run flat

Run-flat tire

A run-flat tire 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 (up to 90 km/h or 55 mph), and for limited distances of up to 100 miles (160.9 km), or even 200 miles (321.8 km) depending on the type of tire. First patented in 1892 (Victorian Age), run-flat tires were re-developed in 1978 and offered as an option in the 1990s mainly for two-seat sports cars with little room for spare tires and jacks. They have grown in popularity for other vehicles, such as high-end luxury cars, because of their safety and convenience, costing only double the price of sports tires.

Technologies

There are three basic technologies currently available, described below.

Self-supporting

The tire is built with stiffer side-walls (sometimes 50% thicker) that can bear the weight of the vehicle even when the pressure within the tire is greatly reduced. The side-walls are typically constructed of layers of rubber and a heat-resistant cord that prevent the side-walls from folding or creasing. The bead around the edge of the tire is also specialized to grip the wheel rim such as to avoid becoming detached from the rim.

Self-supporting run flat tires are fairly common on light trucks and passenger cars and typically provide for the vehicle to drive for 50 miles at around 50 miles per hour. However, if the tires are treated to this kind of punishment, they may still be irreparably damaged in the process. In addition, if the tire is punctured in the sidewall or at the edge of the tread, repair may be impossible or unsafe.

The first vehicle ever to be sold with run-flat tires was the Mini 1275GT in July 1974. It used the Dunlop Total Mobility Tyre (abbreviated to TMT, later known as Denovo) system which required special wheels and featured ultra-low profile side-walls.

In recent versions of the Chevrolet Corvette (C5 and C6), run flat tires are required as there is no accommodation made for a spare tire. As a spare wheel is optional on the MINI One and Cooper and non-existent on the Cooper S and One Diesel, the new MINI also makes use of run flat tires for certain wheel sizes, as do many BMW models.

Self-supporting run flat tires typically carry a 15% - 27% weight penalty over similar standard tires, or additional 2-3 kg (4-7 lb) (samples based on 225/45-17 tires). The additional mass concentrated on the outer edge of the wheels can impose a significant performance penalty due to greater rotational and unsprung mass.

Self-sealing

These tires contain an extra lining within the tire 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 tire is either permanently self-repairing or at least loses air very slowly.

There are also a number of retro-fitted tire sealants which act in a similar way to self-sealing tires. These compounds are normally injected through the tire valve. The rotating force then distributes the compound onto the inner surface of the tire to act as a self-sealing lining within the tire.

Auxiliary-supported

In this system, there is an additional support ring attached to the wheel that can support the weight of the vehicle in the event of a loss of pressure. While these systems generally offer better ride quality because their sidewall's stiffness can be equivalent to a standard tire, the requirement to have both special wheels and special tires increases cost and limits these systems from widespread use.

Performance characteristics

Depending on the design, some run-flat tires perform better than regular tires, and some slightly worse. Some run-flat tires have a 20% higher rolling resistance, in part due to their added structural material and mass; this can worsen a vehicle's fuel efficiency. On the other hand, internal bracing in some run-flat tires reduces deformation, with the opposite effects of reducing rolling resistance and improving fuel efficiency. Also, the overall weight increase of the tires may be offset in a vehicle by the elimination of a spare tire and tire jack. Some manufacturers by 2001 had mandated that run-flat tires provide low rolling-resistance for improved fuel-efficiency, a soft ride, and excellent wet handling.

Criticism

Makers of run flat tires as well as the automobile manufactures that use them for new cars have come under fire for not fully disclosing some of the significant downsides of many run flat tires, including the significantly shorter life of a typical run flat tire (often as few as 10,000 miles versus approximately 40,000 miles for a conventional tire), the significantly higher cost to repair (sometimes as high as $900 per tire, but typically at least $350 per tire versus $125 for a typical family car tire) and the extreme shortage of both replacement tires and trained repair facilities (there are typically only a handful of authorized repair shops in each major metropolitan area in the US). Michelin and Honda have been named in a lawsuit citing various forms of product misrepresentation (read the press release).

Another criticism is that often cars with run flats do not have a spare tire. Manufacturers like BMW do have space for a spare in some of their models (the 5 series for example) but do not include a spare, presumably to save money.

Market share

Run-flat tires accounted for less than 1% of replacement tire sales in the U.S. in 2005. They are gaining in popularity with vehicle manufacturers as a consumer option, and it is expected that this will increase the share of replacement sales in the future.

See also

External links

Sources

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