Multimatic - Ford Focus Aero Development
This is an interesting tale, unearthed by Paul Collins, with
help from Larry Holt at Multimatic.
is often referred to as a "black art." At the Minigrid Motorsports
Meet and Greet on Thursday May 22 (in Toronto), Larry Holt of
Multimatic had an interesting story to relate about the aerodynamic
development of the Ford Focus Daytona Prototype – prior
to the car’s debut race at the Rolex 24.
Typically, before finalizing a car's shape, Multimatic
would prefer to carry out some scale model and full model testing
in the wind tunnel. In this case, a combination of a late start
and early approval timelines imposed upon them precluded any
scale model testing, and Multimatic were completing their first
full-scale model when they had to submit their design to Grand
American. They received approval while in the middle of wind
tunnel testing of their body shape.
It should be noted that the design philosophy chosen
for the car was to 'focus' primarily on drag reduction. It was
felt that this would help to balance the conservative horsepower
allotment as specified in the rules.
Multimatic conducted their wind tunnel testing,
proving that Franco Acacia's design indeed had what Larry Holt
terms "industry leading (drag) numbers." Unfortunately,
they also discovered that the fastback shape with channels between
the fastback and the fenders produced lift in the rear! While
counterintuitive from a common-sense standpoint, it actually
makes sense from a fluid-dynamics (Bernoulli) approach.
The Bernoulli equation states that the energy of
a fluid system remains constant when you include frictional losses,
and the energy is divided into potential, velocity and pressure
energy. Essentially, energy can be traded between velocity and
pressure. Low drag means good airflow, and higher airflow speeds
over the fastback result in lower pressure.
In effect, the classic fastback shape makes the
entire car into a wing – and generates positive lift. Larry
quipped, “We were going to have to hire pilots for drivers.”
In an attempt to reduce this lift, the rear area
channels were filled in, by Bernie Marcus, to provide a more
continuous profile towards the rear of the car – and this
added significant downforce to the rear with, a minimal increase
Since the original shape of the Focus had been
approved by Grand American, another solution was sought to avoid
having to reapply for approval. A gurney was bolted on to the
rear, and tested at various heights.
It was found that this gurney had almost the exact
same effect as the filled in space, and that they gained the
same significant downforce with only a slight increase in drag. “Probably
the most cost-effective improvement we’ve ever made,” Holt
Incidentally, an attempt was made to reduce drag
at the front by filling in the scoops between the nose and fenders,
and it was found that reducing drag also reduced downforce -
at about a 4 to 1 ratio! These infill panels have since been
abandoned. Larry Holt calls the dive planes "the second
most cost effective aero improvement that we found."
Scott Maxwell is hosting Motorsports Meet and Greet
sessions this summer at Minigrid, with various people from within
industry. The next session will be in early July with Ron Fellows.
Those interested can sign up by contacting Minigrid staff by
phone or email as listed on their website.