by Jim Kerr

PAX. The word may not mean much now, but if the tire manufacturers are
right, this word may become as common in our vocabulary as Kleenex is.
PAX is a tire system that offers several advantages over current radial
tires and we will start seeing it on new cars shortly.

The first production vehicle to feature the PAX tire system in North
America will be the 2004 Rolls Royce Phantom, available this April.
Others will soon follow. In Europe, PAX tires have been available on the
Renault Scenic since 2001, while several manufacturers have utilized the
PAX design on limited production automobiles. We have also seen them on
several concept vehicles at international car shows.

The PAX system was first introduced by Michelin in 1996. Michelin may
have invented the system, but with far-sighted vision, it has signed
agreements with Goodyear, Pirelli and Sumitomo as technology partners to
further develop the design of run-flat technology. Each of these tire
manufacturers is including the PAX design in their own product
development plans. The plan is for PAX tires to first come as original
equipment on high end vehicles, eventually working their way down into
mainstream new vehicles and finally the aftermarket. The strategy sounds
fine, but what is this radical new PAX system?

PAX is a run-flat tire and wheel combination. Run-flat tires are not
new. Goodyear has had their EMT (Extended Mobility Tire) and Pirelli
their P-Zero tires for some time now. These types of run-flat tires fit
conventional wheels so their use will likely continue. PAX run-flat
tires require a special wheel assembly, so the tire and wheel are called
a complete system.

In conventional (if you can call them that) run flat tires, the
sidewalls have many plies or layers of material. This stiffens the
sidewall so that the rubber can support the weight of the vehicle if air
pressure is lost. I can attest to the secure handling and decent ride
they offer when flat, but there is a drawback. They are heavy, which
places high loads on suspension components and hinders handling on rough
surfaces. PAX tires use a different design to allow the tire to run
flat.

Inside a PAX tire, a polyurethane support ring is bonded onto the rim.
Sitting just below the inside of the tire tread, the flexible ring is
designed to support the tread and the vehicle when an air pressure loss
occurs. With the tire supported by its tread, the sidewall design can be
different too. The shape of the tire sidewall and bead makes it “lock”
onto the rim when the tire deflates, so that the tire stays mounted on
the rim. With a completely flat tire, the vehicle can continue to be
driven up to 200 kilometres at 80 kph.

The PAX design enables the tire/wheel package to be much lighter,
reducing road noise, rolling resistance, and improving fuel economy. An
added bonus is improved road holding, especially on rough or bumpy road
surfaces.

A unique but necessary design feature of the PAX wheel and tire it that
the inside bead diameter is larger than the outside diameter. The larger
opening on the inside of the tire allows the tire to slip over the
support ring in the center of the wheel. These asymmetrical tires and
wheels also mean we get new numbers on the sidewall to designate tire
sizes. For example, the Rolls Royce comes with Michelin PAX 265x790R540A
tires and Toyota is introducing a new fuel cell vehicle equipped with
Michelin PAX 215x730R560 tires. Low tire pressure warning systems will
be standard equipment on all vehicles with PAX tire systems to warn
drivers of pressure loss.

There are disadvantages of the PAX system. The wheels cost extra, but
this is not a problem on original equipment applications. Special tire
mounting machines and training may be required. Hennessy Industries has
developed the Coates (a well-known brand of tire changing equipment)
HP8020 PAX System Changer that handles tires vertically to ease mounting
and dismounting PAX tires.

Sceptics may remember the ill-fated TRX tires introduced years ago.
Higher cost, special rims, and only one brand of tire supplier limited
the popularity of the TRX tires, which soon lead to their demise. Will
the PAX system be different? I think so. Run-flat technology with
advanced handling and a wide range of suppliers is what will make the
difference this time.

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