. . . and now that we are getting everything dialed in
for accuracy, do we need to account for the tire growth
(in diameter) at speed?
Jack Brooks
---------------------- Forwarded Message: ---------------------
From: "Kas Kastner" <kaskas@earthlink.net>
To: <BillDentin@aol.com>, <mporter@zianet.com>, <fot@autox.team.net>
Subject: Re: Speed equation
Date: Thu, 8 Aug 2002 21:38:01 -0700
This sounds to complicated........just put a chalk mark on the side/bottom
of the tire and another mark opposite on the floor,, roll the car one
roatation and then use your carpenters tape to measure the distance and that
is the TRUE circumfrance......... over Pi and there is D. Rpm over the
gear ratio (whatever, o.d. or other mulrtiplicaion factor) X the wheel
diameter in Inches over 336 and you've got MPH ...........ever time.
----- Original Message -----
From: <BillDentin@aol.com>
To: <mporter@zianet.com>; <fot@autox.team.net>
Sent: Thursday, August 08, 2002 3:33 PM
Subject: Re: Speed equation
> In a message dated 08/08/2002 5:17:11 PM Central Daylight Time,
> mporter@zianet.com writes:
>
>
> > Actually, both of these equations overstate mileage by about 2-3%,
> > depending upon the tire. The most accurate equations for such
(calculating
> > speeds or determining pulse rates for electronic speedometers) use the
> > rolling circumference of the tire, rather than the calculated
> > circumference. The rolling circumference is dependent upon the length
of
> > the tire patch at normal pressure, so the larger the footprint of the
tire,
> > usually the lower the rolling circumference.
> >
>
> Ah so!
>
> How does one get a reasonably accurate rolling circumference?
>
> We've taped tires with a soft tailor's tape to get a circumference, and
> always came up with a number different from one produced with the
> mathematical calculation. But neither allows for a loaded foot print you
say
> comes into play.
>
> I have to assume the tire and wheel would have to be loaded with the car's
> weight. So do you mark the side of the tire, lay a tape on the ground
> parallel to the car, and then roll the car forward?
>
> Bill
>
> PS
>
> Probably be better if I just went on estimating my speed. Sometimes I get
up
> to 160 then. I don't think I can do that with these calculations.
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