Date: Sat, 29 Jun 2002 17:59:41 -0700
Reply-To: mike ingle <mikei@ANCORE.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: mike ingle <mikei@ANCORE.COM>
Subject: Re: Speedo Calibration
Content-Type: text/plain; charset="iso-8859-1"
Thanks, Can't believe I left out the pi * diameter part. oh well.
And, I agree after thinking it through that in fact 2* the rolling raduis
(center to pavement) is more accurate.
Also well aware that the significant figures are way overstated here, but I
always keep precision where I have it, to reduce total error.
The corrected factor becomes:
engine rev (rpm) * transaxle ratio * rolling radius (inches) * 2 * pi *
946.9697 * 10^-6 = miles per hour.
engine rev (rpm) * transaxle ratio * rolling radius (inches) * 5.95e-3 =
miles per hour.
test: 3200 rpm, 1.0 ratio, 14"
266mph.
for an 091 acw transmission:
http://volksweb.relitech.com/helmut/GEARBOX.HTM
3.78 1st gear => 0.05477 ratio
2.06 2nd gear => 0.1005
1.23 3rd gear => 0.1683
0.85 4th gear => 0.2436
the above derived using the given 4.83 transaxle ratio
The only way that these numbers make sense is as final drive reduction,
thus a realistic ratio for 4th gear of 0.2436 gives 64.8mph. Much more like
it
Mike
----- Original Message -----
From: <FrankGRUN@aol.com>
To: <mikei@ANCORE.COM>; <vanagon@GERRY.VANAGON.COM>
Sent: Saturday, June 29, 2002 5:14 PM
Subject: Re: Speedo Calibration
In a message dated 6/29/02 3:59:17 PM, mikei@ANCORE.COM writes:
<< Converion:
engine rev/min * 60 min/hr * transaxle ratio = wheel rev/hour
now,
measure your tires from side to side (to eliminate the little flat spot you
rest on) in inches.
wheel width in inches * 1 ft/12in * 1mi/5280ft = miles / wheel revolution
now,
multiply wheel rev/hr * miles/wheel rev to get miles/hr
simplified : engine rev (rpm) * transaxle ratio * wheel dia (inches) *
946.9697 * 10^-6 = miles per hour. >>
Mike,
Nice arithmetic, bad function, incorrect result. First, C=ÿd=3.1416*d.
Second, measurement of static tire diameter is irrevelant. The rolling
diameter (as in 2*rolling radius) establishes the true dimensions of the
tire. This number is always smaller than the apparent diameter because of
inflation vs. load, tire flexure and tread compliance. For new tires, this
number is given for a specified inflation pressure and load. For example,
805
revs/mile. Now work backwards through the gear train to get the distance
traveled of the wheel as a function of engine speed.
For measurement of the revs/mile for older tires use the chalk mark on the
sidewall method and measure away. Unfortunately the accuracy will not be
better than 2 significant figures.
Frank Grunthaner
