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Formula One Aerodynamics (Part 2)

In the first part of this discussion, we saw how much of the body design of the front of an F1 car is intended to ensure a smooth flow of air to the most aerodynamically important part, the rear. What we rarely see is what happens underneath the car.

In terms of the FIA’s rulebook, the undertray must be flat until it reaches the rear axle. At that point, the designer is allowed to sculpt the underside upwards, creating an enlarged space for the air and thereby forcing it to speed up in its need to fill the available volume. When airflow accelerates, pressure drops – the result is downforce.

This is fine except that there are a few things that get in the way of a completely smooth upturn of the undertray. One is the gearbox, which protrudes beyond the rear axle, and the others are the rear wheels. Leave the gearbox sticking out into the airflow and it will create turbulence, robbing you of some of your downforce. The designers minimize this by dropping two plates from the sides of the gearbox, thus separating it off from their carefully-created smooth flow of air. This can be seen quite clearly in this photograph of the rear of this year’s Toyota.

Toyota rear view

Toyota

Note that there is a wing, not normally seen because it is usually carbon fiber black, at the bottom of the rear wing endplates. The effectiveness of this wing depends upon the smoothness of the air arriving from underneath the car; hence all this fuss about airflow.

The wheels are a different story. Being an open wheel formula, they have to stick out in the breeze and there is little that can be done except to keep as much air away from them as possible. Wheels create a huge amount of turbulence and spin off vortices because they are turning as well as moving through the air. And the lengths designers will go to in the quest to keep air from the wheels can be seen in this photo of the Renault R26.

Renault rear view

Renault

All those protuberances and winglets are to keep the air from going where it’s not wanted. And the flip-ups just ahead of the rear wheels encourage the airflow over the wheels rather than allowing them to meet head on. But notice how the sidepods and bodywork squeeze inwards as they approach the rear, thereby sucking in the flow away from the wheels and directing it to the rear wing.

Finally, there is that sharp-edged airbox to consider. This is where one of the very few laws of aerodynamics, Kamm’s Law, is most obvious. Working in the 1930′s Professor Wunibald Kamm discovered that airflow would follow a backward facing slope until it reached a maximum angle of 7º; any steeper angle would result in the air breaking away and becoming turbulent. This resulted in those strange entries for the fuel efficiency formula at Le Mans in the 1950s and 60s – tiny-engined cars with long, tapering tails.

But the F1 airbox demonstrates that you don’t just have to consider the rearward slope; by sloping the sides as well, you can greatly reduce the length of bodywork required to let the air flow around the airbox without breaking away. And this is the reason why the airbox ends in that razor-sharp spine at the rear.

Car aerodynamics is an arcane but increasingly important part of F1. The wind tunnel has become king and is the reason for the suggestively organic shapes that comprise the modern F1 car. Often it is impossible to understand why one shape works and another doesn’t. But be assured of this: if you see it on an F1 car, it works all right!

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