In 1959, a Citroën ID 19 secured first place in the gruelling Monte Carlo Rally. The car, crewed by Paul Coltelloni, Pierre Alexandre and Claude Desrosiers, had fought off numerous competitive rivals - such as the Jaguar Mark 2 and Porsche 356 - to top the leaderboard.
Citroën had been rallying a DS since 1956 but the ID 19's victory, according to the company, compelled it to enter more races. It quickly established itself in the rallying scene, with ID 19s, DS 19s and DS 21s proving successful in a plethora of events.
Consequently, when the technological masterpiece that was the SM arrived in 1970, the decision was made to enter the new model in the 1971 Morocco Rally. A heavy, complicated coupe might seem an inadvisable choice for such an event but Citroën rarely ever took the logical, straightforward route - and was often all the better for it.
The cars would first campaign in Group 4, and permitted modifications were relatively few and far between. One element the engineers were keen to change, though, was the heavy steel wheels. These weighed in at some 11kg, contributing to the SM's comparatively hefty 1450kg kerb weight and reducing the effectiveness of its suspension and steering.
There were also concerns over durability, as impacts and flexing could bend or break the wheels. This issue also occurred with lighter aluminium alternatives, curtailing their possible use.
Michelin, which was controlling shareholder of the brand at the time, held the answer to Citroën's problem. It had, since the mid-1960s, been working on prototype injection- and compression-moulded wheels that used plastics, resins and fibreglass. There was a glut of potential advantages to such a wheel, ranging from reduced mass through to quick, painless manufacturing.
Reductions in the weight of the wheel, as well as being beneficial from a handling and ride standpoint, could also bring down the cost and weight of a vehicle itself - because the lighter rims would permit lighter chassis and suspension components.
In order to prove the capabilities of the technology, Michelin designed a set of advanced composite wheels for use on the SM rally cars. These wheels were reportedly fabricated out of sheet moulding compound, a reinforced composite produced by mixing fibreglass strands with resin, which was then compression moulded to form the one-piece wheel. The wheels weren't entirely fibreglass and resin, though, as a little metal reinforcement was added around the mounting holes to protect against cracking.
The wheels, dubbed 'RR' for 'Résine Renforcée', were produced at the company's Clermont-Ferrand plant. They measured 6Jx15 and weighed approximately 4.5kg each - which was over 50 per cent lighter than the equivalent steel wheel. They didn't have to be treated with kid gloves, either; according to Michelin they could withstand a load of 500kg each and, when it came to destructive flex testing, they lasted ten times longer than steel wheels.
Further adding to the appeal - from Michelin's perspective, at least - was the fact that they required no labour-intensive machining. The 'plastic' wheel-equipped SM subsequently won the 1971 Morocco rally, during which the wheels proved their durability.
They weren't prohibitively expensive either, despite their advanced nature, with reports suggesting pricing in line with a set of magnesium or alloy wheels. When listed in the options catalogue in December 1971, a set of RR wheels is stated to have cost 1200 French francs. That's equivalent to around £90 or, in today's money, about £1150. The SM, at the time, cost 51,800 French Francs - nearly £3800, or £50,000 today.
Citroën had high hopes for this Michelin innovation and half of all new SMs were expected to roll off the line on these remarkable wheels. Only around 1,250 sets were reputedly produced, though, compared to a total of 12,920 SMs. Seemingly, the price and fairly conventional look of the Michelin RR wheels - and perhaps, more prominently, the limited real-world benefits - worked against them. When production of the SM ended in 1975, so did production of the resin wheels.
Michelin didn't drop the RR project in its entirety, mind. It trialled the wheels on a Lincoln Continental Mark III, presumably in an effort to appeal to a wider array of manufacturers by showcasing the benefits of reductions in unsprung weight. It appears that Michelin may have been slightly ahead of the curve, though, as heat-induced issues were reported. In the SM, the front brakes were inboard and the composite wheels weren't exposed to high temperatures. In more conventional applications, they had to endure the full heat of the front brakes - which could cause cracking and warping.
Other manufacturers, including Dodge, later experimented with composite wheels. It wasn't until the wider introduction of carbon fibre, however, that durable one-piece composite wheels became a reality. Koenigsegg introduced its proprietary 'Aircore' carbon fibre wheels in 2013, which were claimed to save almost 20kg in unsprung mass. Ford followed in 2015, with carbon fibre wheels for the GT350R, and BMW and Porsche weren't far behind.
Suffice it to say that the premium for a set of labour-intensive carbon fibre wheels is significantly higher than Michelin's original fibreglass and resin offerings. When the new Porsche 911 Turbo S Exclusive Series was unveiled in August 2017, for example, the optional 9Jx20 and 11.5Jx20 carbon fibre wheels commanded an almighty €15,232 - equivalent to £13,624, at the time.