Aerodynamics of lamp pods in WRC cars

Night stages have become rather unusual in the current Championship, while, not many years ago (last one was in 1996) even a complete leg could be entirely developed during the night. The most typical example was the famous night of Turini in Monte Carlo, which in many occasions was decisive for the battle for victory. In the last years, apart from some very short night spectacle stages, only winter events still keep night stages in their program. The first two stages of Monte Carlo and at least one stage in Sweden are run with poor or no sunlight, so additional lighting systems are still required, to allow drivers to safely run into the forests.

Additional lamps or lamp pods have been a distinctive sign of identity of rally cars since its eruption. The image of iconic cars such as the Alpine Renault A110, the Lancia Stratos HF or the Porsche 911 Carrera RS that remains in many fan memories includes them as a sign of identity of a rally car.

It was normal for rally cars to include the light pods since early in the morning, and they were covered with protectors for most of the day, for the happiness of publicizers, who rapidly found an additional location to include advertisements. How to forget the names of light manufacturers such as Cibié, Hella, Lucas or PIAA?

The usual location of light pods and light bars, at the front of the car bonnet, represents a significant disturbance for the air flowing above the car, for what their size and design have an impact on the car aerodynamics, which is worth to review.

The current WRC car generation has introduced different alternatives for increasing their lighting capacity, and manufacturers have opted for different solutions, some of which have evolved since their first appearance in 2017.

Rallye Monte Carlo 2017 was the first opportunity to see the new WRC car generation, and lamp pods/bars were already present on that occasion. Three manufacturers, Citroën, Hyundai (with PIAA lamps) and Toyota (also PIAA), opted for the classical Halogen or HID (High Intensity Discharge) lights. An excellent and detailed explanation on the different type of rally car headlights can be found here.

Hyundai was the only team to include a rear cover to the lamps (lamp pod), which surely contributes to have a smoother air flow behind, which in the end reduces its impact on the car aerodynamics. Toyota and Citroën do not use such cover, so the lamps are simply connected to a bar (light bar), and the impact on the aero of their cars is probably higher.

M-Sport was the first to introduce a lightning systema based on LED (light-emitting diode), from Lazer. LEDs have the advantage, amongst others, that are smaller than HID bulbs, thus representing a smaller obstacle for airflow, which in the end means a lower resistance (drag) and a gain in speed. But including two rows of lights reduce such advantage, due to the high frontal area, and it do not help very much in reducing drag.

In addition to lamp pods, additional lamps are usually required to illuminate both sides of the road. Hyundai and Ford opted for integrating such lamps into the body, contributing to a lower drag.

On the contrary, Citroën and Toyota added external lamps for side illumination, thus increasing the drag of the car, due to its prominent position. In the case of the Citroën, the location of the side lights even affected the performance of the dive planes, as they represent an obstacle for air flowing to these planes.

For the 2018 season, Toyota solved the problem of the side lamps of the Yaris, as they were removed from their original location and relocated into a lower position, allowing them to be integrated inside the front bumper and reducing the resistance to air flow (drag) significantly.

Citroën did not corrected it until 2019, when side lamps have been finally integrated inside the front bumper, allowing the dive planes to perform more effectively in their function of generating downforce at the car front. At the same time, Citroën has included two additional side lamps into their lamp pod (which includes now six lamps, as the Yaris or the i20), which has increased air resistance, due to the higher frontal area.

Also in 2019, Hyundai has shifted from HID lamps to a light pod based on LEDs, also from Lazer.

However, the new system possibly did not convince Andreas Mikkelsen, or maybe they suffered a lack of spare parts. In any case, in Rally Sweden 2019 the Norwegian driver came back to the original system, based on HID lamps.

Aerodynamics of lamp pods

On a rally car with no lamp pod, air flows over the car bonnet, following exactly the shape of the bonnet, in what we call an attached flow. The pressure distribution over the frontal part of the bonnet usually shows high-pressure values on this area, due to the impact of air flowing against the car. This means this part of the car also contributes to downforce generation.

At night, due to the presence of lamp pods and bars, air cannot follow the shape of the bonnet, and, as a consequence, it becomes separated (detached). The first impact is that the pressure on the bonnet is reduced. This means that less downforce is generated, as air do not push onto the bonnet once separated.

In addition, air leaving the pod generates an unsteady wake. The smooth flow turns into turbulent, energy-consuming flow, with some recirculation behind the pod and lower air speed. The front area of the pod, together with the energy cost of the wake, translates into a higher resistance to flow, that is, a higher drag.

In conclusion, the presence of lamp pods/bars has a certain impact on the aero performance of WRC cars, both by increasing drag and reducing downforce. For this reason, teams devote significant efforts to minimize the impact of such devices. A clear example can be found in this video released last year by Ford Performance, from a wind tunnel test developed in Windshear facilities located in Charlotte (US) with the Ford Fiesta WRC. Images show how engineers develop visualization tests over the lamp pod, to visually evaluate its impact on airflow.

In addition to the impact on aerodynamics, the addition of a lamp pod on the nose of the car has other impacts. With a weight around 7 kg, lamp pods
also act as ballast, modifying the car’s handling characteristics.

Fortunately for the engineers (but unfortunately for rally fans), lamp pods/bars use is limited to a very few stages during the year. Also, their impact on performance seems to be limited, but not negligible. In any case, two of these night stages are the first two stages of the season (at least it has been in the last five years), so there is no better way to start the season than taking it seriously and optimizing the design of the lamp pods for a minimal impact on aerodynamics.