In addition to the fact that the underside of streetcar is not very clean
aerodynamically speaking you also have to understand that a car is shaped
like a wing. It's fairly flat on the bottom and sort of rounded on top.
That causes air to have to speed up to flow over the top of the car, which,
just like a wing, produces aerodynamic lift. The air dam reduces airflow
underneath the car with decreases the pressure differential between the air
above and below the car. That decreases lift or the floating feeling you
get at high speeds. I don't remember the exact numbers but Porsche measured
the 911's lift in a wind tunnel. The 911 keep in mind is very wing like.
At whatever speed they were working with they measured about 360Lbs of lift
as I recall. They added the air dam and ducktail spoiler (not the later
whale tail) and dropped that number down to about 80Lbs at the same speed.
The ground effects cars used a belly pan as Mike stated. That pan was
actually a venturi, which started behind the front wheels and sloped
downward to a point about 1/3rd of the way down the length of the wheelbase
and then sloped upward to the large diffuser openings you see at the rear.
That inverted wing shape of the venturi accelerated the air underneath the
car to a higher speed that of the air above the car. Again a pressure
differential developed but this time that difference forced the car down,
not up, and had the advantage of doing so without creating anywhere near the
amount of drag that a conventional wing on the nose or tail of the car would
have created. That is why ground effect was such a wonderful solution for
the engineers. The faster you go the more stick you develop without needing
any additional horsepower to push some big wing through the air. If you
read Nikki Lauda's book, you'll see that it wasn't such a great solution for
the drivers. The Chaparral J2, the vacuum or sucker car that Mike
mentioned, did the same thing without the venturi tunnels. It used a couple
of large engine driven fans at the rear of the car to literally suck the air
out from underneath. Brabham used the same solution on one of their Alfa
powered cars in '78 or '79.
Look around on the net for pictures of the 1973 (I think) March F1 car. I
believe it was called a 703. That car used side pods shaped like inverted
wings but they were open on the outside - no skirt. This lets you see what
was going on. It was Colin Chapman's Lotus T-78 (or T-79) that first put it
all together to develop a true passive ground effect.
Brian '69 2000
Tampa, FL
http://web.tampabay.rr.com/oilleak/
-----Original Message-----
I HAVE experienced that
100mph "floating" sensation. Kind of makes you feel like if you hit a
pebble in the road you might be air borne.
I had a 91 Camaro R/S with a factory airdam. took it up north once and
had her up to 140. Instead of floating, the car felt sucked down to the
ground.
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