In recent years, run-flat tyres (RFT) have been gaining in popularity. In fact, the development of RFT has been around for some time. Nowadays, brands such as Michelin, Continental, Goodyear, Bridgestone, Dunlop, Firestone, Pirelli, Yokohama has a range of products, otherwise indicated as the RFT (Run Flat Tyre), SSR (Self Supporting Run-flat) or ZP (Zero Pressure), and so forth.
Run Flat Tyres (RFT) are also commonly known as "puncture-proof tyres," however, that tagline is a bit untrue as RFT are not entirely immune to punctures. RFT at the punctured state (low pressure or the tyre pressure of zero state) can maintain travelling speeds of up to 80km/h for up to 80km or so.
Automakers pitch run-flats as a safety feature because they let drivers avoid stopping on busy highways and other dangerous locations. They also say they like the design flexibility they get by eliminating the space-consuming spare tyre. Even though run-flats are heavier, the lack of a spare tyre can save vehicle weight overall, helping fuel efficiency. Below we analyze the structure and principle of RFT, and its advantages & disadvantages.
Construction and Principle:
Above shows the construction of a run-flat tyre.
The main ingredient of tyre is rubber, but rubber itself is unable to cope with any load, thus the load of tyres mainly depend on layers of a reinforced fibre and steel wire. Rubber elasticity is much lower than fibre and steel wire, thus adequate tyre pressure ensures the process of deformation of rubber tyres will be minimal. However, if the tyre is travelling at low tyre pressure condition, the rubber will be deformed, stretching more than the strength of the fibres and elasticity limit of the steel wire. This could cause significant damage to the tyre, as the pressure will eventually destroy the interface between the different raw materials, and in more serious cases, result in tyre failure (Figure 1).
The Advantage with Run flat tyres is that they can operate without air in them, for a relatively short distance and low speeds, as their basic shape is kept by rigid components. This rigidity helps a driver maintain control of the vehicle if the tyre loses pressure (Figure 2), and allows the driver to continue driving, the pressurised air contained within the tyre supports the weight of the car. (Figure 3) However, recently tyres have been developed which are able to support the weight of the car by themselves, for a short period of time. New cars are also equipped with tyre pressure monitoring devices which warn the driver of a tyre pressure loss.
• Eliminate the need to change a tyre in bad weather or in dangerous roadside areas.
• Safety and reduced risk of accident - blowouts and tyre deflation can be serious driving risks for drivers.
• Tyres can be driven on for 80 kilometres or more with no air in them enable drivers to get to a mechanic without having to change the tyre after a puncture.
• Car need not be equipped with a spare tyre, resulting in more trunk space.
• The absence of a spare wheel contributes to lower vehicle weight which will in turn reduce fuel consumption, reduce harmful exhaust emissions, improve performance, handling and braking characteristics.
• Tyre pressure monitoring devices warn the driver of a tyre pressure loss.
• RFT's sidewall is harder than ordinary tyres, so comfort is compromised.
• To improve this problem, Bridgestone introduced a new RFT tyres with sidewall texture similar to ordinary tyres, thereby enhancing the comfort of the tyre.
• High replacement costs; and limited replacement choices.
• If the material that penetrated the RFT tyres is not removed, continuous driving will result in the hole getting deeper. Once the hole reaches a depth of more than 5mm, there is no way to remedy them.
While the RFT is not so popular in Singapore, increased advances in technology and maintenance of tyres translate to lower prices for them. Do not be surprised by the eventual rise in popularity of RFT that could one day put the history of roadside punctures to bed.
Run Flat Tyre abbreviations:
RFT: Run flat tyre
ZP: Zero pressure
EMT: Extended mobility technology
ROF: Run on flat
DSST: Dunlop self supporting technology
SSR: Self supporting runflat
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