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From: rhaar@albertgmr.com (Robert L. Haar CS50)
Newsgroups: sci.electronics
Subject: Re: measuring vehicle speed
Keywords: car speed
Message-ID: <56669@rphroy.UUCP>
Date: 21 Jun 91 20:26:28 GMT
References: <31487@hydra.gatech.EDU> <1991Jun18.003032.18571@mulga.cs.mu.OZ.AU> <31529@hydra.gatech.EDU> <1991Jun20.024446.18473@en.ecn.purdue.edu> <1254@sppy00.UUCP> <13571@uhccux.uhcc.Hawaii.Edu>
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In article <13571@uhccux.uhcc.Hawaii.Edu>, whinery@hale.ifa.hawaii.edu
(Alan Whinery) writes:
|> In article <1254@sppy00.UUCP> jwm@sppy00.UUCP (Jeffrey W. May) writes:
|> >
|> >Two ideas on this one:
|> >(...)
|> >Since the driveshaft-rotation-to-tire-rotation is a fixed proportion this
|> >would give a reasonable result.
|> 
|> Not Always true -- when the car goes around a corner, the ratio changes.
|> The differential allows each drive wheel to turn at a different speed.
|> 
Even if it was, this would give you wheel speed (rotational), not 
vehicle speed. Ther are long term variations like tire wear and
tire pressure changes that effect the relationship. 

Then there are all kinds of short term variations that come about 
from wheel slipage.  MOST of the time, these are negligble. The problem
is that for ABS or traction control, the times when you want to
do something is exactly when the wheel speeds are not the same
as vehicle speed.


	Bob Haar  InterNet : rhaar@gmr.com 
	Computer Science Dept., G.M. Research Laboratories
DISCLAIMER: Unless indicated otherwise, everything in this note is
personal opinion, not an official statement of General Motors Corp.