Just before the start of the US GP, the FIA demanded Mercedes closes the holes on their rear wheel rims with silicone, to avoid the negligible aerodynamic effect and it looks like it pushed the Silver Arrows team back into crisis they were in before Belgian GP, when they introduced the new rims and hub to aid control tyre temperature.
FIA clarified at the beginning of the week, that the Mercedes rims are legal, despite featuring special holes, similar to the rims Red Bull wanted to use in the 2012 Canadian GP, but were deemed illegal due to the fact the holes have an aerodynamic effect on the air flow. Lewis Hamilton finished third in Austin, after making two stops and suffering heavy blistering to his rear tyres, and his team mate Valtteri Bottas was faking the same fate, loosing 4th place against Ferrari’s Sebastian Vettel in the dying moments of the race.
Mercedes introduced the new rims in Belgium, and they were unbeatable since then, with no sign of the blistering problems they’ve had before. Pirelli increased the tyre pressures by 0.1bar, because they expected problems with overheating as the temperatures have risen by ten degrees compared to saturday.
Hamilton was the only top driver to have made two stops during the race, because his rear tyres quickly developed blisters and begun to vibrate. The interesting part about it is, the FIA made the demand very quietly, without any public communication, but it looks like removing the “negligible” aerodynamic effect the holes had (which is prohibited by the technical regulations) was enough to send Mercedes back into crisis.