RADIALS: A 70-YEAR-OLD STORY
June 4 1946, Michelin patents the modern steel belted radial tire
11:00 A.M., Thursday June the 4th 1946, a Michelin employee walks into the Paris Patent Office located in Rue de Leningrad (now Rue de Saint Petersburg), carrying a rather bulky set of documents, a Patent Application relating to the technical principles and production technologies of steel belted radial tires. Since 1929, Michelin’s engineers have been working hard on this project, but only in the last five years, the radial’s development has taken on new meaning, and the company is now set to reap the benefits.
At the time, soon after the Second World War, radial tires were not entirely a novelty. Thirty years before, in fact, G.H. Hamilton and T. Sloper had patented something similar for the Palmer Tyre Company (patent n° 467 filed in London on January 7 1914) and shortly after that, November 7 1916, Arthur W. Savage, a small tire manufacturer in San Diego, California, practically did the same (patent n° 1203910) in the USA.
Nevertheless, radial technology is still viewed as somewhat “exotic”. The vast majority of vehicles are equipped with cross-ply tires, laid at 35° and distributed as uniformly as possible. Yet because of the excessive heat produced by the number of plies needed beneath the tread as well as in the sidewalls (4 on average on cars and up to 20 on transport vehicles), the French manufacturer started considering the possibility of reducing that heat, without compromising on strength and duration. Thus the Metallic, the first steel belted tire was born.
Robert Puiseux drives the change
Encouraged by the positive results, at the death of Edouard Michelin (25 April 1940), Robert Puiseux, Anne Michelin’s husband, succeeded the father in law as president of the company and decided to focus on the new technology. The reason is simple. This kind of tire promises to last two or three times more than any other product on the market and therefore is bound to have a great economic impact on the market.
Their production, though, is certainly not free from problems. At the time, for example, working with tolerances of a millimeter was quite a feat since this value was five times lower than any machines could guarantee. Raw materials and production processes had therefore to be rethought and redesigned from scratch.
The advantages, however, outweigh by far the disadvantages. The new radial tires are head and shoulders above the competition from every point of view: comfort, wear, grip, handling, resistance to puncture ... So, in the end, the interests of customers and a community of users that placed great importance on strength and duration prevailed. “Creating the first radial tire cost us large amounts of time and efforts not to mention huge investments in the short run, but looking back, the decision to persevere with this project played a decisive contribution in making Michelin what is today” declared the French tire maker.
The “bookkeepers” hand
Marius Mignol is a key figure in the development of Michelin radials. After working for a time in a printing press, he becomes bookkeeper in Michelin’s export department. Here he is noticed by Edouard Michelin while using a straight edge of his own invention to calculate the exchange rates between currencies. Edouard is particularly impressed by his creativity and his ability to turn his ideas into real objects. After talking to him about tires, "le Patron" decided to transfer him to the research department. "There he will be much more useful to the company," he said to himself.
During the German occupation of France in the Forties, Mignol leads a series of experiments to demonstrate that most of the heat and rolling resistance is generated in the sidewalls and not the tread. He therefore proceeds to manufacture a “tire without sidewalls” which, unfortunately, proves to be lacking any stability and is impossible to drive. To solve the problem, two steel belts, the so-called “cage à moche” (fly trap) are added to the prototype, a solution that has two main benefits: a reduced heat build-up and the car has a more stable behavior. We are in the Spring of 1941, and, perhaps unwittingly, Mignol comes up with a new and innovative way of manufacturing tires, a big step forward in automotive engineering: the radial tire.
A global success
In any case, Michelin deserves the credit for having created and commercialized the radial tire on a global scale also thanks to the fact that, from the middle of the thirties, the Clermont Ferrand based company controls Citroen and therefore can supply as original equipment its new radial 185-400 SP (the name " X "was adopted starting from 1949 to highlight its cross-ply structure) on two revolutionary and widely popular models, the Traction Avant and the 2CV, both front-wheel drive.
Even Lancia, another automotive factory traditionally sensitive to innovation, was among the first to adopt it as original equipment on its Aurelia, the same Aurelia B20 that, equipped with Michelin X radial tires and driven by Giovanni Bracco and Giovanni Lurani, came twelfth overall and first in its category at the 24 hours of Le Mans in 1951. After this success, the superiority of the Michelin X is such that many drivers adopt it even though the French company does not officially participate in motor racing.
At this point all the industry’s largest manufacturers looked for ways to enter the radial tire market, but, not being able to use the steel belts that Michelin had secured with a twenty-year patent, many resorted to other solutions. Already in 1951, for example, Pirelli unveiled its Cinturato (first example of a radial reinforced with fabric plies), soon to be followed by similar proposals by Continental, Dunlop, Firestone and Uniroyal, all stimulated by the growing interest of the European car market as a whole in this new tire. Not surprisingly, at the beginning of the seventies, as soon as the Michelin patent expired, 80-90% of new cars were equipped with radials.
Meanwhile the radial technology starts finding new applications. In 1952 Michelin introduces the first Michelin X Radial for trucks and in 1977 makes its debut in Formula 1 with Renault winning four GPs the following year with Ferrari. It is the beginning of a revolution that goes beyond competition. In 1979 the first Michelin BIB X radial for farming machinery is presented and in 1981 Michelin X Air, the first aircraft radial, makes its debut on a Dassault Mirage III fighter. On motorcycles, the first radial was the Pirelli MP7 which equipped in 1983 the European version of the Honda VF1000R but soon after Michelin introduced radial technology in competitions. And nowadays tires are almost exclusively radial.
Radial or cross-ply? Here are the differences
In a cross-ply tire the plies are disposed in pairs overlapping at different angles (less than 90 degrees) crossing each other. In this kind of tires the tread is integral to the sidewalls and influenced by the latter's flexion movements. This results in a deformation of the contact patch that reduces lateral handling and increases tread-wear. Friction between the plies in the casing also increases the tire's operating temperatures.
Radial tire construction uses body ply cords extending from the beads and across the tread at approximately right angles to the centerline of the tread. Its structure is symmetrical and relatively unstable and therefore reinforced with a stabilizer steel belt beneath the tread. In this way the tread and the sidewalls work independently. The flexion of the sidewalls will affect the tread to a lesser extent.
The advantages of radial structure are represented by a larger footprint on the ground and a better absorption of bumps in the road surface. The absence of friction between the ply cords reduces the tire's operating temperature and the energy absorbed by the tire’s rolling resistance.