The controversial Mercedes rear wheelrims first appeared at the Belgian Grand Prix, and at the Mexican GP the team asked stewards to investigate them, resulting in them being ruled legal. It’s always difficult to quantify what is a moveable or moving aerodynamic device. The basic principle of cooling a brake disc by passing air through the holes in the disc means that, like the wheel, which at the very least has spokes, they are a moving aerodynamic device. But they are not moveable and the geometry is consistent when the car is stationary. Adding holes in the wheels is just exaggerating the wheel-spoke design.
Managing the tyre temperatures for both one lap in qualifying and over the full race is not easy. You want the tyres, especially the fronts, to warm up quickly for a qualifying lap, and over a race distance you want the rears in particular not to overheat. It is fairly easy to get the rears to come up to temperature by just spinning the wheels, but you don’t have
that option with the fronts. So getting them up to temperature is all about using the brake-disc temperatures to influence the wheelrim temperatures, which increases the tyre temperatures. That’s the reason why Mercedes has concentrated on only the rear wheels. Basically, it has vented some cool air directly from the brake-duct inlets through the wheel spokes just outside the diameter of the wheel-retaining nut. This acts like an insulator, reducing the heat transfer that the hot brake disc and axle have on the wheel mass.
With the brake discs running at something like an average of 650C and even higher just at the end of braking at corner entry, the cooling air is then carrying some of that heat with it through the wheelrim.