EcoModder Forum help with simplified cDA/cRR formula

Register Now
 Remember

 07-12-2008, 11:07 AM #1 (permalink) needs more cowbell     Join Date: Feb 2008 Location: ÿ Posts: 5,038 pimp mobile - '81 suzuki gs 250 t 90 day: 96.29 mpg (US) schnitzel - '01 Volkswagen Golf TDI 90 day: 53.56 mpg (US) Thanks: 158 Thanked 267 Times in 210 Posts 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 __________________ WINDMILLS DO NOT WORK THAT WAY!!! Last edited by dcb; 07-12-2008 at 12:10 PM..
 Today Popular topics Other popular topics in this forum...
 07-13-2008, 06:21 PM #2 (permalink) needs more cowbell     Join Date: Feb 2008 Location: ÿ Posts: 5,038 pimp mobile - '81 suzuki gs 250 t 90 day: 96.29 mpg (US) schnitzel - '01 Volkswagen Golf TDI 90 day: 53.56 mpg (US) Thanks: 158 Thanked 267 Times in 210 Posts shameless bump (maybe this belongs in instrumentation?) __________________ WINDMILLS DO NOT WORK THAT WAY!!!
EcoModding Lurker

Join Date: May 2008
Location: Vancouver, BC
Posts: 9

G-metro - '95 Geo Metro
90 day: 54.43 mpg (US)
Thanks: 0
Thanked 2 Times in 2 Posts
Quote:
 Originally Posted by dcb 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
Well, the numbers that you have quoted are for 'metric' units. Read on for explanations...

In standard metric units (mks - metres/kilograms/seconds) there should be 2/1.2 in stead of "6" and 1/9.8 instead of "(28.2.../(1000...".

So, re-written:
cDa=((2*m)*(a1-a2))/((1.2)*(v1*v1-v2*v2))
rr=(a2*v1*v1-a1*v2*v2)/(9.8*(v1*v1-v2*v2))

From the (hand-derived) math:
- the "2/1.2" is the reciprocal of the standard 0.5*'rho' ['rho' being the density of air @ 1.2 kg/m^3]
- the 1/9.8 is the reciprocal of acceleration due to gravity [9.8m/s^2]

As you have stated the speed units as km/h, there are factors of 3.6 (1 km/h = 1000 m / 3600 s = 1m/3.6s). So, used appropriately, one will achieve the original equations with the "6" and "28.2".

And, as you have already suspected, the results will be an indication of real values but not absolute. I don't feel like doing the calculus to get the exact results - too time consuming and the external influences when taking the readings will not justify its precision!

Also note that there is another component that is often overlooked: a 'viscous' drag - a velocity component in the equation:
F = 'rho'/2*cDA*v^2 + cV*v + m*g*cF

BTW = this is bringing back memories of my master's thesis...

 07-14-2008, 11:21 PM #4 (permalink) needs more cowbell     Join Date: Feb 2008 Location: ÿ Posts: 5,038 pimp mobile - '81 suzuki gs 250 t 90 day: 96.29 mpg (US) schnitzel - '01 Volkswagen Golf TDI 90 day: 53.56 mpg (US) Thanks: 158 Thanked 267 Times in 210 Posts Thanx CO, it sounds like it might be good enough for our purposes then. I hope to take a lot of the timing/speed variables out of the equation by having the computer do all that, but I also didn't want to make the user jump through too many hoops about current temp/humidity/pressure/locust density/etc __________________ WINDMILLS DO NOT WORK THAT WAY!!!
EcoModder

Join Date: Jul 2008
Location: North Central Alabama
Posts: 572

Big Salsa - '04 Toyota Sienna LE

Silver - '10 Toyota Prius III
Thanks: 110
Thanked 123 Times in 71 Posts
Viscous Drag

Quote:
 Originally Posted by co_driver Also note that there is another component that is often overlooked: a 'viscous' drag - a velocity component in the equation: F = 'rho'/2*cDA*v^2 + cV*v + m*g*cF
Is there a simple way to get the viscous drag component? I have seen where people get the rolling resistance by warming up their car and stopping in a level area (parking lot), and then either push or pull the car with a scale to see what the resulting rolling resistance would be. Would a person be able to get the rolling resistance this way, and then calculate the viscous drag from the equation above? It should work. Is there a more simple way? Does viscous drag account for much, or is it able to hide in the cDA component ealily?
__________________

EcoModding Lurker

Join Date: May 2008
Location: Vancouver, BC
Posts: 9

G-metro - '95 Geo Metro
90 day: 54.43 mpg (US)
Thanks: 0
Thanked 2 Times in 2 Posts
Quote:
 Originally Posted by wyatt Is there a simple way to get the viscous drag component? I have seen where people get the rolling resistance by warming up their car and stopping in a level area (parking lot), and then either push or pull the car with a scale to see what the resulting rolling resistance would be. Would a person be able to get the rolling resistance this way, and then calculate the viscous drag from the equation above? It should work. Is there a more simple way? Does viscous drag account for much, or is it able to hide in the cDA component ealily?
As one is bound to find (from all of the discussions in this forum), there are no simple solutions. But there are rules of thumb.

In a spreadsheet that I have developed (its a beast!), I do include the viscous component - and guestimated it to be ~1 for a 2-wheel drive vehicle (where F(viscous) = cV*v (v in [m/s]). It could be ignored, but will have an influence on the standard pair of coast-down tests (high speed and low speed). Hint: (greater than) 3-speed coast down tests will show this minor factor (did this in uni many years ago w/ driveshaft torque measurement and data logger).

This is notably small when compared to the other two inputs:
eg. '90 Firefly:
@ 25 m/s:
F(friction) = 81.3N (cF = 0.010, m = 830kg)
F(viscous) = 25.0N
F(aero) = 236.3N (cD = 0.36, A = 1.75m^2)

[Disclaimer: YMMV, above numbers may or may not simulate real life, ...]