There has been a lot of focus on Mercedes wheel rims since images (via SomersF1) appeared to show a hollow section inside their rear wheels. However looking up close at both Mercedes front and rear wheel rims at Monza showed a new and unique solution. All the wheels sported cross-cut pattern on the inside of the wheel and a black paint coating to this inner face of the rims.
Closer examination proved the hollow rim theory to be unfounded. What appeared to be an opening on the rim into a double-walled hollow section, provide to simply be tape placed inside a recess made into the rim. This recess is used to place the lead weights used to balance the wheel when fitted with the tyre and valves. All teams use this sort of design. Why Mercedes chose to place tape over this recess\wheel weights isn’t clear. My assumption is that low friction tape was used to prevent the brake duct rubbing on the rim and to protect the duct from scratching from the wheel weights.
From these pictures it’s clear that the all rims sport this finished, which from its soft edged texture must be made when the rims are forged by Advanti. This surface extends from the wheel weight recess to the spoke within the rim. This is the area largely filled by the cake-tin brake ducts. I crosschecked this wasn’t a brand new wheel specification with Ross Brawn and he confirmed they are not new, but are an in-season development. Images from Hungary showed the wheel were used there too (SomersF1)
The purpose could be two fold, either aerodynamic or thermal. Due the wheel spinning around a static brake drum, the air between them is sheared by the movement which will create drag. The pattern could be a means to break up the airflow and reduce the friction and drag between the wheel and brake duct. This could be a valid explanation but doesn’t explain the black paint on the inside face of the rim, not seen on other specification of Mercedes wheel. Thus thermal management could be the more likely explanation. This year thermal management of the tyres has been critical and controlling heat transfer from brake to wheel to tyre has been a key practice in managing this. The greater surface area provided by the texture and the heat absorbing black paint would each contribute to greater heat transfer between brake and tyre. Mercedes have historically run very high tyre temperatures, so this explanation may at first be contradictory to the teams needs. But wit the new specification Pirelli tyres running cooler and the onboard thermal image cameras from free practice showing tyres temperatures falling between corners, there may be a need at Monza to retain more heat in the tyres. This wheel specification would achieve just that.