CdA - (Drag co-efficient) Anyone calculated for a 2001 Quattro 225?
09-01-2001, 01:59 AM
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CdA - (Drag co-efficient) Anyone calculated for a 2001 Quattro 225?
Hi, I've been trying to calculate the drag co-efficient for my TT, and am getting an unexpected high number. (The higher the number the more drag !)
Method. (We need 2 times)
TIME1
Drive past 60mph say to 65mph , coast (in neutral) from 60mph down to 55mph. and take the time in secs.
TIME2
Do the same for 15mph down to 10mph.
Next take the weight of the vehicle add fuel+drivers weight (in Kg's), and use the following..
CdA = (Mass * (Time2-Time1)) divided by (169*Time1*Time2)
I'm getting about 0.85 what do you guys get?
Method. (We need 2 times)
TIME1
Drive past 60mph say to 65mph , coast (in neutral) from 60mph down to 55mph. and take the time in secs.
TIME2
Do the same for 15mph down to 10mph.
Next take the weight of the vehicle add fuel+drivers weight (in Kg's), and use the following..
CdA = (Mass * (Time2-Time1)) divided by (169*Time1*Time2)
I'm getting about 0.85 what do you guys get?
09-01-2001, 05:09 AM
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Traditionally coast-down tests are run over a larger delta V than 5 mph.
You appear to be calculating total drag (Dt).
Dt= -m * (V2-V1)/(t2-t1) which is Newton's 2nd law of motion (F=mA).
The next step is to subtract tire rolling resistance and account for inertial effects due to wheel rotation, which is a challenge in data gathering.
Next it gets fun: divide Dt by the dynamic pressure and the frontal area to get Cd, or coefficient of drag.
I refer you to a great book: "Race Car Aerodynamics Designing for Speed" by Joseph Katz.
It's published by Robert Bentley Publishers, the same as our TT Factory repair manual.
My $.02
Dt= -m * (V2-V1)/(t2-t1) which is Newton's 2nd law of motion (F=mA).
The next step is to subtract tire rolling resistance and account for inertial effects due to wheel rotation, which is a challenge in data gathering.
Next it gets fun: divide Dt by the dynamic pressure and the frontal area to get Cd, or coefficient of drag.
I refer you to a great book: "Race Car Aerodynamics Designing for Speed" by Joseph Katz.
It's published by Robert Bentley Publishers, the same as our TT Factory repair manual.
My $.02
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