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uploaded 1/8/2001

Ground Effects

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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.

Things Change

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.



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