The run flat tyre has been around since 1934 when it was first used for military and armoured vehicle purposes. The current incarnation has been around for a while now and is becoming increasingly standard on cars up and down the budget range, particularly brands like BMW and Mercedes. If you are a regular visitor or contributor to car forums one of the subjects that get a lot of coverage is motorists wishing to swap their OEM (Original Equipment Manufacture) run flat tyres with standard tyres.
There are a multitude of reasons that drivers wish to make the change but most common is the perceived difference in ride quality, cost and longevity of the tyre.
So lets have a look at run flat tyres in more detail and help to address the question of should you swap your run flats for normal tyres.
What is a run flat tyre?
A run flat tyre is similar to a normal tyre in that it is inflated with air or gas with sufficient pressure to optimise the tyre for the load and speed ratings applicable for the wheel and the car. Where run flats differ is their ability to remain structurally effective as a tyre even in the event of a puncture. To clarify, run flat tyres enable the driver to continue on his/her journey even with a puncture at speeds up to 56 mph for up to 50 miles. This is sufficient time to get you to a garage or request a mobile fitter to have the tyre examined, repaired or replaced.
Run flat tyres are abbreviated RFT on the sidewall of some tyres signalling their reinforced nature. The sidewall of the tyre is reinforced to be able to take the weight of the vehicle in the event of a sudden or gradual deflation. The tyre also has a specialised beading to ensure it grips the wheel and does not detach easily from the rim.
The most advantageous attribute of the run flat tyre is safety and its ability to provide continuous driving and control in the event of the loss of air pressure due to a puncture. Tyre Pressure Monitors (TPM) are used to notify the driver of any loss in tyre pressure. In some cases the loss of pressure can be imperceivable to the driver, especially where pressure is being lost of a longer period of time. As such it is a good idea to check your tyre pressure monitor regularly as you should with normal tyres to ensure they are running at their optimum.
What are the benefits associated with run flat tyres?
Before you make up your mind to make the switch from run flat to normal tyres let us take a look at some of the benefits of a run flat tyre. It is also worth mentioning that some people make the switch from run flat to normal and then back again offsetting the perceived comfort, sportiness, reduction in noise and increased fuel economy (just some the reasons people swap) with plain and simple assurance and safety that run flats deliver.
- Depending on the car manufacturer run flat tyres are a crucial design component of the car. And therefore all of the testing including performance, economy and safety would have been done using run flat tyres.
- Tyre blowouts at any speed can be a shockingly frightening experience. Run flat tyres are able to withstand blowouts and sudden deflation from a puncture much better, offering increased safety and a reduced risk of an accident
- Reduced anxiety associated with driving with only four tyres on the car and no spare. Although tyre repair kits and pumps are carried by some people with normal tyres not all of them no how to replace a tyre. And we all know when your more likely to need to change a tyre
- Increase in luggage space as there no need for a spare wheel. This also equates to a decrease in kerb weight, which we all know improves performance, efficiency and stopping
- Run flat tyres are always accompanied with tyre pressure monitors to notify the driver of a loss in pressure. This information can then be acted upon early to avoid a potential accident
- The added benefit of knowing that in the event of a puncture you can continue merrily on your journey or at least until you are prepare to stop and have the tyre repaired or replaced
And the negatives associated with run flat tyres?
Having been through some of the benefits of driving a car with run flat tyres what should we consider as potential negatives. Below is a list of the concerns in no particular order:
- The cost of run flat tyres as a direct comparison to standard tyres offering the same performance characteristics will cost upwards of 20% more
- Fitting a run flat tyre is more complicated and often needs a dedicated tyre fitter to install
- Run flat tyres weigh more increasing the unsprung weight of the car, which will affect the handling and performance
- The sensors that are necessary with run flat tyres can fail, which will give you erroneous readings on the dashboard. Changing them can be expensive
- Unlike normal tyres run flat tyres typically have to be discarded if they suffer a puncture as the structural integrity of the tyre may have been compromised
- Stiffer ride, poor handling and reduced performance are objectively mentioned by car enthusiasts who make the swap from run flat to normal tyres
These concerns are real and present in the thought process for some who find themselves in the dilemma of choosing new replacement tyres. BMW, Mini, Mercedes and Audi are the biggest proponents of run flat tyres. And overtime anecdotally they have been getting better as both tyre and car manufacturers improve the combined package.
