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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
Comments: To: vanagon@VANAGON.COM
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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