

03202006, 09:55 AM

#31

Registered Member
Join Date: Dec 2005
Posts: 4,223
Country: United States

Re: CD
Quote:
Originally Posted by JanGeo
Actually I have heard that you can ignore the CD Drag Coeffient at lower speeds (under 100mph) as it is such a small portion of the rolling resistance and frontal area drag as to not matter much.

100 mph? (hopefully i'm misunderstanding your point. you're not saying that aero drag is unimportant below 100, are you?)
have a look at the spreadsheet or the web page and you'll see the aero penalty begins to rear its ugly head relative to rolling resistance at between 3035 MPH.
http://metrompg.com/toolaerorr.htm
below that speed you burn most of your fuel to overcome rolling & mechanical drag.
above 35 mph, you burn most of your fuel fighting a losing battle against aerodynamic drag.
(a "losing" battle because the amount of fuel needed to overcome aero drag quadruples as speed doubles.)
__________________



03202006, 12:27 PM

#32

Registered Member
Join Date: Jan 2006
Posts: 2,442
Country: United States

YEs you hAvE
Yes you have misunderstood  the drag coefficient is the factor of the streamlineness of the shape i.e. square box vs teardrop but that has only a small influence on the aerodynamic drag  the more important factors are frontal area first and rolling resistance due to weight second. This is why the really fast sportcars are very small and low to the ground, pointy or not they only go fast because of their small frontal area and a few extra HP as well of course. Once you get up over 150mph then the shape becomes more important as well as surface treatment.
__________________



03202006, 12:54 PM

#33

Registered Member
Join Date: Feb 2006
Posts: 1,480
Country: United States
Location: Raleigh, NC

Re: YEs you hAvE
Quote:
Originally Posted by JanGeo
Yes you have misunderstood  the drag coefficient is the factor of the streamlineness of the shape i.e. square box vs teardrop but that has only a small influence on the aerodynamic drag  the more important factors are frontal area first and rolling resistance due to weight second. This is why the really fast sportcars are very small and low to the ground, pointy or not they only go fast because of their small frontal area and a few extra HP as well of course. Once you get up over 150mph then the shape becomes more important as well as surface treatment.

The formula for calculating drag contradicts what you are saying. CD*A*RHO/2*V^2. The CD and frontal area are EQUAL factors. Since they are multiplied by each other, you could decrease either by a certain percentage with the same results. Also, since aero drag increases at the rate of velocity squared, both CD and FA are much much much more significant than rolling resistance at speeds over 40MPH.
__________________



03202006, 03:41 PM

#34

Registered Member
Join Date: Sep 2005
Posts: 933
Country: United States

I am taking a statistics
I am taking a statistics class in college right now. I think you guys could teach the class.
__________________
2008 EPA adjusted:
Distance traveled by bicycle in 2007= 1,830ish miles
Average commute speed=25mph (yes, that's in a car)



03212006, 06:43 AM

#35

Registered Member
Join Date: Jan 2006
Posts: 2,442
Country: United States

drag drag drag
Apparently the equations are not as simple as one is led to believe according to one site I found. I will have to go back to my college days when I modeled hp vs speed for cars and got some good results with a basic equation. Found some Cd for various cars on line also.
referring to the classic equation . . .
http://hypertextbook.com/physics/matter/drag/
R = .5 Cd roe A v^2
"Simple, compact, wonderful. A nice equation to work with  or is it?
Well, yes and no.
Yes, but it works only as long as the range of conditions examined is "small". That is, no large variations in speed, viscosity, or crazy angles of attack. The way around this is to reduce the coefficient of drag to a variable rather than a constant. (I can live with this.) Cd depends on some yet to be specified set of factors. It is totally acceptable to say that Cd varies with this that or the other quantity according to any set of rules determined by experiment.
No, since speed is squared. [Gasp!] Recall that speed is the derivative of distance with respect to time. Have you ever tried to solve a nonlinear differential equation? No? Well, welcome to hell. Wait, let me rephrase that  Welcome to Hell! [Carack! Boom!] Ah ha ha ha ha haaaa! [Rumble] You fool! No one can manage the square of a derivative. The mathematics will consume you. [Carack! Boom!] Ah ha ha ha ha haaaa! [Rumble]
Whew. What the hell wasthat all about? I might not know how to solve every kind of differential equation off the top of my head, but so what. I can always look for the solution in a book of standard mathematical tables or an online equivalent."



03212006, 07:40 AM

#36

Registered Member
Join Date: Dec 2005
Posts: 4,223
Country: United States

Re: drag drag drag
Quote:
Yes, but it works only as long as the range of conditions examined is "small". That is, no large variations in speed, viscosity, or crazy angles of attack.

fortunately, using the equation to figure out drag for a road vehicle, there is no variation in viscosity or crazy angles of attack. so we don't have to worry about the writer's concern over those variables.
so we're left considering the writer's concern over the "large variation" of speed. and with no definition of what a "large variation" is, it's not a particularly useful point to have raised. (is 3070 mph through air considered a "large variation"?)
i don't think the utility of the equation is seriously in doubt for the application we're looking at.



03212006, 07:43 AM

#37

V32D
Join Date: Mar 2006
Posts: 1,186
Country: United States

Re: CD
Quote:
Originally Posted by MetroMPG
http://metrompg.com/toolaerorr.htm

great tool man! took me a while to figure out the general area of my engines efficiency. unfortunately this also assumes the engines efficiency is constant throughout the rpms. still very cool though!
__________________
don't waste your time or time will waste you



03212006, 08:58 AM

#38

Registered Member
Join Date: Jan 2006
Posts: 2,442
Country: United States

back to college
I will get the old punch cards out of the box from my college days and bring in the equation that I used for my Rambler which pretty much nailed my top speed dead on. It took into effect weight frontal area and drag cooefficient and yields either HP or speed in MPH.



03212006, 09:39 AM

#39

Registered Member
Join Date: Dec 2005
Posts: 4,223
Country: United States

punch cards!! i caught the
punch cards!! i caught the very tail end of the punch card era:
my very first computer class in high school was about the last year it used cards (the pencilintheboxes kind). at that time my "PC" was a Commodore 64
yes, if you can decipher the cards, i'd be interested in seeing another version of the equation.



03212006, 05:33 PM

#40

Registered Member
Join Date: Mar 2006
Posts: 162
Country: United States

Results look good...
I did a spreasheet a week or so ago that does essentially the same calculation and my results agree with yours. ~40mph is the point where rolling resistance becomes secondary and air resistance takes over. This also agrees with my informal observations with the scangauge. On a nice flat road I can get some really crazy instantaneous FE numbers when I'm driving between 35 and maybe 42mph, 35mph being the slowest I can reasonably cruise in fifth gear. After that my numbers decrease from 4050mpg+ to the high thirties and down.
__________________
__________________
'07 Toyota Prius





Thread Tools 

Display Modes 
Linear Mode

Posting Rules

You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts
HTML code is Off




» Car Talk & Chit Chat 





» Fuelly iOS Apps 





» Fuelly Android Apps 