Insurance and Warranty
Disclaimer: You are advised to read your insurance and warranty policies and where applicable seek their advice to ensure you are compliant with the policy.
There are plenty of accounts on various forums where people contact their insurance companies to make them aware of the modification from run flat to normal tyres. The overwhelming consensus is that the insurance companies may record a note but on the whole, as of the time of writing it does not affect the insurance policy. Nor should it, providing the chosen tyres meet the correct speed rating and is the correct size (width, profile and diameter) for the wheel. After all we don’t call the insurance to notify them if we need to change to winter tyres.
Just remember that the move from run flat to normal tyre could be argued as a modification. For peace of mind a quick call to the insurance may be worthwhile.
With regards to the vehicles manufacturer warranty, again the consensus is that the changeover from run flat to normal tyres will not invalidate the cars warranty.
Can you mix and match run flat tyres?
Run flat tyres work in a similar way to normal tyres in that they provide contact with the road through patches not much bigger than a mans hand. The four patches have to flex in all directions to accommodate braking, cornering and acceleration. These contact points also have to spread the weight of the vehicle as evenly as possible on the road. This sometimes results in different size wheels and tyres to the front and rear to improve balance and performance, particularly for performance cars.
Tyres are an intrinsic part of the cars design, which take into consideration the location of heavy components such as the engine and where occupants and their luggage need to reside. The following attributes are just some of the considerations that tyre manufactures need to work collaboratively with the car manufactures to perfect:
- Braking Distance
- Rolling Resistance
- Complement key components such as suspension, brakes and dynamic control systems
- Performance (dry and wet)
In answer to the question of whether or not to mix and match standard tyres with run flat tyres, the consensus answer is no. The recommendation by tyre installers is to replace all four tyres from run flat to normal tyres at the same time. This is because of the potential imbalance that will inevitably occur if you have two tyres of different construction on either side of the car. Consider the flexibility of the side walls when cornering or under heavy braking if one tyre has 50% more side wall strength than the other. Similarly, tyre wear of RFTs (run flat tyres) is often greater than standard tyres which will cause uneven wear between the two.
Run flat tyres are known by many names
Depending on the run flat tyre manufacturer you may come across different acronyms describing similar technology. Here are a few:
- Pirelli – Run Flat tyre
- Dunlop – SSR or Self Supporting Run Flat
- Bridgestone – RFT or Run Flat Tyre
- Continental – SSR or Self Supporting Run Flat
- Michelin – ZP or Zero Pressure
- Goodyear – EMT or Extended Mobility Tyre
Can you place run flats on a car not designed for them?
In simple terms, no. There are some fundamental differences including the wheels and how they retain the tyre. Cars with run flat tyres also have the mechanism to monitor air pressure installed, which will give the driver the necessary signals that the tyres are in need of attention. For example a puncture or low air pressure. Cars that have been designed without run flat tyres do not have the Tire Pressure Monitoring System (TPMS) installed to provide the warnings from the tyre. Therefore, a driver could be driving and not know they have a puncture for sometime. Which could result in poor performance or worse, if the recommended speed limit for a punctured RFT is exceeded it could result in an accident.
Other considerations include the cars components such as suspension and brakes. Such components may not have been optimised during the design process to accommodate the use of run flat tyres.
Fundamentally, you need to ensure the balance of the car is not upset especially in an emergency braking scenario.
Do you need a spare wheel to meet the MOT?
