The Skinny on Run-flat Tires
By Aaron Robinson
Using a common tire shop tool, the engineer for Italian tire manufacturer
Pirelli reaches down and unscrews the air valve from the left front tire of a
new Mini Cooper S. It hisses flat in seconds, emitting a plume of tire odor
that smells remarkably like dirty socks. The Mini sags sadly on its deflated
“Now,” says the engineer, “go take a lap.”
What? Here we are in the pits of the famous Circuit Paul Ricard grand prix
race course, a roiling ribbon of asphalt set amid the green hills and rocky
outcrops of southern France with a view of the sun-dappled Mediterranean that
almost cannot be believed, and this guy wants me to drive a lap with a flat?
Ah, but this Mini is wearing Pirelli’s new line of “Eufori@"™ run-flat tires,
and as the description suggests, they are engineered to run without air at
speeds up to 50 mph for distances up to 100 miles. I take the Mini out, and
except for some buzzing from the flat tire and looseness in the steering, the
super-cute Mini laps the track safely and without incident.
The nuisance of slow leaks and the danger of high-speed blowouts are what
self-sealing and run-flat tires have been fighting for more than two decades.
As far back as the late 1970s, Pirelli was equipping the Lamborghini LM002, a
monster sport-utility with the V-12 engine, with a giant self-sealing tire
called the Scorpion BK. In recent years the race to develop better and more
affordable run-flat tires has intensified.
All of the major tire companies, including Michelin, Bridgestone/Firestone,
Goodyear, and of course, Pirelli, offer some kind of tire that can run without
air for a limited distance. No spare tire necessary. If you suffer a puncture,
these tires won’t leave you stranded on the freeway shoulder. They can get you
a bit further down the road, up to 100 miles in some cases. The automakers
think this is a dandy idea, and have included run-flat tires on their option
sheets. On some models, including the all-wheel-drive version of the Toyota
Sienna minivan, the Mini Cooper S, Lexus SC430, and the Chevrolet Corvette,
run-flat tires are standard equipment. Over the next few years BMW plans to
make run-flat tires an option or standard equipment on all of its models.
One big reason for the trend is the danger of freeway-speed blowouts. Another
less apparent but very important reason: the ongoing war for space under the
car. Increasingly popular amenities such as fold-flat seats and
all-wheel-drive are causing a crowding problem below the car’s floor. Extra
driveshafts, wells for folded seats, the fuel tank – there isn’t room for
everything. Engineers are salivating over the huge hole consumed by the spare
tire. Get rid of the spare, jack, and associated tools and you not only free
up a ton of space, you also trim both the vehicle’s weight and its cost.
With such incentives, the spare tire could be an endangered species. You must
have noticed how the spare has already shrunk on most passenger vehicles from
a full-size copy of the road tires to a tiny “doughnut” – usually a
high-pressure pneumatic or a solid-rubber ring, almost small enough to slide
down the slot of a Coke machine.