On the technical front:
Two decades of run-flat mousse
Twenty years ago, the Michelin Group chose the Acropolis Rally for the
first public airing of its new ATS system (Appui Temporaire Souple, or
Flexible Temporary Support) that would go on to revolutionise world
rallying since it was designed to enable drivers to continue without
stopping despite one or more punctures. Two decades on, with the system
now perfectly mastered, and with 90% of punctures covered for enhanced
crew safety, the FIA has announced that the system is to be outlawed
from 2008. Is run-flat mousse a victim of its own success?
Inspired from the run-flat solutions employed on presidential cars and,
famously, the Popemobile, the ATS system was first used on a trial
basis in 1987 on the Acropolis Rally, notorious for being Europe's most
punishing fixture for tyres. The honour of using the system for the
first time in competition fell to the works Renault 11s of Jean Ragnotti
and François Chatriot. Those around at the time can still recall the
look of surprise on the face of the great Markku Alen, then with Lancia,
when the ever-facetious Ragnotti pretended to stab one of his tyres with
a pocket knife prior to the start of the first stage!
In reality, this wasn't exactly a first. For the past three years, the
French firm had been successfully campaigning a run-flat system known
as Bib Mousse on motorbikes in enduro, rally-raid and motocross events.
The ATS system used in rallying which differed from Bib Mousse which did
not use air was officially unveiled at the Paris Motor Show on a Renault
Mégane prototype and won the Technological Innovation Award in 1988.
A difficult incubation period
How does the ATS system work? At first sight the principle appears
simple: a mousse ring is inserted between the tyre and rim to full the
space inside should air be lost for any reason. Yet its incubation
period which began in 1981 proved long and fraught as the Michelin
Group's engineers sought to overcome several problems, including
the mousse's reaction to heat and the unavoidable increases in its
equivalent pressure.
In 1987, when deployed, the mousse represented a pressure-equivalent
of 1.2 bar when hot, while the time- loss for divers was estimated to
be 1s/km and the maximum recommended distance for its use was 30km.
Meanwhile, centrifugal force meant that the mousse ring tended to
move about inside the tyre, resulting in the transmission of fairly
unpleasant vibrations via the steering wheel. Chassis engineers had
to modify suspension systems, too, in order to compensate for the
extra unsprung weight represented by the mousse insert (approximately
3kg, i.e.10% of the wheel's total weight). The mousse also caused
higher pressures and prevented the use of nitrogen to inflate the tyres
because of the risk of thermal conflict, even though nitrogen could have
contributed to more constant pressures.
By 1990, the equivalent pressure of the deployed mousse was 1.7 bar
(compared with 2 bar in normal use) and cars could run at up to 160kph
for 50km. But even though the system was by now far better understood,
it continued to be used exclusively for the season's roughest fixtures,
essentially for logistical reasons. Indeed, to prevent the mousse from
increasing in volume when cold which would make its fitment extremely
difficult it is essential to store it at cold temperatures. Today, two
refrigerated semi-trailers travel to all European rounds of the WRC,
while for long-haul events the mousse rings are put in containers as
soon as they arrive on site.
The special challenge of the Safari Rally
Over the years, use of the ATS system spread to all gravel rallies, then
all asphalt events and then the wintry fixtures, which basically covered
the entire World Rally Championship calendar... except the Safari
Rally. For a long time, the Kenyan round stood out as the exception.
The problem was that the mousse was incapable of standing up to the
heat, the sustained high speeds and the distances covered by each set of
tyres. These same three criteria continue to prevent the system being
used today in rally-raids and circuit racing.
The Safari Rally finally fell in 2001 when Michelin's engineers
succeeded in developing a mousse that was capable of withstanding all
these constraints. After a major set-back in 2000, Bibendum took sweet
revenge the following year when Tommi Mäkinen claimed victory with his
Mitsubishi Lancer running on the latest-generation mousse that had the
ability to withstand speeds of more than 200kph.
For competition use only
After winning the Technological Innovation Award in 1988, the Michelin
Group-developed ATS mousse clearly had a bright future in motor sport
and thoughts also obviously turned to employing the principle in
day-to-day motoring with a view to doing away with the need for spare
wheels (for a space and weight saving), not to mention the prospect of
enhanced passenger safety. For a variety of reasons, however, this never
happened. At the time, for example, the system's life expectancy was too
short (50km), while its fitment required specific tools and the mousse
was also difficult to stock. On top of that, Michelin's engineers didn't
have enough hindsight to be able to say whether or not the mousse would
survive the four years of a tyre's normal working life. The traditional
spare wheel was therefore kept on (above, the specific equipment
required to fit the mousse rings).
What substitute for rallying?
