The studded tyres mystery
- 21st February 2007, 11:34am
There’s no doubt that today’s super-hero drivers are the best in the world, but even they recognize that it’s not just skill and daring that helps them drive in such difficult conditions where temperatures sometimes drop as low as minus 30 degrees Celsius. Tyres play the greatest part in the equation and, thanks to studded tyres, off-road incidents are few and far between.
The Swedish and Norwegian rounds of the WRC take place on snow and ice for which the sport’s governing body, the FIA, permits the use of studded tyres. For both events, the grip produced by these studs – which measure no more than 20mm in length – enables the WRC cars to reach speeds of up to 200kmh!
With the exception of 1973, when the regulations banned studded tyres, studs have always played a decisive role on the Swedish Rally, allowing the cars to take the event's icy country or forest lanes at truly incredible speeds. Anyone who hasn't spectated or driven on the stages would have difficulty believing just how much grip is provided by these tiny metal tips which have been especially developed for rallying.
"The grip they give is identical, if not superior to what we have on gravel rallies," says three-time World Champion and 2004 Swedish Rally winner, Sébastien Loeb. "Even though the conditions are so slippery, our studded tyres enable us to brake extremely late and corner particularly hard. It’s simply gob smacking at first."
"Our data shows that the cars benefit from more lateral grip and stopping power on ice than they do on gravel," confirms one engineer. "The grip ensured by the studs on sheet ice is equivalent to that of a rain tyre on smooth, wet asphalt. That said, traction is less good than on gravel."
In the 1970s, the regulations were significantly more liberal in terms of studs. The so-called pyramido tyres bristled with 600 studs each and their tips formed a sort of claw. The studs were inserted from the inside of the casing and could be angled with a spanner. In the 30 or so years since, things have evolved a great deal.
Today's BFGoodrich g-Force Ice tyres come with 'only' 384 studs each. Strict FIA regulations specify that the studs must be of a cylindrical form with a maximum length of 20mm and a maximum weight of 4g. Their diameter may be no more than 9mm at their base and no less than 2.5mm in their middle section. The studs themselves must be inserted into the blocks from the outside. The FIA also dictates a maximum number of studs per tyre: 20 for every 10cm of the tyre's circumference which, in the case of the g-Force Ice, amounts to 384 studs per tyre. According to tyre pressure, the cars sit on between 18 and 20 studs per tyre, i.e. between 70 and 80 for the entire car.
For the past two Swedish Rallies (and this year Norway), the FIA has authorised just one tread pattern. Given the fact that snowploughs now pass through the stages before and during the rally, the drivers opt for the so-called 'ice' pattern, the g-Force Ice, the longitudinal blocks of which optimise traction and braking, while the oblique blocks contribute to the tyres' outstanding cornering grip. The g-Force Ice is 40 per cent narrower than an asphalt WRC tyre 145mm/225mm) in order to increase the pressure exerted on the contact patch by the car's weight.
More than the pattern, however, it is the studs (which strike the ice 17 times per second at 120kph) that play the most decisive role on Scandinavia's winter fixtures. The studs of BFGoodrich's tyres are glued inside the tread blocks, a delicate operation necessitating approximately 15 minutes per tyre and which is carried out by the Swedish company Däckproffsen. The tyres are produced in France and then sent to Däckproffsen who drill the tread blocks and insert the studs. Performance depends on the quality of the bonding between the metal studs and the rubber blocks.
Although it specifies a maximum length of 20mm for the studs, the FIA makes no restrictions concerning the proportion of the stud that protrudes from the tread block. BFGoodrich proposes three options regarding protrusion length: 'short stud' (5.5mm), 'normal stud' (6.5mm) and 'long stud' (7mm). In the case of a normal stud, 6.5mm of metal stands proud of the block and 13.5mm are glued inside the tyre. As a rule, drivers go for the long stud option in snowy conditions with a view to the stud piercing the top-coating of fresh snow and biting into the ice underneath. The normal stud option is used on sheet ice, while the short stud is only chosen in the case of stones mixed with frozen gravel.
The outcome of the recent Swedish Rally was decided when Grönholm and Loeb went for different stud options for SS11 and 12 which were predominantly icy with little snow covering. Marcus Grönholm opted for the normal stud (6.5mm), while Sébastien Loeb went for the long stud (7mm). The difference of 0.5mm may seem insignificant but the drivers can definitely feel it at the wheel.
"The extra 0.5mm produces a corresponding increase in the contact patch of about 11 per cent which has a considerable influence on traction and road holding," explains BFGoodrich developer Thierry Kindt. "Now, you could be forgiven for thinking that the more metal you have protruding from the tread the better performance will be, but this is not the case because certain limits apply."
The first concerns the extent by which the studs penetrate the ice. They need to bite totally into the surface if the tyre's tread is to be in contact with the ground. It is not difficult to conceive that if the car rests solely on its studs then it will be impossible to control at high speeds.
The second limit relates to the proportion of the stud that is inside the tread block and the way it is anchored in the tyre. "There must be a sufficient amount of metal inside the tread block if the stud is to be held firmly in place. Otherwise there is a high risk of it being pulled out," adds Thierry Kindt.
Last but not least, the third factor concerns the conditions. The longer option comes into its own when there is a sufficiently deep layer of fresh snow (2-3cm). Otherwise, the studs tend to move about in their housing (the tread block) and this makes the car unstable (which is what happened to Loeb on SS11 and 12 in Sweden). On a predominantly icy stage, the studs risk being broken or pulled out. So although the difference between the various options amounts to mere millimetres, the drivers' choices can play a decisive role in the final result.
For consistent performance in Sweden and Norway, the overriding objective for tyre manufacturers is to ensure that the 384 studs per tyre are still in place at the end of each group of stages. That may seem simple on paper, but it is more difficult in practice given that the 70 to 80 studs in contact with the ground at any one time not only have to transmit more than 300hp onto the ice, but they must also withstand fierce acceleration and braking forces and soak up the punishment dished out by corner cutting, deeply rutted ice and heavy landings after jumps.
The studs might be made from a tungsten alloy, the hardest metal on Earth, but their strength is not infinite.
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