas and though the price is slightly higher, it’s not as high as it was when they were introduced many years ago.

There are three types of run flat tyres on the market, namely:

  • The self-supporting run flat tyre;
  • The self-sealing run flat type that uses a gel-like substance lining the inside that will seal a small puncture by being pushed into the hole by the pressure on the inside; and
  • Auxiliary-supported run flat tyres which have a ring installed inside the wheel, so that when pressure is lost, the weight is transferred to the ring, requiring special tyres and wheels and they’re more expensive.

Mercedes-Benz is getting on the RFT train, developing their own version of the technology in conjunction with tyre manufacturers, called MOExtended or Mercedes-Benz Original Extended run-flat safety tyres with pressure loss system, introduced in 2012 on the W246 B-Class.

MOE tyres are now original equipment on the new A-Class, B-Class Sports Tourer, CLA-Class, C-Class, GLA-Class, S-Class, all AMG variants and Mercedes-Benz commercial vans.

They have run flat like characteristics but don’t meet all of the International Organisation for Standardisation (ISO) run-flat tyre operating limits under all permitted vehicle loads so they can’t be branded as RFT’s.

They have been specifically tuned for Mercedes-Benz vehicles, and challenges were faced during the development as engineers battled to retain ride comfort. Mercedes-Benz engineers collaborated to develop the lighter-duty extended run flats to improve cargo space and provide a more compliant ride compared to full run flats. But MOE can only get you up to 80km at speeds of up to 80km/h – a significant reduction compared to the distance RFTs offer.

Another point of difference is that MOE tyres are fitted to conventional rims, so technically can be replaced with conventional tyres without the need to replace the rims, though this isn’t recommended by Mercedes-Benz.

Mercedes-Benz claims that MOE offers reduced rolling resistance, less of an impact on car body and chassis due to the softer sidewalls and greater driver comfort compared to 1st generation run flats.

Audi on the other hand doesn’t offer any RFTs or tyres based on the technology, instead opting to offer a space-saver in the majority of models.

Run flat tyres aim to reduce the need for stopping immediately if you score a flat tyre. Monthy Motor reader Jack wanted to know why his tyre replacement cost was so high and whether he should keep run flat tyres on his BMW X5.

Q: Hello. I own a BMW X5 and it is due for a tyre replacement. The price for these tyres is quite expensive and when I asked why, the dealer told me it was due to them being ‘run flat’ tyres. What does this mean? And why are run flat tyres so much more expensive?

A: Run flat tyres feature on a number of BMW vehicles and were originally designed to help minimise the need to pull over immediately when you puncture a tyre and it begins deflating.

These tyres feature stronger sidewalls — often steel reinforced — that remain intact when the tyre is punctured. The purpose and idea is that you will still be able to drive on the tyre until you are able to get to a service centre to have the tyre fixed or replaced.

Given the nature of the tyre construction, these tyres are often more expensive — generally around 25 per cent more — but come with the added value of convenience. They also reduce the vehicle’s running weight (as you don’t need to have a spare tyre and jack in the vehicle), but can affect the ride quality (due to the construction of the tyre).

Run flat tyres normally have a recommended speed limit of 80km/h and can be driven with for up to 150km when punctured. If you aren’t keen on getting your hands dirty when you get a flat tyre, run flat tyres offer a middle ground between convenience and practicality, but come at a cost when it comes to replacement.