Sipe activators The sipe activators that have been found to work well in several Nokian Hakkapeliitta winter tire models are also included in this tire. They are located on the shoulder area and centre surface of the tire. Sipes are the cuts on the tire tread that improve grip. Sipe activators ensure driving safety under rapidly changing weather conditions, in which the wider sipe activates the narrower sipe. This functionality adds grip for extreme situations in the most slippery weather, when the feel for the road is nearly non-existent.