F1 drivers are threatening to strike if their teams do not pay them, reports Germany's Sport Bild.Bild reports that Kimi Raikkonen's manager Steve Robertson was seen arguing in the paddock with Lotus team boss Federico Gastaldi at the 2014 Bahrain GP in April.Reportedly is was due to the fact that Raikkonen, who switched to Ferrari at the end of 2013, still has not been paid in full by Lotus for the 2013 season.DRIVERS TO STRIKE?The magazine claims that Romain Grosjean, Nico Hulkenberg, Adrian Sutil and Kamui Kobayashi are all waiting on overdue payments.It is believed that, under the auspices of their union, the Grand Prix Drivers' Association (GPDA), the F1 drivers have signed a document vowing to strike if the growing trend of not being paid continues.Sport Bild claims that Raikkonen (not a GPDA member) and one of F1's highest earners, Mercedes driver Lewis Hamilton, have refused to sign the document.Hulkenberg admitted that unpaid wages is an issue for the drivers: "Teams are aware of the situation, drivers can be easily replaced. Maybe not with the same quality but they (the teams) do take advantage of that."Hulkenberg also expressed some understanding for the situation of the teams: "They are not doing it (failing to pay drivers) for fun. The money just isn't there. The sport is too expensive."