BFGoodrich on the technical front

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