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Old 07-12-2008, 11:07 AM   #1 (permalink)
dcb
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help with simplified cDA/cRR formula

I saw a coastdown calculator and wanted some help understanding it before I try and integrate it into the mpguino.

Basically you (or your computer) times a coastdown at high speed (say 55mph to 50mph) and another at low speed (say 7mph to 2mph) and put the speeds and times and weight in and it tells you your cDA and rolling resistance. But I'd like to understand what it is doing a little better. I appreciate that they have approximated things like pressure and temperature for the sake of simplification, but can anyone determine what the "6" on the cDA line represents or the "28.2" on the rr line actually is?
The crux of it is:

a1=(va1-vb1)/t1;
a2=(va2-vb2)/t2;
v1=(va1/2)+(vb1/2);
v2=(va2/2)+(vb2/2);
cDa=((6*m)*(a1-a2))/(v1*v1-v2*v2);
rr=(28.2*(a2*v1*v1-a1*v2*v2))/(1000*(v1*v1-v2*v2));

abbreviations:
a1: high speed deceleration
a2: low speed deceleration
v1: high speed average velocity KPH
v2: low speed average velocity KPH
cDA: coefficient of drag * area
rr: Rolling resistance
va1: high speed start KPH
vb1: high speed end KPH
va2: low speed start KPH
vb2: low speed end KPH
m: mass in Kilograms

Other conversion factors
1 mile = 1.609344 KM
1 pound = 0.45359237 kilograms

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Last edited by dcb; 07-12-2008 at 12:10 PM..
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