Date: Thu, 18 Aug 2005 00:59:44 -0500
Reply-To: Joel Walker <jwalker17@EARTHLINK.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Joel Walker <jwalker17@EARTHLINK.NET>
Subject: Re: How much fuel efficiency gained by shedding 300 lbs?
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> (According to some tech geeks, the measured coefficient of drag for
> the
> Vanagon body is 0.75, and the rolling coefficient is 0.014. See
> http://www.goshen.edu/physics/horsepower/1998_LabReport-87Vanagon.gif )
0.75??! i kinda don't think so. perhaps for the entire bus ... but
that calculator
> http://www.bgsoflex.com/aero.html
only wants the FRONTAL drag coefficient. which vw was giving out to
the magazines
as 0.44 when the vanagon first appeared in 1979.
and that was touted as being lower than a 1979 corvette.
trick is ... the bus has a frontal AREA of probably two or three times
that of the corvette. :)
and to get the drag, you have to multiply the coefficient by the
frontal area.
so what's the frontal area of a vanagon?
excluding the rearview mirrors, it's 1845mm wide (widest at the
bottom) and 1950mm tall.
but we have to subtract the ground clearance of 190mm, so we get
1845x1760mm.
or 72.6 inches wide by 69.3 inches tall (for purposes of calculating
drag).
or 6.1 ft wide by 5.8 feet tall = 35.1sq ft.
by contrast, a 1979 corvette would be about 5 ft wide by 3 feet tall
(including windshield)
or about 15 square feet. probably less.
figure the rearview mirrors, especially the powered ones, would add an
additional 0.9 sq ft, so
we get a nice round 36 square feet to use in the calculator. :)
so watch what happens according to that calculator page ..
using 0.44 coeff of drag,
36 sq ft frontal area,
70F temp
30in Hg barometric pressure
3600 lbs
40 psi tire pressure
and 0.075 frontal lift (no idea if this is even close for a vanagon)
we get the following table ...
mph(input) aero hp (out) rolling hp (out) frontal lift (out)
40 7 hp 4 hp 11 lbs
50 13 hp 5 hp 17 lbs
55 18 hp 6 hp 21 lbs
60 23 hp 7 hp 25 lbs
65 29 hp 7 hp 29 lbs
70 36 hp 8 hp 34 lbs
75 45 hp 10 hp 39 lbs
80 54 hp 11 hp 44 lbs
90 77 hp 14 hp 56 lbs
100 106 hp 17 hp 69 lbs
now, the way i read that is that it takes just about all of your
engine's hp to overcome aero drag at 90 mph :) but at 60-65 or even up
to 75, you're ok. more or less. less than half anyway.
then take a look at a table i came up with for my 86 4-speed bus
(which did weigh 3600 lbs)
for fuel consumption. i used a zemco zt-4 trip computer that would
display the instantaneous mpg, so by using a stop watch on some flat
interstate, i came up with gallons per hour and so forth.
but i did it by rpm, not mph.
rpm calc mph gal/hour mpg max range
3200 52.3 2.3 23.1 368 miles
3400 55.6 2.4 23.5 375 miles
3600 58.9 2.7 21.8 347 miles
3800 62.1 3.2 19.2 306 miles
4000 65.4 3.4 19.2 306 miles
4200 68.7 3.7 18.8 302 miles
4400 71.9 4.3 16.8 267 miles
one thing to remember is the Max Torque RPM for the 86 2.1 liter
engine was around 3400 rpm.
so running along at 3200 rpm is actually worse, cause you aren't
developing all the torque you can (to get up even small hills). notice
also how the mpg starts to drop off rather sharply above 4000 rpm.
didn't get any readings higher than 4400, cause there was too much
traffic that i caught up with at that speed. ;)
anyway, the tables seem to agree with each other. but one thing to
remember ... buses do NOT have smooth bottoms. like beetles and
porsches. and smooth underbellys DO make for less drag. buses do not
have such things cause they are TRUCKS, and not really designed to
roam up and down the highways at high speeds. :)
i THINK the Eurovan DOES have a smooth bottom, or at least, a lot
smoother than a vanagon or breadloaf.
always wondered what it would do to the mpg and to the handling if i
put flat sheets of aluminum to cover the bottom. :)
and then there's the drag along the sides of the bus. and turbulent
drag at the rear.
just for funsies one night, when i was REALLY bored and it was too
dark to go outside and put more reflective tape on my bus, i sat down
and calculated the complete surface area of a vanagon, and figured
that if it were raining, and there was water alllll over the bus to a
depth of only 1mm, it would add 80 lbs to the weight of the bus. :)
more drag!! ;)
but always remember two things:
1. there are three kinds of Lies: Lies, Damned Lies, and
Statistics. <Mark Twain>
2. 52.7 percent of all statistics are made up on the spot. :)
3. "Statistician" is an anagram for "Satanic its it"!!! and we all
know what that means!!
unca joel