Today, as it has been for the past fifteen years or so, run-flat mousse
is a perfectly mastered technology that permits drivers to win stages
and even rallies despite one, two or even three punctures. In many
cases, they don't even realise they have punctured! Indeed, current
generation WRC drivers have always run with mousse and find it difficult
to imagine starting a stage without this trusty ally. Even so, from next
January, they will have to accustom themselves to the idea of competing
without it.
For 2008, the FIA regulations state that no solid matter may be inserted
between the tyre and the rim. This effectively means that alternative
systems developed for the championship by the Michelin Group such as
the Pax System will not be permitted since this system employs a rigid
support between the tyre and rim. So what substitutes for mousse exist?
One solution is to produce puncture-proof tyres, as seen on earthmovers.
The down side of this however is that they don't stand up well to heat,
give little grip and are extremely heavy. They would also lead to
chassis and suspensions having to be modified since the tyres would no
longer serve as a 'fuse' under impact and would consequently pass on all
energy to the suspension components, mounting points and chassis. To a
lesser degree, the same disadvantages would apply to a self-supporting
tyre with reinforced sidewalls, a solution that would call for rims with
specific profiles.
Tyre and chassis engineers clearly have plenty of work on their plate
if they are to come up with a workable replacement for the mousse
system, not to mention the costs required to develop a substitute
for today's well- mastered and economical system (less than 100 per
mousse). Meanwhile, the drivers will have to radically alter the way
they make pacenotes and also the way they drive. After pretty much
being a phenomenon of the past, the risk of puncturing will rear its
head once more, including the risk of slow punctures that formerly
obsessed competitors. True, the outlawing of mousse led to stages
becoming degraded after the WRC cars had been through, but the move will
certainly lead to a painful transitional phase in terms of performance
and safety. Finding a substitute that is as reliable and effective
as mousse promises to be a major fresh technical challenge for tyre
manufacturers.
RUN-FLAT MOUSSE IN DETAIL
What is it?
Despite their banal appearance, these black rings are packed with
advanced technology. They are made from a special mousse whose
micro-cavities are filled with a special chemical agent that is
incorporated during the cooking process, a particularly delicate moment
in the mousse's life which governs the life expectancy and quality of
the end product. The volume of this compressible, low density mousse
is more than halved when the tyre is inflated. Then, should air be
lost, it expands again; very slowly in the case of a slow puncture and
instantaneously in case of a big cut. In the latter case, it deploys so
quickly that there is no loss of 'pressure'. Today's mousse ensures a
pressure-equivalent of 1.3 bar when cold and is capable of withstanding
temperatures of up to 150°C. In 2001, mousse development focused on
producing an even lower density, a bigger volume and a higher equivalent
pressure once deployed. The system is currently capable of resisting
speed peaks of up to 200kph and can be used more than once.
How does it work?
Phase A
The unassuming 'doughnut' of flexible foam that makes up the system (coloured here, but in
reality black) is in fact a highly sophisticated piece of technology. BFGoodrich's fitters slide
the mousse inside the tyre before fitting the cover to the rim. So long as the tyre hasn't been
inflated, the insert takes up all the space inside.
Phase B
When the tyre is inflated to its working pressure of approximately 2 bar, the mousse is compressed; its
volume is roughly halved. For the moment, the insert is cold and therefore inactive.
Phase C
Once the stage has started, the heat generated by driving at speed transforms the physical
properties of the mousse. The system is now primed and capable of withstanding the sort of
dynamic loads all rally tyres must soak up. So long as everything goes according to plan 'on
the outside', the system remains inactive, but ready to be deployed at the slightest alert.
Phase D
In case of air loss (because of a cut, a hole or, very often, a damaged or broken rim!),
the system is instantly deployed. With air pressure no longer compressing it, the insert
automatically expands to fill the inside of the tyre. Thanks to its special properties,
notably its flexibility, the mousse perfectly takes over the role of the air that has
escaped.
Phase E
The system has been deployed and the driver is able to continue the
stage without having to stop to change the wheel. And in the majority of
cases, he can carry on pushing as hard as before. It is frequent that
he doesn't even realise he has punctured, especially on gravel. Fastest
times set after the system has been activated are not rare. Nor are
examples of drivers going on to win a rally thanks to the system!
WHAT THE CHAMPION SAYS...
Sébastien Loeb, three-time World Champion
"I competed with run-flat mousse for the first time on the 2001 Sanremo Rally which was my first WRC drive with
the Citroën Xsara WRC. It didn't take me long to work out how to make the most of the system when I saw the
lines the other drivers were taking! On asphalt, you can feel when the mousse kicks in because the car becomes
slightly less precise and the rear end starts to move a little more. On the loose, however, you don't notice at all;
punctures just aren't something you bother about!"
-credit: bfg
