F1 Technical: Why rear brake ducts are important

In this new episode of the “Technical Eye” series by f1i.autojournal.fr, we take a look at a seemingly insignificant, but crucial element: the brake ducts. While they capture air to cool the discs and calipers, their role is also aerodynamic… and thermal.

1 – COOLING

It is an almost invisible part. Its carbon black colour blends in with the dark rubber of the tyres, and it takes a trained eye to distinguish it. Despite its discreteness, the rear brake duct is at the intersection of several functions.

The first is obvious. Like its front counterpart, the rear brake duct is responsible for cooling the carbon discs, which is essentially done by convection.

It is through a network of ducts that the fresh air is guided to the parts to be cooled: the carbon disc and pads, the aluminium caliper, and even some electronic sensors. The parts on which the air is blown are numerous and carefully chosen, which explains the large number of internal ducts.

The inlet of the duct distributes the air to different ducts, each of which has a different function. The first of course is to cool the brake disc, so some of the air is directed towards the inner diameter of the disc and through the holes in it. This only serves to keep the temperature of the disc within its optimum window. Another duct captures the air and directs it to the caliper, which has a much lower operating temperature than the discs. So it needs quite a bit of air to be cooled.

2 – AERODYNAMICS

As always in F1, aerodynamics is a complicating factor, as there is a trade-off between sufficient cooling and efficient aerodynamics. The air that flows over the car is used to generate performance, whether it is cooling or aerodynamics.

Another part of the air captured by the duct is blown through the rim, as fast as possible. This blown air will take with it, in its wake, the turbulent air generated by the rotation of the wheels and move it as far away as possible from the flat floor and the diffuser. In addition, the rear brake ducts are equipped with a whole series of fins and small deflectors. Some are used to produce downforce on their own, others are placed near the diffuser to alter the airflow in that area. They help to close, seal, the diffuser, and as such play an extremely important role. This is why there are so many iterations in the wind tunnel. It really is a critical part, much more so than the front ducts.

The operation of these fins and other deflectors is particularly complex, insofar as these parts are in close proximity to elements that rotate (wheel) or that deform (such as tyre rubber).

3 – THERMAL MANAGEMENT OF TYRES

The design of the inside of the ducts therefore takes into account the cooling requirements and the aerodynamic effects sought, but also the thermal constraints linked to the operation of the tyres.

Indeed, the ducts not only supply fresh air to certain parts, but they must also redirect some of the hot air produced by braking to certain specific areas, such as the rims, in order to heat the inside of the tyre.

As well as looking for aerodynamic gains by directing air through the rims, teams also use the hot air released by the brakes to heat the rims and tyres. This air doesn’t have much velocity, but it is very hot (several hundred degrees) as it passes through the carbon brake discs. The rim radiates the heat and transfers it to the tyre carcass, which in turn transfers it to the tread.

Often coated with thermal primers, the inside of the rims are covered with turbulators and other protrusions to increase their surface area – either to radiate heat (this is the case at the front) or to dissipate it (at the rear), to put the tyres in the right operating window.

To withstand such temperatures, the ducts have to be insulated, using various techniques.

The brake drums are thermally insulated. If you let the heat from the disc radiate to the rim without insulation, the rim would literally melt and risk breaking. Similarly, some of the ducts inside the duct feature a sandwich construction, consisting of a layer of gold foil, a layer of silica aerogel* and a layer of carbon fibre.

The design of the ductwork is based on CFD (Computational Fluid Dynamics) results and bench testing to simulate the stresses experienced during a Grand Prix.

It should be noted that the analysis of flows in closed circuits (such as ducts) is historically the first field of application of CFD software, which was then perfected for use on open surfaces such as the bodywork.

As we have seen, ducts work closely with brakes.

(*) An aerogel is a nanostructured material that resembles a gel but where the liquid component is replaced by gas. This makes aerogel a particularly low density solid. With a very low thermal conductivity, silica aerogel is a very effective thermal insulator.