When the time comes to consider purchasing new tires for your car, whether it be online or in store, the various letters and numbers decorating a brochure or even the sidewall of your choice can be confusing. Tyre size, speed rating, load index, date of manufacture and more are all to be considered so you can see where people can go astray from the offset.
One of the acronyms you may have come across is RFT, or run flat tire to give it the proper title. Although they were introduced into the mainstream in the 1980s, run flat tires are now more popular than ever with some manufacturers such as BMW making them standard on all new vehicles. And such is the demand more consumers are asking about run flats, we thought it best to answer what are run flat tires, how to identify run flat tyres, their advantages as well as the disadvantages, and how using them impacts your driving.
In everyday operating conditions, run-flat tyres work like conventional tyres. They still contain air under normal use but it is when the tyre is flat, it allows you to continue driving so that you can get to a garage, your driveway or find a safe, level ground to change your tyre. Run flat tyres also reduce the dangers of a potentially dangerous tyre blowout due to their unique sidewall construction.
They is no solution to all your problems once the air in your tyre decides to escape for good though. Different manufacturers specify how fast and how far you can drive on your run flat tyres once you get a puncture so it is always good to know the specifications once disaster strikes. For example, Pirelli P Zero run flat tyres will allow continued operation even after a loss of some or all inflation pressure for up to 50 miles (80 km) at a maximum speed up to 50 mph (80 km/h.)
Car dealers may fail to mention that your new car comes with run-flats or may mistakenly indicate that a car has run-flat tires when it does not. To find out whether your car has run-flat tires, check the sidewall. The International Organization for Standardization (ISO) has developed a universal symbol for self-supporting run-flat tires, represented by a flat tire rolling down the road (pictured below)
In addition, tyre manufacturers do love a good acronym as mentioned before and they have created special branding to indicate run-flat capability. Here is a list of some of the branding you may come across:
- Bridgestone: RFT
- Continental: SSR
- Dunlop: DSST or DSST CTT or DSST RunOnFlat
- Firestone: RFT
- Goodyear: EMT or RunOnFlat
- Michelin: ZP
One thing to note though when it comes to opting for an RFT. Since they continue performing even when flat, all run flat tyres may only be used on a vehicle equipped with a Tyre Pressure Monitoring System (TPMS). The TPMS alerts you through a light on the dash or on board computer as soon as one of your tyres loses pressure. Without it, you might not know you were driving on an underinflated tyre. If this were to occur, it could lead to a complete failure of the tyre and cause even more damage.
And also to note, it is ok fitting non run-flat tyres even if your car comes from the factory with RFT equipped. Just make to sure check your manufacturer warranty before hand.
But what is the difference in construction between a run flat and conventional tyre on the rack at MicksGarage.com? There are two leading types of run flat tyre systems: the self-supporting system and the support ring system.
Types of Run Flat Tyres
The Self-Supporting Runflat (SSR) technology is based on a self-supporting reinforced sidewall. When there is a puncture, the sidewalls stiff construction prevents it from being crushed between the rim and the ground, as well as the tyre from slipping off the rim, which can make it impossible to fit a spare tyre.
The internal support ring is a less common design of run-flat tyre. When the tyre becomes deflated, it rests upon the internal ring, which supports the vehicle and maintains both the tyres shape, and its contact with the road. The support ring is made of rubber, bonded to a lightweight metal ring that is fitted to the wheel inside the tyre. In case of air loss the ring supports the vehicle and fixes the tyre bead onto the wheel rim.
Advantage of Run Flat:
1.You don’t have to change your tyre in dangerous or uncomfortable conditions. This is perhaps the biggest benefit of run flat tyres and is the one of the reasons why they were designed. With conventional tyres, you have to replace a flat on the spot or have your car towed to a safe location.
2.In a puncture situation, run flats are more stable than conventional tyres. Since they’re made to support your vehicle even when they contain no air, run flat tyres will help you maintain better control in a complete air loss situation than conventional tyres.
3.Lower vehicle weight: With the spare and tyre repair tools eliminated, vehicle weight should theoretically go down. But it’s not as much as you might expect, since run-flat tyres weigh more than regular tyres, due to the added sidewall reinforcement.
Disadvantages of Run Flat:
No spare: Vehicles equipped with run-flat tyres carry no spare, which means they don’t have the jack or tools either. In fact, eliminating the spare and reallocating that space to some other purpose (styling, third-row seat, interior room, etc.) is a big reason why car manufacturers offer run-flats.
Harsher ride: The stiff sidewalls that make a run-flat work also result in a harder ride. If the vehicle came with run-flat tyres from the factory, the automaker usually tunes the suspension to offset the harsher ride.
Reduced tread wear: Studies have found that people were replacing their run-flat tyres an average of 6,000 miles sooner than owners using conventional tyres. Professional opinions differ on why this is, but one theory is that tyre makers put a soft tread compound on a run-flat tyre to counter the hard ride. A side effect of the softer compound can result in how long run flat tyres last.
Blowouts are still possible: If a driver fails to heed or notice the run-flat warning and drives beyond the zero-pressure range or above the speed limitation, the tyre can begin to disintegrate, with the same destabilizing effects. Additionally, if the puncture occurred on the sidewall or if the tyre hits a large object, the driver would have to call a tow truck.
Hard to tell if it is low on air: A side effect of the stiffer construction is that the sidewalls do not bulge if the air pressure is low. This means that it is critical to have a tire pressure monitoring system and check your tyre pressure frequently. You can find tyre pressure gauges from MicksGarage here.
Cost of Run Flat Tyres: The price of run-flat tyres can be off putting at first. Due to their unique construction in comparison to conventional tyres, an identical model of RFT will generally be 20-30% more costly to your wallet.
Replace rather than repair. Can a run flat tyre be repaired? Short answer, yes. But should it be? In the case of a nail or small puncture in a conventional tyre, they can be repaired to a certain degree. But with run flat tyres, it is strongly recommended that the tyre is replaced as its durability will have been weakened after being repaired. Some run flat tyres also need to be replaced in pairs.
Less availability: Because run-flats aren’t a big-selling tyre, having your tyre in stock can’t always be a guarantee and may need to be ordered in, so act fast once a puncture occurs.
So to conclude, although the disadvantages do overshadow the advantages numerically, the health and safety aspect that comes into play with the advantages help show why more and more manufacturers are opting to offer run flat tyres from new. It is said, that a standing pedestrian on the side of a motorway has the life expectancy of mere minutes, meaning a flat tyre can actually be a matter of life and death.
That approach may seem drastic, but the extra few pennies spent in the garage towards the end of the month can be difficult at the time but worth this in the long run.