Over the years many companies have sold the consumer on the benefit of having a run-flat tyre. In fact for a while, they were standard fit on many Austin and Morris Metro cars in the UK during the 1980s.
There are advantages and disadvantages of having these types of tyres fitted – the clear benefit is that should the tyre be punctured, the driver can still continue their journey to a tyre centre to have a refit, whereas with a normal tyre the driver would have to change the wheel on the spot – or call a roadside assist service to do it.
The structure of the tyre or rim means that should the tyre fail even at reasonable speeds, there should be a reduction in risk of loss of control provided the driver doesn’t panic. Another advantage touted by the tyre sales companies is that with them fitted you do not need to carry a spare – however to counteract that, there is a disadvantage!
The main disadvantages are that the tyres are expensive and have to be replaced. You cannot easily fix a small puncture as you can with a regular tyre. I had a friend who bough a new BMW some years ago with run-flats fitted and within a few weeks had several punctures due to the construction sites on his regular commute. After the second, he changed to regular tyres that could be fixed or replaced at a cheaper price.
Another disadvantage to these tyres is the weight and structure means that they can worsen fuel economy by having a higher rolling resistance and heavier weight than a normal tyre. With car manufacturers looking at ways to lighten a vehicle, run-flats are not considered a great way to improve the car’s efficiency. The combined weight of four run-flats might be heavier than simply carrying a single spare space-saver wheel onboard! For light trucks and buses where the overall weight is higher it may not be such an issue.
The structure and compound also appear to be an issue with higher wear resulting in a shorter life. Honda stopped fitting them to some cars because owners were complaining of an unreasonable life – the change in fitment was the result of a class action lawsuit a few years ago!
If the tyre is damaged on the sidewall, it compromises the structural integrity of the product and as such will not provide any benefit to being a run-flat – it will be similar to a conventional tyre and need replacing immediately.
There are three different types of run-flat tyres:
1 Self Supporting
These are the most common and were initially developed during the 1930s by Michelin in France. The idea is to have the tyre support itself with the remaining pressure inside to keep the tyre structure intact – very similar to an airless tyre. Michelin’s concept had a ring fitted to the rim inside the tyre with the inner part of the tyre formed to mould around the kidney shaped ring. However like modern run-flats, were very expensive so didn’t sell well.
During the late 1950s, Goodyear had another go in conjunction with Chrysler to market a tyre that didn’t deform too much. Then in the 1970s Dunlop developed the “Denovo” tyre – the brand that I most remember from my youth. This was fitted to British Leyland cars including Rover, Austin and Morris cars.
In recent years, as described above, BMW have offered a version supplied by Bridgestone or Pirelli. The interesting thing is that many of the designs actually could be called Auxiliary Supported because they used a combination of rim support as well.
2 Self Sealing
This method allows the driver to continue without knowing until the next tyre check that the tyre has been slightly damaged by a foreign object like a nail or other sharp object. The tyre has a membrane fitted that can help seal the outer layer of rubber. I keep cans of a similar substance in both cars and have an emergency kit when I’m motorcycle touring that helps to repair basic damage. Luckily I’ve not had to use them, however it is a safety measure I always have with me.
Continental Tyres use a proprietary product that is a sticky gum under the tread area of their tyres and they claim it can seal up to 80% of puncture damage.
3 Auxiliary Supported
These types of tyres also need a special wheel fitted as they have a thick rubber strip fitted directly to the rim to support the weight of the vehicle. As described earlier, many of the earlier Self Supporting versions could also be called Auxiliary as the designs needed a different rim design to make it work efficiently.
So there are pros and cons of run-flat tyres (sometimes called mobility tyres). Many manufacturers recommend using them with a TPMS (Tyre Pressure Management System) fitted so that you can get a warning when a tyre is losing pressure. This is especially important for speed and usage duration. It is advisable to reduce speed when notified of lower pressures and to plan for a tyre test as soon as possible. You cannot simply keep driving on them when they have a lower pressure than recommended!
I think it is dangerous to believe all the marketing hype that comes with these tyres regarding accident minimisation – there are often too many factors involved in a road accident, not just one simple reason. I would also question the cost benefit for many buyers although I am more and more aware that most drivers don’t really understand their vehicle or their abilities to know how to change anything for the better! I would also question any manufacturer who states their products are new and revolutionary when the idea has been around for 80 years!
My view is that run-flats have a place in the market and for me wouldn’t suit the usage of my vehicles, however if your driving consists of low speed commuting, perhaps these tyres might provide a benefit.
If you are interested in this technology then (as you would guess) each manufacturer calls them a different name: Dunlop call then ROF (Run On Flat) or DSST (Dunlop Self-Supporting Technology), Pirelli use the “Euforia” name and Michelin use the name “Zero Pressure”. You might also see manufacturers call them self-supporting run-flats or extended mobility tyres.