WRC briefing: Snow tyres

On Sweden’s specialised stages, which normally consist of layers of snow and ice on top of gravel tracks, drivers will rely on studded tyres that bite into the surface to generate grip.

On Sweden’s specialised stages, which normally consist of layers of snow and ice on top of gravel tracks, drivers will rely on studded tyres that bite into the surface to generate grip.
Here’s is some essential info on these very special tyres:
Sweden is the only round where studded snow tyres are compulsory. Only one pattern and compound is allowed.
Each tyre is fitted with tungsten-tipped metal spikes that protrude from the tread by between six and seven millimetres. FIA regulations limit the number of studs to 20 for every 10cm around the circumference.
The number of studs per tyre varies by manufacturer. Michelin’s X-Ice North tyre and DMACK’s DMG-ICE tyre feature 384, while Pirelli’s Sottozero Ice tyre has 380.
All but one of the World Rally Car crews will use Michelin tyres. The exception is Henning Solberg, whose Ford Fiesta RS WRC will be shod with Pirellis. This will be the first time Pirelli will equip a World Rally Car in the WRC since 2010.
WRC-spec snow tyres fit the same 15 inch wheels as gravel tyres. The basic construction is similar to a gravel tyre but the tread is around two centimetres narrower.
WRC and WRC2 drivers are allowed 24 tyres for the rally plus another four for shakedown.
Snow tyre tread design is a compromise. While an open pattern is good at clearing fresh snow, it can also have a negative impact on the car’s stability because of the flexibility of the tread blocks.
The amount of each stud that protrudes also varies. The more they protrude, the better they bite into the ice. However, this also increases their chances of being pulled out of the tread or snapping off as they hit the ground.
The expert view:
George Black, M-Sport tyre engineer: “This is the easiest rally in the championship for tyre choice, because there isn’t any! There’s no stud option, no compound option and no tread pattern option. Nothing.
“Instead, the skill to master here is tyre management. This will be especially important on Saturday because with a section length of over 70km crews will ideally need six new tyres. With only four available, they’ll need to bring tyres forward, reusing ones from shakedown for example. In this weather, the drivers are also going to be busy rotating tyres between stages to even wear and allow them to cool down.
“Without a packed snow or ice base the studs face two different issues. The first is down to the gravel underneath. When frozen it is as hard as concrete, but even if it’s loose it can break off the stud tips. Okay, this is bad news, but at least there is some stud left, so they still work to an extent.
The second problem is heat-related. On longer stages the tyre temperature goes up and, especially in milder weather like we have now, the stud can start to move in the rubber blocks or come out altogether. If this happens before an icy section, then the driver could end up with no grip at all...”

wrc.com