It is not compulsory to have a spare wheel in the boot of the car in order to pass an MOT. The MOT requirement relates to ensuring each tyre has sufficient tread across 3/4 (three quarters) of the tyre at a depth of 1.6mm. Therefore if you do swap from run flat tyres to standard tyres you do not need to carry a spare wheel. If a space saver wheel is fitted to the car during the MOT the car will not pass. All wheels and tyres must be uniform, front and rear.
A tyre repair kit is recommended for peace of mind and could be a gift for anybody who does not have a spare tyre in the their car.
My experience of run flat tyres
Apart from the rock hard tyres on my first bike my first experience of what was then called the Total Mobility Tyre, which became the Dunlop Denovo tyre was fitted to my friends fathers Rover 3500 SD1. Spencer was a friend at school back in the mid 80’s who had it all including the first Sony Walkman I ever saw or heard. I recall this because not only did Spencer make a point of telling me all about his dads new Rover SD1, the power, the fact that the police used them but he went onto explain everything one could know about these Denovo tyres that could continue to drive even when punctured. However, as good as the Dunlop Denovo tyres were they didn’t stop Spencer’s younger brother Adam from engaging the automatic gearbox into drive while the car was idling on the driveway. This resulted in the car ploughing straight through the front of the house. Adam was under 5 years old at the time.
Back to the point, I have needed to use RFTs on two occassions in the past and they have proved to be time savers if not life savers. One occasion was a trip back from a UK holiday fully laden with the family and our paraphernalia . The TPMS light came on and I could feel a leaning effect to the nearside tyre. Despite this inconvenience we were able to drive at least 10 miles to a tyre fitter to have the tyre replaced. The only pain I recall was the cost of the tyre fitted to a 2009 Ford S-Max. The cost of the tyres are one of the biggest factors they are at least 20% extra but it is a price to pay for piece of mind and safety, especially with a family in tow. I have a different mindset admittedly when driving performance cars.
On another occasion my wife went for a night out in St Albans in Hertfordshire using the Ford S-max. While travelling on the M25 (London orbital road) she experienced the light come on for the Tire Pressure Monitoring System and she noticed that she had to wrestle the steering wheel a bit more as one of the tyres lost pressure. My wife managed to continue on her journey, called me when she arrived at her destination to explain what had happened and enjoyed her evening. She drove the car home cautiously via an A-road and the following day I sorted out the replacement. My wife and I were unsure what caused the rapid deflation, it could have been a pothole or a screw.
On both occasions the speed was kept to below the recommended maximum of 50mph. But the point is the run flat tyres came into their own and did what they were meant to do.
I recently ordered a set of 4 new regular tyres online for my BMW E85 Z4. This was to replace the existing run flat tyres, which I had read on various forum posts make a noticeable difference. I arrived at the garage to have them fitted and following the brief inspection by the tyre technician I was told it would invalidate my insurance if I place normal tyres on a car designed to have run flats. Being naive I took his word for it and made my way home to do my due diligence and place a new order. My due diligence led me to write this article.
In the end I double checked the tyres which were still legal but had been tramlining rather a lot lately. I checked the tyre pressure and realised they were significantly under their recommended values. I quick pump using a portable cigarette lighter fed pump was enough to make a whole lot of difference. The ride improved in so far as it became softer and the tramlining all but disappeared.
Tyre pressure is a real factor as is the due diligence we must all do as motorist to ensure we are not fed misinformation.
What have we learnt about run flat tyres? Should you swap?
Hopefully this article has helped to distinguish between misinformation and fact and has helped in your decision making. The point is you do have a choice of tyres to place on your vehicle and more importantly that choice is yours. On performance cars the decision to swap may be easier. However, do take into consideration the safety and convenience aspects associated with run flat tyres for family vehicles or where potentially vulnerable drivers could benefit from the technology. As in late at night, autumnal or wet weather. Changing a tyre or requesting the breakdown services in such conditions is not ideal. So removing the associated anxiety may be a price worth paying.
If you are in the market for a new car do bare in mind you may have the option to choose normal tyres with a space saver, car jack and brace in lieu of run flat tyres.