The Underwing and Diffuser
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This is an excerpt from Inside Racing Technology.
Here is an underwing cross section for reference and two sketches
showing cross section shapes and rear views of a tunnel car and
a flat-bottom car with diffuser. Indy car sidepods and '95 Formula
1 flat bottoms have 2 inches ground clearance because the rules
mandate the bottom of the sidepod at 2 inches above the bottom
of the car.
The lower sketch shows an underwing similar to that of a current
Indy Lights car and that of an F1 car prior to the "plank
rule" created in mid-1994. It's a flat plane starting with
a "splitter" and ending in a diffuser section. The
splitter is a lip at the front of the underwing that causes a
clear division of air going under the car and air going around
the car. Maximum downforce demands minimum ground clearance for
the splitter and a specific "rake" of the flat part
of the underwing.
The purpose of the diffuser is to allow the air that has been
accelerated (so that its higher speed can produce a lower pressure)
under the car to decelerate back to close to the same speed and
pressure it was before the car ran into it. The better the diffuser
works, the less drag the underwing produces.
You also see lateral fences or strakes in the diffuser area
at the rear of the car. These have been banned from Indy cars
(that means they worked), but Indy Lights and F1 cars still use
them. The strakes probably do several things, but one of the
most obvious is that they prevent low-speed, high-pressure air
getting in along the sides of the car from disturbing the flow
in the central area of the diffuser.