A run-flat tyre is a pneumatic vehicle tire that is designed to resist the effects of deflation when punctured, and to enable the vehicle to continue to be driven at reduced speeds - under 56 mph (90 km/h) - and for limited distances - generally between 10 mi (16 km) to 50 mi (80 km), depending on the type of tire.
Video Run-flat tire
Technologies
There are three basic technologies currently available, described below.
Self-supporting
The origins of the commercial self-supporting run-flat tyre started in 1935 with a tyre that had a fabric inner tire. The tyre was advertised as a protection against blow outs, a common and dangerous occurrence in the 1930s.
In 1934, Michelin introduced a tyre that was based on technology developed for local commuter trains and trolleys. It had a safety rim inside the tyre which if punctured would run on a special foam lining. The tyre was sold for military use and for specialized vehicles like bank armoured cars. It was advertised as "semi-bullet proof". While the tyre performed as advertised it was far too expensive to be a feasible option for most private automobile users.
In 1958, Chrysler teamed with Goodyear Tyre and Rubber Company to offer Captive Air run-flat tires using an interlining to carry the weight.
In 1972 Dunlop launched the Total Mobility Tyre (later Denovo) "fail-safe" wheel and tyre system that became optional equipment on the Rover P6 3500 in 1973, and by 1983 evolved into the TD/Denloc which became standard equipment across the whole Austin Metro range.
Most recently, Bridgestone and Pirelli run-flat tyres are supplied on some new model BMW cars. The automaker promoted these as a safety feature and as an alternative to carrying a spare tyre.
Self-supporting run-flat tyres are now common on light trucks and passenger cars and typically provide for the vehicle to drive for 50 miles (80 km) at around 50 miles per hour (80 km/h). However, if the tyres are subject to this kind of misuse, wheels may become damaged in the process, and repair may be impossible or unsafe, especially if the tyre is punctured in the sidewall or at the edge of the tread. These tyres carry a 20 to 40 percent weight penalty over similar standard tyres. The thicker sidewall also means higher rolling resistance, which reduces the vehicle's fuel economy.
Self-sealing
These tyres contain an extra lining within the tyre that self-seals in the event of a small hole due to a nail or screw. In this way, the loss of air is prevented from the outset such that the tyre is either permanently self-repairing or at least loses air very slowly.
There are also a number of retrofitted tyre sealants which act in a similar way to self-sealing tyres. These compounds are normally injected through the tyre valve. The rotating force then distributes the compound onto the inner surface of the tyre to act as a self-sealing lining within the tyre.
Auxiliary-supported
In this system, there is an additional support ring or insert attached to the wheel that can support the weight of the vehicle in the event of a loss of pressure. The runflat insert, because of its unsurpassed ability to carry heavy vehicle loads for long distances at high speeds, is the normal runflat selection for military vehicles, high-level executive protection vehicles, and "armored" vehicles used by government, aid groups, or private contractors in conflict zones.
Maps Run-flat tire
Standards of performance
The basic benefit of using run-flat tyres is continued mobility in case of a loss of air pressure, due either to a 'normal' puncture or to a hostile deliberate act including a bullet strike while the vehicle is traveling at high speed. Performance criteria are therefore in terms of distance and speed at which the vehicle can escape without becoming immobile and the steering control over the vehicle during this process.
The usual standard of performance, especially for military or security vehicles, are the Finabel standards.
Run-flat tyres accounted for less than 1% of replacement tyre sales in the U.S. in 2005. In 2006, it was expected that such tyres would gain popularity with armored vehicle manufacturers, but growth figures were slow with one major model, the Michelin PAX System, no longer being developed by the manufacturer (though replacements will be produced for the foreseeable future). A Michelin study released in 2008 found that 3 percent of drivers worldwide want run-flat tyres. U.S. market share is well below 1 percent. American Honda Motor Co. announced that the 2009 Honda Odyssey Touring and Acura RL were its last models available with run-flat tyres and with Honda no longer using run-flats. This leaves only a handful of volume manufacturers offering them as standard fittings and only on some models. An exception is BMW, who are the largest fitter of run-flats as original equipment.
Former US President Barack Obama's presidential limo, "the Beast", had Goodyear Kevlar lined run flat tyres.
Besides the cost, which can be more than double other tyres of comparable size, run-flat tyres can not be run flat if the flat is due to sidewall damage, a common cause of flats. Also, under the best circumstances, the speed and range that the run-flat tyres can be run flat is very limited. Run-flat tyres cannot be driven over 50 miles per hour and usually offer only up to 50 miles of extended mobility. These limitations lower the value of the extra expense for many buyers. In certain applications, depending on the vehicle, specific tyre design, and driving surface, a run-flat tyre can provide from 25 miles to 200 miles driving while flat with limited speed.
See also
- Airless tyre
- Direct TPMS
- Tyre manufacturing
- Tyre mousse
- Tyre-pressure monitoring system
- Tweel - Michelin's air-free tyre
References
Template:Tyres
Source of article : Wikipedia
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