Date:         Mon, 20 Jun 2005 16:21:12 +0200
Reply-To:     bus.mail@IAE.NL
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Jaap Nauta <bus.mail@IAE.NL>
Subject:      Re: Speedometer -- the straight scoop
Content-Type: text

Sorry, pressed send too early :-)

Michael Elliott wrote:
> Looks like the answer, all right.
>
> Maybe someone can help me with Mechanical Speedometer 101:
>
> 1. How do they work?
>
> 2. What can shift or age internally to cause them to read higher than
> manufacturing tolerance sez they should when used with the correct tires?
>
> I assume that the tires are the correct size (185/14) because the
> odometer reads pretty darn accurately (2.5% error). I have a second
> reason to make this assumption. I base it on another assumption, a quick
> measurement, and some rough math: The little calibration number on the
> face of the speedo is 805. I assume that this means that it is set up
> for a tire that rotates 805 times in one mile. (A mile has 63,360
> inches, the diameter of my tires are (roughly) 25 inches, one rotation
> of the tire covers (roughly) 78.5 inches, if it rotated 805 times it
> would cover (roughly) 63,192 inches . . . coincidence? You be the judge!)
>

The 805 is indeed the conversion factor from rotations to miles, for metric
speedometers it is 500. As far as I know the synchro has a different factor,
I persume the speedocable is not directly connected to the left frontwheel.

Since I didn't have a clock, I used a bicycle speedometer, which gave me also
an accurate speedometer and extra odometer. If I use the same conversion
factor I get exactly the same odometer reading, and roughly 5% lower speed.
See also http://home.iae.nl/users/jnauta/bus/clock.html

Why they read higher? A little spring is pulling the needle back to zero,
maybe this spring gets a bit weaker.
A magnet is mounted to the shaft, and rotating. Around this magnet is a
metal cup, connected to the needle.

Jaap
'87 Caravelle diesel
 [2~



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