Images copyright Frederic Le Floc'h (reproduced from the 1999 ACO Le Mans Annual), Franco Pizzagalli and Michael J. Fuller
Text copyright Michael J. Fuller
 The
ACO introduced the 2004 LMP 1 & 2 chassis rules in order to
address
some of the aerodynamic stability concerns that flat bottomed sports
cars
have encountered over the years. In 2002 the FIA released a
report
called, "Aerodynamic Instability of Sportscars at Abnormal Yaw
Angles".
The research was empirically compiled through scale wind tunnel testing
of a 40% model on a rolling road wind tunnel. The model was
yawed
through various angles relative to straight running and the research
determined
that the current flat bottomed prototype had a potentially dangerous
instability
when yawed past a certain degree. The instability was such
that the
vehicle could easily become airborne. Note that the yaw
instability
was a different phenomenon than that which was encountered by the
Mercedes
CLR at Le Mans in '99, the BMW LMR at Petit Le Mans in '00, and the
Porsche
GT98 at Petit Le Mans '98. Those incidents, whereby the car
flipped
end-over-end, were caused by a combination of external conditions in
conjunction
with the large flat bottom area of the prototype. The
blow-over accidents,
as well as the FIA's report, prompted further research at reducing
aerodynamic
instability and from that research came the core of the the 2004
aerodynamics
modifications. |
 For
new cars built to the 2004 LMP1 & 2 rules, the ACO
has eliminated
the traditional flat bottom replacing it with a regulated tunnel
system.
The idea was to reduce downforce generated outside of the car's
wheelbase.
With the old flat bottom, the rear diffuser started at the rear wheel
centerline.
For '04 the rear tunnels start 1000 mm in front of the rear wheels
(yellow).
Therefore the primary suction peak generated by the tunnels is well
within
the car's wheel base and not at the rear axle centerline.
This, coupled
with a reduction in rear overhang (to a maximum of 750 mm) as well as
an
increase in front over hang (to a maximum of 1000 mm) should reduce the
pitch sensitivity of the cars and minimize the chance of a blow-over
type
accident. |
 Additionally
the ACO is attempting to reduce the overall downforce generated by the
cars in efforts to curb speeds. One of the techniques used to
achieve
this is the mandate of a 20 mm plank on the bottom of the car (red
plank
in above Pizzagalli image). The plank is intended to force an
increase
in ride heights and therefore reduces the effectiveness of the
underfloor
aerodynamics. Secondly, the front splitter section forward of
the
front wheel centerline must be raised 50 mm over 1000 mm of
width.
Again the aim is at slightly reducing the ability to produce
downforce.
Lastly, the rear wing cord has shortened from 400 mm to 300
mm. The
shortening of the chord will reduce the efficiency of the
airfoil.
The rear wing has also been moved closer to airflow disrupting
structures
such as the cockpit and/or roll over hoops by the reduction in rear
overhang
as detailed above and therefore will be operating in a less efficient
flow
field. For '04 and beyond, open top cars will be required to
provide
roll over protection for the driver and theoretical
passenger.
The obvious implications being increased drag for '04 open top LMPs
over
previous years. |
 To
reduce yaw induced instability, the ACO have adopted a unique chamfered
floor section. In cross section, the bottom of the car is
effectively
wing-shaped. Therefore, when the car is induced into a yaw
situation,
the underfloor will be encouraged to generate downforce where as in the
past the top surfaces have generated lifting forces in excess of the
flat
bottom's downforce. An additional yaw stabilizer will be
large rear
wing endplates. The endplate rules specify a maximum size of
300
mm x 765 mm but also a minimum area of 1000 cm^3. |
