‘Run-flat’ safety tyres

First generation run-flat technology

Though they have yet to make a significant impact in either the automotive OE or
replacement tyre markets, one of the most notable tyre innovations of the 1990s has
been the concept of the ‘run-flat’ or ‘safety’ tyre. More than 30 US Patents have been
awarded to various tyre companies since 1990 related to ‘run-flat’ or ‘safety tyre’
capability. Unlike puncture sealants that are ‘self-repairing’ and limit the loss of inflation
pressure due to punctures, the run-flat tyre concept offers the capability of operating
without inflation pressure, at least for a limited duration at a specified speed, thus
eliminating the requirement for a spare tyre.

Bridgestone began producing the Expedia S-01 as early as 1992, but the first OE placement was the Goodyear Eagle GS-C EMT on the 1994 Chevrolet Corvette. Both of these tire lines were confined to very low aspect ratios (40 series) with short sidewalls that contained stiff rubber inserts between the carcass plies as illustrated in Figure 11.9. Though both the Bridgestone and Goodyear tyres could be mounted on industry standard rims, they both had the drawback of requiring the use of an expensive low-pressure warning device .In 1995, Michelin entered the run-flat market with the MXV4-ZP 60 series tyre line with sizes that fit several popular auto platforms, but which required the use of a special SH-M tyre rim with a taller hump in the flange area. It too required the use of a low- pressure warning device

The typical run-flat capability of these ‘first generation’ run-flat products at zero inflation
pressure generally included:
• Operating distance limited to about 81-162 kilometers.
• Operating speed limited to no greater than 89 kilometers per hour.
• Handling characteristics as good as a ‘normal’ tyre under ‘normal’ inflation.
• Replacement of the tyre (no repairs permitted) after operating at zero pressure.
• Relatively poor fuel economy due to the tyre’s heavy construction and high hysteresis
rubber inserts.

Because of these limitations, combined with the high cost of the mandatory low-pressure
warning system and the relatively low occurrence of flat tyres, sales of these early run-flat products never reached a significant portion of the market during the 1990s

New run-flat concepts

In the mid-1980s, Continental developed the ContiTireSystem (CTS), a radical tyre concept
eliminating the conventional tyre bead and apex construction with an ‘inverted rim’.
Instead of the typical wire bead, the lower sidewall rubber of the tyre was clamped and
bonded directly to a special wheel rim. In the 1990s, this concept was again employed in
Continental’s IWS (Integrated Wheel System), and still later in their ContiWheelSystem
(CWS) in which it was combined with a supporting shelf built into the special wheel rim
that provided run-flat capability while the rim clamping/bonding system kept the tyre on
the rim [78]. Though providing a larger diameter wheel with the same tyre diameter
leaves room for larger brakes and additional wheel and tyre sensors, the use of a non-
standard wheel is a drawback for the adoption of this concept in either the OE or
replacement tyre market.

Continental has introduced another concept, which provides the advantages of a ‘safety
tyre’ but compatible with a standard wheel rim. The ContiSafetyRing (CSR) is a multi-
piece, reusable, lightweight (3.2 kg) steel ring insert that is mounted inside a conventional
tyre on a standard wheel rim. In profile, the CSR has a corrugated shape with rounded
humps that supports the tyre if inflation pressure is lost. Unlike most ‘run-flat’ tyres, no
low-pressure warning device is required with the CRS because vibration and noise is
increased sufficiently to alert the driver if pressure has been lost.

In addition to the ‘conventional’ MXV4-ZP run-flat tyre, Michelin has also developed a
new run-flat concept similar in some ways to Continental’s CWS. Like the CWS, the
Michelin PAX (formerly called PAV based on the French acronym for ‘tyre anchored vertically’), utilises a larger diameter non-standard wheel rim with a supporting shelf
insert as seen in Figure 11.10 [79]. Introduced in March 1997 the PAX concept is claimed
to offer superior run-flat capability (up to 200 km at 88 kph), improved ride, and lower
rolling resistance versus conventional run-flat tyres. Though Renault agreed early on to
offer the PAX in small volumes on a production car model, the OE automotive market
has been slow in accepting the PAX concept, not only because of its non-standard wheel (which requires a complete new system for sizing both tyres and wheel rims), but also
because of the weight penalty. A set of four PAX tyres weighs 12% more than five (four
plus a spare) conventional tyres. However, General Motors have produced PAX concept
tyres in three sizes on its Cadillac Evoq and Oldsmobile Recon concept cars.