Run-Flat Mousse – Victim of its Own Success

As we reach the mid-season break in this year’s FIA World Rally Championship, it is worth pausing to remember a small and seldom celebrated development that for two decades has kept competitors in the action and even seen drivers take first place when disaster would otherwise have loomed large. This saviour of many a racing team’s competition hopes is the mousse insert, an innovation that can keep a tyre in roadworthy condition even after receiving substantial damage.

It is twenty years since the Michelin Group first publicly showed off this mousse insert, which it christened the ATS (Appui Temporaire Souple, or Flexible Temporary Support), at the gruelling 1987 Acropolis Rally. Initially the system had its limitations, with post-puncture use restricted to a maximum of 30 kilometres and centrifugal force caused by the moving mousse ring creating unpleasant steering wheel vibrations, but despite these teething troubles, numerous improvements were made in subsequent years and today the system is said to enable vehicles to keep driving on 90 per cent of all punctures.

The system’s mousse inserts are made from a flexible foam ‘doughnut’ containing a chemical agent within its micro-cavities. The insert is placed inside the tyre before it is fitted to the rim and at this stage takes up all the space within the tyre cavity. However, when a tyre is correctly inflated the volume of the mousse insert is halved, and it will remain in this inactive state so long as the tyre functions as normal. But should a tyre lose pressure through a puncture or damaged rim, the mousse will rapidly expand back to its original size. This expansion can be either very slow, as in the case of a slow puncture, or virtually instantaneous should the tyre be cut or ripped. In the event of a sudden loss of tyre pressure, the mousse insert expands so rapidly that the driver may not even be aware that anything is awry.

The mousse currently employed in WRC tyres ensures a pressure-equivalent of 1.3 bar when cold and is capable of withstanding temperatures of up to 150°C. The system is currently capable of resisting speed peaks of up to 200kph and can be used more than once. BFGoodrich, current sole tyre supplier to the WRC, report that in most instances a driver can continue pushing the vehicle as hard as prior to the puncture, and frequently drivers do not realise a puncture has even occurred, especially when driving on gravel. Furthermore, the tyremaker claims that it is not rare for a car’s fastest times to be set after the system has been deployed.

However the very effectiveness of the mousse insert may yet prove to be its downfall. It has become so reliable that the FIA has announced the system will be prohibited as of the 2008 WRC season. Drivers will no doubt find it a painful transition as, after years of it being all but an unpleasant memory, the risk of being stopped by punctures once more looks set to rear its ugly head.