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Re: Tire Rolling Radius

To: Bob Palmer <rpalmer@ames.ucsd.edu>
Subject: Re: Tire Rolling Radius
From: Steve Laifman <laifman@flash.net>
Date: Fri, 26 Mar 1999 09:55:14 -0800
Tigers,

With the talk about speedo calibration, I thought I'd dig up
some of the "rolling diameter" given by tire companies.
Now, these assume the wheel width is appropriate for the
tire aspect ratio ( 98, 95, 70, 60, etc.) as the diameter
will decrease if it's spread too far.  Most are appropriate
for the Tiger, in the moderate width ratios.

Per Mike Taylor (page 223):
Stock Original Tiger-

Tire Circumference 15.90x13
D (in.)= 23
Miles/rev = 0.001140413


Popularly used:

Tire Circumference 185/70R13
D (in.)= 23.2
Miles/rev = 0.00115033

Some handy formulae:

rpm@60mph=
(mph)*(revmotor/revaxle)/((60min/hr)*(revmotor/revaxle)*(miles/tirerev))

mph=((revmotor/min)*(60
min/hr)*(Miles/tirerev))/(revmotor/revaxle)

(miles/tire revolution) = (3.14159*rolling diameter,
inches)/(12*5280)

mph @ rpm = ((rpm*60)/(gear ratio/rear end
ratio))*(3.14159*rolling diameter inches)/(12*5280)

This last formula can be used to calculate the (mph/1 rpm)
for each gear.  Then, just multiply this number times the
rpm in any gear, and you've got the speed in that gear at
that rpm.


For those not familiar with equation notation used in spread
sheets:

Notation:  "*" is 'multiply'
                  "/" is 'divide'
 Procedure:

Always solve for the number within a set of parentheses
"(12*5280)" before applying that result to the next
operation.  That's why you see "(( " multiple parentheses.
There should be an equal number of ones facing one
direction, as the other or the equation is incorrectly
formatted.

These can be plugged into a spread sheet, with the proper
cells referenced, and curves of the resultant speed vs rpm
can be made.  Pick your shift points by knowing your engine.

Steve





Steve Laifman         < One first kiss,       >
B9472289              < one first love, and   >
                      < one first win, is all >
                      < you get in this life. >


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