Technofile - the slick tyre

The slick tyre was one of the greatest revolutions in racing car history, yet it crept into Formula One almost unnoticed. Keith Howard discovers its origins.

If you know your maths you’ll know what an asymptote is: a value that is approached but never actually reached, like the hapless frog whose every jump across the pond is half the distance remaining to the other side. The slick racing tyre was an asymptote of sorts. Although it was, of course, a destination eventually reached, it took many years of painstaking development to do so. Moreover, as the treadless tyre era approached it was heralded by semi-slicks with very little and to the casual observer, vestigial tread pattern. So obvious, so infinitesimal seemed the final step to no tread at all that the slick eventually made its entrance without great fanfare, or even much comment.

When I began researching this piece I felt certain I’d quickly discover a potted history of the slick in my modest library of technical and motor racing tomes and cuttings, on the Internet or, surely, in back numbers of MOTOR SPORT. But no. Only careful reading of the magazine’s Grand Prix reports from 1971 finally turned up this tantalising snippet from the Monaco GP: “…Peterson using some new Firestone tyres on his March, as were the two Lotus drivers [Wisell and Fittipaldij. These were a new compound smooth tyre, reckoned to give increased cornering power…”. Although it’s an equivocal remark, I take it to mean this was the first occasion a true slick was used in Formula One. But for the fine-toothed comb I was wielding that day, I might easily have missed it.

In itself the treadless tyre was nothing novel, of course. It had been used on Land Speed Record cars and in drag racing years earlier, but those applications were significantly less challenging. Removal of tread on high-speed tyres was simply a logical step to counter heat build-up; in drag racing the slick was used because of the superior grip it offered, but under conditions of traction and braking only. For the racing slick it was cornering that was to prove the really big problem. Why want a slick at all? It’s a reasonable question because what you learn about friction in school physics suggests it’s a senseless exercise. You may recall having it drummed into you that the frictional force developed between two sliding surfaces is dependent only on the clamping load between them, not on the contact area, and that the maximum possible coefficient of friction (friction divided by load) is One. If that rings a bell then cast such notions aside. Although this classic view of friction holds good where both materials are hard and unyielding, it means nothing in the world of tyres.

Frictional interaction between a flexible material (rubber) and a hard material (road) is fundamentally different, as a consequence of which two of the three statements above become untrue. Frictional force remains dependent on load, as before, but the area of contact is now a significant factor too and Unity no longer represents a theoretical maximum for friction coefficient. In fact a modem slick typically has a coefficient of friction around 1.8.

Contact area being a factor makes the attraction of a treadless tyre immediately obvious. Any form of tread pattern results in a reduced area of contact between rubber and road. Remove it and, provided you allow yourself the luxury of reverting to a treaded tyre when it rains, you have a tyre capable of generating more grip.

The story of the F1 slick is often reckoned to begin with the arrival of the two American tyre giants Firestone and Goodyear in F1 during the mid-1960s, and the Indycar tyre know-how they brought with them. Certainly the renewed competition acted as a wake-up call to incumbent Dunlop. But two technical developments were crucial to the slick tyre’s development, one of which – the introduction of synthetic (particularly styrene-butadiene) rubbers – took place earlier, beginning with the Dunlop R6 (CR48) tyre of 1962. Synthetic rubbers, because they were created in the lab rather than bequeathed by nature, really kick-started the art of modern tyre compounding, opening the way to the development of a new breed of soft tread materials that were to prove vital to the slick. Racing tyres of the time were notoriously hard, Jim Clark once remarking that he was certain they grew rather than wore during a race as they picked up detritus from the track.

As tyre widths rapidly increased in F1 during the latter half of the ’60s, a second crucial development occurred. Up until this point the crossply casings of the tyre had been formed in a circular cross-section, necessitating the shoulders of the tread to be created by increased rubber thickness. As tyre widths and cornering knees grew, this caused overheating. The solution lay in what Dunlop called ‘reverse crown’ or ‘depressed crown’ casing in which the tread area was formed in concave section, the tyre assuming its correct shape only when inflated. This allowed the thickness of shoulder rubber to be reduced from up to 20mm to 6-8mm. The Americans used something similar, derived from their Indycar experience.

From this point, you could say the slick became an inevitability, although a deal of development was still required, particularly in the compounding area, befbre it became a practical reality. You might suppose that removing the tread pattern should make a racing tyre more predictable in its responses, by banishing undesirable tread squirm, but early attempts at the slick defied such expectations.

Jackie Stewart, for example, tried a treadless Dunlop in private testing with the new March team in 1969/70. The tyre’s 970 wet compound – soft for the time but hard by today’s standards – conferred excellent braking and traction performance, but cornering grip lacked the necessary progression. Sudden breakaway made the tyre too difficult to drive at the limit. Only by adding small tread features, like the so-called crows’ or sparrows’ feet in the pictured 1970 CR92, could the tyre be made driveable.

Dunlop quit F1 at the end of the 1970 season, of course, so the final step of removing all vestiges of tread was left to its American competitors. Not, as I’ve noted, that putting the final piece of the jigsaw in place exactly made the headlines. It must seem ironic to those from all three F1 tyre suppliers of the period, who strove to make the slick work, that the FIA in its wisdom should now have banished it from Formula One altogether.

Our thanks to Alec Meskell, ex-Dunlop International Racing and Rally Service, for sharing his recollections