LONDON, England - The plan to ban tyre warmers in 2015 is in doubt.This week at the Bahrain test, after Williams test driver Felipe Nasr tried an experimental Pirelli tyre, the Brazilian said it was "very difficult to get them up to temperature, even in the Bahrain heat"."It will be even more of a challenge at tracks such as Silverstone or Spa," Nasr warned.ON AGAIN, OFF AGAIN?Pirelli boss Paul Hembery said the main issue is tyre pressure, as the difference between a cold and a hot tyre is significant.Hembery told Speed Week: "At present we believe it will be difficult to implement the FIA's plan for 2015."F1's governing body wants to get rid of expensive tyre warmers on cost grounds, but Hembery said safety is a consideration."We need to consider how the tyre behaves during, for example, a safety car period, when the pressures and temperatures fall," he said.Told, however, that other racing series cope without tyre blankets, Hembery responded: "Yes, as in GP2, but we are talking about different tyre sizes, and the pressure at the start is much higher."One possible solution is that, along with the warmer ban, F1 simultaneously ditches its "old-fashioned" tyre design and switch to a low-profile.Williams' Pat Symonds said: "If we had a lower profile tyre with a stiffer sidewall and a lower-volumeair cavity, it would certainly be much easier to manage."Mercedes' Paddy Lowe, however, warned: "Introducing the low profile (tyre) is a very non-trivial task in terms of engineering and cost, so not an easy one."Stay with Wheels24 for the 2014 F1 season – fresh reports every day.