by James Kraus
Today, radial-ply tyres are virtually universal, while back in the mid-twentieth century, they were considered exotic. The vast majority of cars still rode on tyres of bias-ply construction, the basic layout of the pneumatic tyre since the heyday of the bicycle.
Bias-ply tyres had reinforcing plies running across the entire tyre from bead to bead, each at an approximate angle (bias) of 35 degrees from the direction of travel. The design dictated that the ply configuration be uniform throughout the tyre: if there were four plies under the tread, there was by necessity four plies throughout the sidewalls.
Most passenger car tyres were built with up to four plies of cotton cord, while trucks and buses rode on tyres with as many as twenty plies. The downside of a high ply-count was excessive heat build-up within the tyre. In order to reduce the ply-count while retaining the necessary strength, Michelin developed the first tyre with steel cord plies in the late 1930’s for large trucks and buses. This breakthrough allowed reduced ply-counts back down to the two-to-four range, greatly reducing heat generation and prolonging tread life.
Although early experiments with radially braced tyres date back to the early 20th Century, the development of the radial tyre as we know it today began in the 1940’s when Michelin embarked on a major fundamental research program in basic tyre technology.
The radial ply-orientation approach actually came about through the design of a special test tyre built under the direction of Marious Mignol to facilitate performance evaluations of tyre treads in isolation (i.e. without a sidewall attached.) In order to connect the tread to the drive hub, loops of wire were utilized in a radial configuration from the hub to the tread. Due to its appearance, the test tyre was dubbed the Cage À Mouche (flytrap), or CAM. This was the genesis of the very first production radial tyre.
Testing with the CAM revealed that, contrary to popular belief, the majority of heat built-up in conventional tyres was not from the tread area but was being generated in the sidewalls, where the multiple opposing-angled plies were generating intermolecular friction as they flexed within the carcass.
It became obvious that the key to an improved design lay in separating the design of the tread and sidewall areas according to their functional roles. Exactly, in effect, as what was done in creating the test tyre mock-up.
A decision was quickly reached to build prototype tyres based on these findings. They were constructed using a single cotton ply of radially-oriented fibres, replicating the wire loops used in the test tyre. To stabilize the tread, reinforcing belts using the steel cord of the heavy-truck tyres were added. Early test results indicated both superior grip and tread-life as compared to conventional designs.
The patent of radial concept was registered in Paris on 4th June 1946 in the name of Pierre-Marcel Bourdon, Michelin’s Directeur Technique, two months to the day after Frank Sinatra released his first record album.
In 1949, the revolutionary Michelin “X” radial made its public debut and was offered as optional fitment on Peugeots, the Citroën 11 and Simca 8.
It was the Italians who led the charge to fully embrace the advantages of radial tyre construction. Lancia was the first, specifying the X tyre as exclusive factory equipment on their new Aurelia.
Alfa Romeo soon followed suit, fitting the X to Alfa 1900 models. Due to the commercial success of their new radial, Michelin discontinued all further development of traditional tyres in 1952.
In 1956, the firm took a further step by ceasing production of bias-ply tyres completely in order to focus exclusively on their radial models. At the same time, the X was given a lower 78-series profile, and the cotton body-plies were replaced by synthetics. As the public grew to appreciate the handling characteristics and long-life of steel-belted radial construction, the X continued to increase in popularity.
It became standard issue on nearly all Lancias and Alfa Romeos, as well as Citroëns and Peugeots. Other tyre manufacturers had no choice but to respond, most notably Pirelli, with the textile-belted Cinturato. As competitors continued to catch up, Michelin announced a revised X in 1967, the ZX, with a softer–riding carcass and improved tread pattern.
Michelin’s second radial line, introduced in 1965, was aimed squarely at the sporting driver. The XAS was H-rated for sustained speeds up to 210 km/h (130 mph) and wore an innovative new tread pattern, the first commercially successful all-season asymmetrical design.
The centre of the tread was composed of a pair of large jagged ribs, with the inboard portion of the tread featuring separate blocks and large voids for enhanced traction and water dispersal. The unique part of the tread was the outboard shoulder that was formed of two more jagged ribs spaced little more than a millimetre apart.
This put more rubber on the ground where it is most needed in hard cornering. Beyond that, as the tread distorted under high lateral g-forces (which would only be achieved on dry pavement), the two nearly adjacent ribs would actually lock together forming a continuous band of rubber at the road surface. This not only offered improved corning power and steering response, it also reduced tread chunking, improving tyre life under hard driving conditions.
The 1960’s saw the completion of many key links in the new network of high speed autoroutes, autobahns and autostrada interconnecting the population centres of Europe. Following in their wake came a crop of new cars which could travel at sustained speeds of well over 210 km/h (130 mph); a group including the Lamborghini Miura, Ferrari 365, Dino 206, Porsche 911S, Mercedes 300 SEL 6.3 and others.
To address this market, Michelin introduced the XWX radial in 1970. To cope with continuous high cruising speeds, the belts were reinforced and yet another tread design was created. To forestall aquaplaning at elevated speeds, evacuation of water was give priority over noise abatement. As a result, no continuous ribs were utilised, only small tread blocks with a high void-to-tread ratio. Extensive hand-work was utilized in the production process of the XWX making them quite expensive; they sold for nearly twice the price of an XAS.
Through the 1950’s and 1960’s, tyre manufacturers continued adding capacity to produce more radial-ply tyres to meet demand. Michelin alone built fifteen new factories for radial tyre production between 1956 and 1970. As availability increased, radials began to be supplied on several additional automotive models as original equipment.
By 1971, all French cars, and all Alfa Romeos, Audis, BMWs, Ferraris, Jaguars, Lancias and Porsches came fitted exclusively with radial tyres. Fiat fit them on their more sporting models as well as the 127 and 128, and VW specified them on the 411 and 914. American manufacturers were slower to adopt the new technology: the only U.S. car to be series-fitted with radial tyres that year was the Lincoln Continental Mark III, which came equipped with 225-15 Michelins.
The radial tyre, along with the disc brake, was one of the key mid-century breakthroughs in automotive performance, providing much improved handling and steering response. After the Arab oil embargo of 1973, another radial advantage, lower rolling resistance, came to the fore. The increased fuel efficiency this provided finally led to adoption of the radial tyre throughout the industry, with American and Japanese manufacturers joining the Europeans in fitting radial tyres to all models.