Run flat tires
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, 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 double the price of sports tires.
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 Tire (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.
In 1958 Chrysler and US Royal (a tire company) offered run flats using an interlining to carry the weight. They did not sell well.
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. However, such sealants often make it more difficult for the tire to be properly repaired, which may result in an extra fee if the repair is done at an automotive repair shop.
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.
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 slightly offset in a vehicle by the elimination of a spare tire and tire jack, although the fact that the run-flats are "unsprung" weight makes this an uneven trade. Also, the excess tire weight is around the perimeter of the tire, which increases inertia/momentum of tire rotation, slowing acceleration and increasing stopping distances. 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. Of course, this is a objective point, since a regular tire with the same ratings will still be superior to a run-flat. Even if a high-dollar run-flat gives superior performance to a lower cost standard tire, achieving equal performance will still be cheaper on a standard tire.
Pros and Cons
The differing design and structure of run flat tires are such that run flat tires generally cost more to replace than conventional radial tires, however manufacturers cite the various safety advantages of run flat tires as justification to counter this argument (see 'advantages' paragraph below). A 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) is a further criticism. Michelin and Honda have been named in a lawsuit citing various forms of product misrepresentation.
Another criticism is that often cars with run flats do not have a spare tire, though vehicle manufacturers of run flat equipped vehicles consider a spare to be redundant (surplus to requirement). 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. The reason for this is that run flat tires became standard fitment on BMW vehicles when the 5 Series (E60 Series) was already in production and part way through its production life cycle. Later designs such as the BMW 3 Series (E90 Series) do not have space to carry a spare tire since the vehicle design was conceived from the outset to be used exclusively with run flat tires.
The use of run flat tires on passenger cars does provide several distinct advantages. The first is dynamic safety, since the run flat tire has a very strong carcass and is also designed to remain intact on the specially designed wheel rim (deflated tire will not separate from the rim). The second is personal safety since the user does not have to stop the car to change wheels on a fast highway, in poor weather, in the dark or in a high crime area. Another obvious advantage is convenience since the journey can be completed without stopping.
Further advantages are gleaned from not needing to carry a spare wheel: The space can be used for other purposes. Also, 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. However, these may be negated by the increased weight of the tires if they are self-supporting.
A further advantage is that all production vehicles equipped with run flat tires are also fitted with a tire pressure monitoring device which warns the driver of a tire pressure loss. Many cars which use conventional tires do not have tire pressure monitoring systems.
The future strategic direction of the automobile industry also favors manufacturers which use run flat tires, because alternative fuel vehicles require careful use and optimization of storage space and the absence of a spare wheel is a major advantage. For example, space for additional secondary fuel tanks is required for petrol powered cars with secondary CNG, LPG or Hydrogen fuel systems. Hybrid petrol/electric type vehicles require storage space for battery packs. Removing the provision of a spare wheel in the design phase of such vehicles can therefore contribute to improved integration of alternative power in future generations of automobiles.
Run-flat tires accounted for less than 1% of replacement tire sales in the U.S. in 2005. In 2006, it was expected that such tires would gain popularity with armored vehicle manufacturers, but growth figures were slow with one major model, the Michelin PAX, being discontinued by the manufacturer.
- The Michelin PAX auxiliary supported run flat system
- The Pirelli Self-Supporting Run Flat Technology
- Run-flat tires draw complaints and a class-action lawsuit
- Tire Rack article: Run Flat Tires