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This is an excerpt from Inside Racing Technology.
Denny Hulme on McLaren's Accidental Downforce
Maybe most people don't think about technical stuff when they
go to the Monterey Historic races at Laguna Seca, but I do. I
like to look at the basic design and construction of the cars
and what type of engines and suspensions were used during various
eras of motor racing. Some designs, like the twin overhead cam,
four-valve-per-cylinder engine, are not as new as you might think.
At the 1992 Monterey Historics, Denny Hulme was there to drive
a 1972 McLaren M-20. Hulme was, of course, Formula 1 World Champion
in 1967 driving for Brabham, but is better known in the United
States for his achievements in the Can-Am series driving a McLaren.
I wanted to ask him about a story in an English magazine where
he had talked about how, in the late '60s, they almost stumbled
onto ground-effects aerodynamics with the McLaren M8.
In that piece in Racecar Engineering, Hulme said, "At
Goodwood one day, we squared off the bottom of the tub with a
piece of aluminum and she picked up some time. We popped it off
again and dropped time, so we put it back on. We didn't understand
what was happening, but looking back now, technically, I think
we got very close to a ground-effects car. We didn't run hard
suspension on the McLarens, but it was hard with a full load
of fuel. The springs were a joke and the shaft of the shock absorbers
had Aeon [bump] rubbers and, with a full load, the car sat solidly
on these which meant that there was fixed suspension. The M8
always ran its quickest when you put the 80 gallons of fuel in
and jammed it down onto the ground.
"The first couple of laps were always your quickest of
the race. We put that down to having new tyres on, but what we
totally overlooked was that we had made the car into a ground-effects
car and it clamped itself onto the road through running so close
to the ground. When you could hear it scrape, you felt the car
was going at its best, but we didn't like doing that because
we were wearing the tub out. We didn't have rubbing strips underneath,
so the grounding was knocking rivets off."
I looked around for Denny Hulme that day at Laguna Seca, and
found him standing in the paddock talking to a guy. I waited
patiently until they paused in their conversation, and Hulme
looked toward me expectantly. He was about 6 feet tall, hefty
but not fat, and had that familiar hawk nose and jutting jaw
that I'd seen in all those photos in magazines.
I introduced myself and asked if he could tell me more about
the ground-effects car that almost happened. "We were there,
we just didn't know it," he said. "We had a big radius
on the body along the sides of the tub, and we squared that off
and it went faster. We knew it was aero, because it went faster
in fast corners than in slow ones.
"We had to get to America for the race season and so
we stopped. We should have given it to some boffin [that's Olde
Country slang for a scientist or engineer] to work up a program,
but we didn't. Those cars would have been really fast because
there was a lot of body area and lots of power. We should have
gone on with it, but we didn't." It was a friendly conversation,
and Hulme seemed to enjoy talking about it. He died a few months
after our conversation. I'm glad I got to talk to him.
The cars of the late '60s had rounded body work, probably
because it looked good, and they thought it was aerodynamic.
Actually, the rounded areas under the radiator air inlet on the
nose just packed air in under the car and caused lift. The rounded
lower corner of the body sides between the wheel wells likewise
let air under the car and resulted in more lift. A decade later,
race car shapes had changed drastically. Air dams at the front
kept air out from underneath, and slab sides with lips or movable
skirts on the bottom helped seal under-body areas. As a result,
lap times came down, and there was really very little drag penalty
for this benefit.
Ground effects enhances the efficiency of a wing because the
air under the wing is accelerated by interaction with the ground.
I've used the example of air flowing past an open window to illustrate
how a fluid speeds up and loses static pressure as it gains dynamic
pressure. The underside of the racecar drags the air up to speed,
creating lower pressure under the car.
The result is phenomenal. Without ground effects, a good racecar,
say a Swift DB-1 Formula Ford, can generate 1.2 to 1.4 Gs of
cornering force. An object dropped from some height aove ground
accelerates at 1 G--one unit of gravity. In 1993, Formula 1 cars
routinely generated 4 Gs in medium speed (100-120 mph) corners.
Downforce is the reason for the high cornering speeds. Ground
effects is a big part of the story.