ATLAS F1   Volume 7, Issue 9

  Taking the Lid Off F1

Formula One Technical Analysis

  by Will Gray, England

Last year, Atlas F1 ran a series of articles that investigated the technical areas involved in design, development, and construction of an F1 car. Now, a year later, Will Gray picks up where he left off, and dives deeper into the technical analysis of Formula One.

Part C4: "Passing Problems: The Overtaking Phenomenon"

Formula One racing has recently come under scrutiny due to a lack of it, and rules are constantly being changed in an effort to reduce speeds and increase it: We couldn’t finish a technical series without discussing the buzzword of the moment - overtaking!

There are many factors influencing the performance of a Formula One car, and current changes concentrate on the mechanical grip of the car. However, it is an acknowledged fact that aerodynamics plays a big part in the problems of overtaking, and since the introduction of aerodynamic principles into Formula One, they have become of ever increasing importance to the car's performance. As explained in a previous part of the series, the aerodynamic requirements of the F1 car are high downforce at minimum drag - but the more downforce you have, the more drag you incur. This drag is due to air being disturbed as it flows over the vehicle, and as the air does not have the energy to sort itself out, it ends up following the car as its wake.

Any protrusions into the airflow can affect the flow over the car, and therefore the wake following it. In an F1 car all the aerodynamic devices are so close together that there are large interactions, and the rear wing, rear wheels and floor-level diffuser contribute significantly to the shape of the wake. As the air travels over the car, it will lose energy, and cannot return to its initial values of speed and pressure. Therefore, by moving through the air, a Formula One car (like any car) will trail a region of low momentum air behind it, and this is what the wake is.

Any vehicle which produces lift (be it positive in most road cars, or negative in racing cars) will produce vortices - the phenomenon you see when water swirls down a plug hole. The F1 rear wing, for instance, will produce a significantly large pair of strong vortices called trailing vortices which last a large distance behind the wing which created them. There are often seen as swirls of white on the edges of the rear wing when the air is moist, and were a regular sight back in the early 1990's. In an F1 car, they rotate in such a way as to produce an upwash (air travelling rearwards and up relative to the ground) in the centre behind the vehicle and a downwash on the outside. In addition to this, the flow underneath the car plays a large part in the shape of the wake. The F.I.A. Regulations state the major percentage of the car floor must be flat, but diffusers are allowed. The air channels out of the diffuser and is immediately sucked upwards due to the influence of the rear wing, adding to the already large upwash in the centre of the wake, which creates a low pressure in that area. As the wake moves with the car, this causes a drag upon it, and will also cause problems for the vehicle behind.

The effect on the following car is beneficial in a straight line as it will get a 'tow' from the car in front, and its drag will be reduced by following in this turbulent air. Drivers say the 'tow' from the low pressure wake (often referred to as a slipstream) can be felt up to six car lengths behind, but it is only significantly large when trailing by approximately one to two car lengths. However, it has been suggested that tows from older F1 cars were larger (which makes sense, as they had more downforce), and this is one problem suggested for the lack of overtaking in present day Formula One. Down a straight, a bigger tow makes it easier to close on the car ahead, and if you have less tow, it is harder to get close to the car at the end of the straight when you are coming into the corner, so harder to get into a good position to overtake.

In cornering, however, the effect of the wake is very detrimental, and drivers complain of understeer (brought about by a loss of downforce due to wake effects) up to three or four car lengths behind. The car's downforce decreases because the air the car (especially the front wing) is working in has less energy than normal, and the air separated from the wing surfaces earlier than normal. The car cannot sustain wheel side forces similar to the car ahead, so cannot travel as fast through the corner. It will therefore drop back and not be in a position to overtake once on the straight - another aerodynamic problem once again giving the upper hand to the car in front (not the best plan if overtaking is required!). A very public display of car-to-car interactions appeared when a new aerodynamic device was run in the American Champ Car series. The Handford Device is basically a vertical plate spanning the rear wing, aimed at killing its downforce and increasing its drag. The aims were achieved, and as is obvious from the previous comments in this section, a significant wake was created behind the device. Because there was very little upwash (downforce creates upwash, and this is reduced by the device), the following car runs straight into the wake and experiences a significant 'tow'.

It is important for teams to understand their car's behaviour in following another, and there have been many methods used in attempting to physically simulate the situation. These include full size car to car tests, car to car model wind tunnel testing, wake simulation using turning vanes and solid metal blocks (to simulate the wing and blockage at the rear end of a car), and CFD testing. However, as time is seriously limited in Formula One, most teams neglect this area of study, and put passing problems down to just being an awkward consequence of downforce.

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Will Gray© 2007
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