Date:         Tue, 30 Apr 1996 08:54:00 -0700 (PDT)
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         "Maher, Steve (SD-MS)" <SMAHER@gi.com>
Subject:      Re: aerodynamics (Seriously...)


>>But I would guess that the main reason for buses getting lower gas mileage
>>than a same-weight sedan, and especially that little round sedan first
>>brought out in '47, is that large, tall, flat front; and especially the
>>large, tall, flat rear end.
>
>I thought that the early Buses actually had a lower drag co-efficient than
>the Beetle sedans. I read that somewhere.

I would be astounded if that were true.

The main reason the beetle is shaped the way it is, is aerodynamics.
When it first came out, it had the lowest drag coefficient of any
car, bar none, and it retained that distinction for many years, despite
the development of cars such as the Citroen 2CV, the Jaguar X series,
the Corvette, etc.

Drag coefficient is defined as the aero drag at any given speed, divided
by unit frontal air pressure at that speed, then divided by the area of
(something). For cars, that (something) is usually the projected frontal
area of the car-- the size of the shadow it throws when there's a light
way behind it. (In airplanes, it's the total area of the wings, looking
down from above.)

The drag coefficient tends to go way up when air flow burbles over the
car like eddies in a creek, rather than flowing smoothly like water under
a water ski. on almost any car, even including out beloved bus, this smooth
flow is easily accomplished on the front of the vehicle. But on the back
half, it's a lot tougher to persuade the air to continue flowing smoothly.
An abrupt, flat cutoff, such as a rear window that doesn't slope or a flat
rear end of the trunk, is almost guaranteed to produce turbulent flow and
higher drag. The back end of a bus is especially guilty. Pickup trucks
are even worse-- entire back of the cab is flat, and then the tailgate
sticks up like an upside-down flap on an aircraft wing-- which aircraft
deliberately use as brakes!

The beetle, on the other hand, has a smooth, continuous curve from the edge
of the windshield, all the way back and down to the rear bumper. If you
tuft the car as described earlier, you'll find almost no turbulent flow
anywhere on it, other than the wheel wells. Even the fenders taper gently
to the rear, for the same reason. The only turbulence on the car, is at
the sharp juncture where the front fenders meet the body in front of the
doors.


>Don't know about pound-for-pound comparisons between Buses and sedans of
>any make; that would be interesting to know. If a sedan were to weigh this
>much and use 1930s engine technology, could it hit 20 mpg? I wonder if
>there are any double-valved watercooled monoblocks out there that can do
>what we do...

Weight is a major factor in gas mileage, mostly during slowdown/speedup
times (traffic), and going up hills. On a flat road at steady speeds, most
power goes to overcoming aerodynamic drag. Drag contributes to most of the
difference in gas mileage between, say, a 1970 beetle and a 1970 bus with
identical engines. The bus's larger frontal area and flat rear end, are
the main reasons why the bus gets 20 mpg while the beetle exceeds 30 mpg
under the same conditions. Another, lesser factor is the lower gearing of
the bus's tranny-- the bus engine is turning faster than the beetle engine
at the same speed, so throttling losses are higher-- the pistons have to
work hard to pull in the air/fuel mixture from the partial vacuum in the
intake manifold. The more "pulls" per mile, the more energy you spend per
mile doing it.

Drag of the transmission is surprisingly high IMHO-- caused by friction
in the bearings, viscosity of the oil, etc. Almost one-third of your engine
power is spent overcoming this. In fourth gear, beetles rear wheels turn
at about the same speed as the engine itself, due to the transmission's
gearing. If the car had no transmission, and the engine crankshaft were
attached directly to the wheels, gas mileage would go way up. Of course,
it would be a little difficult to get it rolling...  :^) Some electric cars
have an electric motor driving each rear wheel directly, to take advantage
of this savings in drag from having no transmission.

Other factors, such as drag of the wheel bearings, rolling resistance of
the tires, etc. are much smaller, especially since the development of
radial tires.

This post is getting long. Shut up, Steve.

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