24

Secrets of Racing

Nevertheless, no F1 designer would ever underestimate the benefit to be
gained by reducing the frontal area of the car. This prompted 1986 Williams
FW11B drivers Nelson Piquet and Nigel Mansell to try and sit as low in
the car as possible, each hoping to out-do each-other. Mansell discovered
that by removing his seat, he lowered his position in the car by just 1.5
centimeters, which translated into an additional 25kg of downforce. (F1
Nutter [2])

24.1 Downforce

Formula One racing cars have the power to produce wheel spin for velocities
higher than 100 mph. To allow the tires to transmit a greater thrust without
wheel spin, and thus to increase acceleration, a major design goal for a F1
racing car is to maximize downforce, that is negative lift. downforce is also
crucial for increasing the cornering ability.

The key to create downforce for a F1 car is the design of the front and

rear wings, which are regular wing profiles turned upside down and thus gen-
erating negative lift. The front wings contribute to 25–40% of the downforce,
and the rear wing to about one third [3]. Although, there is a trade-off be-
tween downforce and drag (Fig. 24.1), so that increasing downforce through
the design of the wings result in higher drag. Thus the specific design of the
wings is frequently modified depending on the driver and the track.

Another important part of the car for downforce is the diffuser, which is

the rear section of the car’s floor where air flowing under the car exits. The
exit speed of air is controlled by the design of the diffuser, with a higher speed
corresponding to a lower pressure under the car, by Bernoulli’s Law, and thus
a higher downforce.

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24 Secrets of Racing

Fig. 24.1. Downforce and drag for F1 car.

24.2 The Wheels

The wheels of the car are considered to be the major sources of drag, and
in addition the exposed wheels result in a significant lift force, reducing the
downforce with about 11% for a typical F1 car [3].

Wind-tunnel testing of the wheels is associated with several problems:

(i) letting air flow past a stationary car in a wind tunnel will induce a boundary
layer near the floor, not present for a moving car, and (ii) the flow past a rotat-
ing wheel and a non rotating wheel is different. More advanced wind tunnels
aim to overcome the difficulty (i) by trying to eliminate the boundary layer
by techniques of blowing and suction, and (ii) by constructing a moving belt

24.3 Drag and Fuel Consumption

197

so that the wheels can rotate. Even so, some problems remain and new ones
are created, for example, typically a moving belt construction cannot carry
the weight of a full size car leading to additional constructions supporting the
car influencing the validity of the flow measurements.

The use of CFD is thus very attractive; with a reliable and efficient compu-

tational method, a computational model of the car is built in the computer for
which the aerodynamic performance of various designs is easy to investigate.

In Fig. 24.2–24.3 we present results from a G2 computation of the flow

of air past a model of a rotating wheel. We compare with a computation
for a corresponding stationary wheel, modeling wind tunnel testing. We find
that the two flows are completely different, indicating that the results from
a simple wind tunnel testing may be completely misleading. We describe the
computations in detail in Chapter 35.

We note that drag is significant, and that also the generated lift from the

wheel is high.

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

−

0.5

0

0.5

1

1.5

2

Fig. 24.2. Aerodynamic forces on the wheel: c

D

≈ 1.3, c

L

≈ 1.2 for the rotat-

ing wheel, and c

D

≈ 0.8, c

L

≈ 0.5 for the non rotating wheel. The side force is

approximately zero for both cases.

24.3 Drag and Fuel Consumption

For a regular car, downforce is not the critical design criterion. On the
other hand, the drag of a car directly couples to fuel consumption, and thus

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24 Secrets of Racing

minimizing drag of a design is crucial for fuel economy, and reducing the
environmental load from carbondioxide emission.

Fig. 24.3. G2 solutions of the flow past a wheel: snapshots of magnitude of the
vorticity for the rotating (left) and the stationary (right) cylinder.