Why Mercedes' wheel rims have become F1's latest tech controversy

Both teams have pored over their own and each other's cars in pursuit of tiny performance boosts, which has led us to F1's latest tech controversy - wheel rims.

Shortly after Mercedes' surprise United States Grand Prix defeat, in which it struggled to manage its tyre temperatures, it emerged the team covered up controversial holes in its rim design amid questions about them being moveable aerodynamic devices.

Ferrari queried the use of the holes, but the FIA told Mercedes on the eve of the Austin race it was satisfied the concept was fully within the regulations.

But Ferrari questioned that interpretation, and with Mercedes aware a rival team could lodge a post-race protest to get a definitive ruling it elected to cover up its holes to guarantee any potential title celebrations would not be overshadowed.

Mercedes made its first step towards its current wheel-rim design ahead of the Belgian Grand Prix, when it made a fairly innocuous change to its rear rims.

A ribbed concept, which featured large fins added to the rim's outer face, making the wheel act like a heat sink.

This helped use the neighbouring airflow to alter the rim's temperature, which in-turn has an effect on the tyre's core temperature.

It was something Mercedes had for a number of years on the inner face of the rim, albeit with much smaller fins due to the small gap between them and the brake drum.

But the Spa change was only the first step, and from Singapore the team raced a new design - featuring a series of holes in the wheel - that worked in conjunction with new wheel hubs to improve its rear-tyre temperature management.

The use of holes on the inside of the wheels caught the attention of rival teams, which questioned whether they qualified as a moveable aerodynamic device - a catch-all statement that has been core to many FIA bans over the years.

It was suggested the Mercedes concept resembled Red Bull's blown axle concept that was banned by the FIA in 2012.

At the time, the FIA determined that holes drilled in the side of the wheel that mated with the ones in the axle were considered to be a moveable aerodynamic device.

Mercedes' design does appear to be different, though, because the wheel hub itself is not moving.

The first piece of the puzzle is how airflow is transported between the brake assembly and the wheel rim. For Mercedes, it is not done through the axle.

Instead, it appears airflow passes through the brake bell holes that are not occupied by the wheel's locating lugs.

The shim, or ring, that is usually mounted on the inner face of the wheel rim (inset) and features holes that are plugged by the team, has been replaced by a diffusion ring.

This has much smaller holes drilled into the surface that allow air to enter the wheel rim's core chamber and then pass out through holes in the wheel's centre bore (arrowed).

Taking into account an overriding design feature of the old- and new-specification wheels, we can see how the two can be used to transfer heat.

Mercedes has long used a hollow wheel spoke design (red arrow), which gives more freedom for the air to move around between the rim and the tyre, helping to control the core temperature.

However, note that in line with the new spoke and outer fin design, the aperture has been extended to match the indentations that now exist within the rim to maximise the transfer of heat from the fins (white arrow).

Considering the proximity of the hollowed-out spokes at their other end, they essentially meet with the wall of the aforementioned central chamber. This then becomes a conduit for which the core temperature can be exchanged, altering the core temperature of the tyre which can result in a performance and durability improvement.

Specific details about why the FIA deems the design legal, and what grounds Ferrari have to question it, have not been released - but there could be some debate about the primary function of what Mercedes is doing.

The problem for the FIA is that the primary function of Red Bull's original design was aerodynamic, allowing it to use the catch-all 'moveable aerodynamic device' clause.

But the aerodynamic effect of what Mercedes is doing is actually a secondary effect, with temperature control for the rear tyres appearing to be the primary purpose.

It remains to be seen whether Mercedes will use the design again at this weekend's Mexican Grand Prix.