Date: Tue, 8 Dec 1998 16:23:13 -0600
Reply-To: Blue Eyes <lvlearn@MCI2000.COM>
Sender: Vanagon Mailing List <vanagon@vanagon.com>
From: Blue Eyes <lvlearn@MCI2000.COM>
Organization: Vexation Computer
Subject: Re: Distance per tire rotation
Content-type: text/plain; charset=us-ascii
Larry asked for comments that go to the issue of distance traveled per tire
rotation.
When you look in a tire rack, you see tires that appear to be round. Pick the
kind you prefer and have them mounted on rims, and they're still round. Talk
of tire circumference and diameter all seem to fit wonderfully. But that's
about the last time those tires which support your vehicle's weight are
round. Two differently manufactured tires with identical unloaded diameters
may have different rolling distances after they are loaded on the ground.
It's common to talk of diameter as though a loaded tire had a circular outside
shape despite the fact that it has a "foot print." I was told long ago by a
friend who did road tests for a magazine that my simplistic assumptions were
close but not quite accurate in trying to calculate distance traveled per
revolution of the driving axle.
I hate to repeat myself like this, but I was told to mark a line on the ground
onto a loaded tire, then roll it for some arbitrary number of turns (since
it's so easy to improve accuracy, why not at least roll it 10 times), then
position the tire mark at the same rotation position and mark the ground
again. Measure the distance between your ground marks, divide by the tire
rotations, and you have your low speed rolling distance..
Is it just that differently constructed tires crush to form their foot prints
with some slight variation? Is it because tires with different widths are
affected by different air pressure differently? Hell if I know. Blame it on
the Bosinova. Measure it and you don't have to estimate it.
What I do know it that if you watch rail dragster tires warmed before a race,
they visibly change diameter by several inches when they are spun hard. I had
photos of my drag racer motorcycle rear tire that was visibly taller at speed
going through the traps than is was standing still. I've seen photos of tires
at high speeds where the leading edge is so deformed as it piles into the
ground that nobody would call it round. Our Vanagons don't show much
variation from the fixed size round tire assumption, but they are not immune
from those effects. At this URL, Bruce Bowling takes a stab at estimating
tire size vs. distance.
http://sura1.jlab.org/~grippo/effectivediam.html
Then you might go to his index page and play with some of his other
estimators, like the MPG
meter. You'll need a coefficient of air drag index, and .44 was supposed to
be VW's own measure according to an early Road & Track test report that you
may find in the List Archives. As for BSFC figures, the indirect injection
diesel Vanagon's best was 264 grams of fuel pre horsepower hour. If anyone
has a direct injection TDI diesel, you can aspire to get by with only 200
grams for the same work. But neither Vanagon is geared properly to get these
best values that their motors could yield, so they will burn more fuel than
those ideal values. I can't help you with the water boxer's BSFC, but it's
probably about 390 grams per hp-hr, judging by some indicators. I just don't
know.
Go to
http://sura1.jlab.org/~grippo/
then pick the mpg estimator or prowl around some of his other estimators.
They aren't perfect, but he is trying.
Anytime you're given easy choices between measuring reality and estimating it,
pick measuring first.
That is unless you believe your estimate is better than reality. Or, as the
Hitch Hiker's Guide to the Universe said, "The Guide is definitive. Reality is
frequently inaccurate."
John
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