It?s the ultimate show on ice: watching the best drivers in the World Rally Championship charge down the frozen tracks deep in a Scandinavian forest at speeds in excess of 120kph. That is what?s in store on the Swedish Rally (8-11 February), and the extreme conditions make the Karlstad-based event one of the most spectacular on the WRC calendar.

It takes skill, precision and a rather special design of tyres to keep the Subaru Impreza World Rally Car on a road that is covered with sheet ice. For the Swedish Rally and the following weekend?s Rally Norway, the cars of Petter Solberg and Chris Atkinson will be fitted with studded tyres from BFGoodrich?s g-Force ice range.

There are 380 studs in each tyre, and 50 of those tiny studs are in contact with the ground at any one time. At a speed of 120kph, each stud strikes the ground 17 times per second, biting into the icy road surface and providing an astonishing level of grip on a surface that is treacherous and slippy.

?The ultimate level of grip is quite similar to a gravel rally,? explains Richard Thompson, senior rally engineer for the Subaru World Rally Team. ?Sometimes the event organisers will spray a lot of water onto the stages to make them freeze, the reason is two-fold: firstly, in order to protect the gravel road underneath. If the cars wear through the protective layer the roads become damaged quickly. Secondly, it is to try to maintain constant conditions for all competitors. If there?s a good ice base, the grip level will go up. When you?ve got soft, heavy falling snow on and warmer conditions the grip level tends to be a bit lower.

?The tyres we use are very narrow ? that ensures we have the highest pressure area on the road to enable the tyre to bite into the snowy and icy surface. The blocks of rubber on the tyres still work in a conventional manner and provide some grip when there is snow, but they also have the studs set into them which reach through the snow to bite into the ice.?

The key decision is how far the studs should protrude from the tyre, a decision that is dictated mainly by the conditions on the stages.

?The studs can protrude out of the rubber to a maximum of 8mm,? says Richard. ?We would use that on a very snowy stage, to bite through the snow deep into the surface. However because the studs protrude more, you also have more leverage on the block and, when the tyre warms up, the studs can start to flex. So tyre choice is a balancing act of having good drive from the studs, but also good wear and stud resistance.

?Also stud retention is very important. In certain conditions the studs get worked very hard and can get ripped out of the tyre block. Generally the studs are put into the tyres by hand, using a machine that pneumatically fires the stud into the block, and then are set in place with glue. The tyre manufacturers are continually working on different compounds of glue and different stud-setting techniques to try to retain the studs for as long as possible.

?If it turns out to be a warm event where the layers of snow and ice are thinner, on the second run through the stages some patches of gravel might show through on some of the slower corners. The studs are given a really hard time in those conditions. If we have conditions like that we would use a tyre which has the studs set in deep, perhaps with only 5mm of the stud protruding, so it will last longer.

?You can also go for an asymmetric stud pattern, where the studs on the inside of the tyre are set a little bit deeper into the rubber than those on the outside. The ones on the outside work more in cornering and thus are not so active. When the car is in a straight line, under traction and braking, you?re working the inside of the tyre more and therefore the duty cycle and ultimate wear is greater. It can help if the studs are set deeper to counter this?

Another important factor on the snow is how comfortable the driver feels. The Swedish Rally has the second-highest average speed of all the WRC events, so a well set-up car is also vital.

?The most important thing is to pick a tyre that will give the driver the most amount of grip, response and confidence he needs,? Richard says. ?Over one stage it is quite easy, but over a loop of three stages it?s a lot tougher. If it is warm and we see some frozen gravel coming through the snow, then road position becomes important too, it could be beneficial to be nearer the front, however this may not be the case if there is a lot of loose snow on the surface!

?The Swedish stages are very smooth, so we run the cars quite low. You don?t see the damage on the underside of the car that you would on a gravel rally. There?s a couple of famous jumps where the fans go and watch, particularly Colin?s crest, a very high speed jump with a left and right corner over the blind crest! and you?ve got to factor in that and make sure the car is controllable over the jumps, but you really need a car that will make the drivers feel confident in long, fast corners. The driver needs to know the car is going to keep in line so he can keep the throttle planted and ultimately turn in the fastest stage time.?