Drag Coefficient wrt speed - Fuelly Forums

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Old 06-11-2006, 12:58 AM   #1
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Drag Coefficient wrt speed

I remember reading somewhere that drag coefficient depends on the speed at which it is measured.

And this makes sense - for example, see how airplanes are designed. The higher the speed, the less deviation away from a flat angle is allowed.

Although it's indeed possible to conserve fuel by slowing down, this seems to me to be a quick fix. Better to optimize the highways we have and reduce the time it takes to get from place to place and also design around that rather than plan for congestion.



To this end it would be nice to see Cd/ speed curves from the auto manufacturers. I doubt that the econobubbles are going to be that good above 50mph, whereas something very low and sporty could likely have a best fuel economy at 80 mph.

It's something to think about.
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Old 06-11-2006, 02:42 AM   #2
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the problem is not really aerodynamics. The problem is speed. Aerodynamics isn't going to help a lot for fuel economy when the car is driving 100mph. Right now freeways are getting built for 100mph speeds because in the future that is what's going to happen. We need more gears. 4 speeds and 5 speeds are not cutting it anymore.

Toyota is already working on 7 and 8 speed automatics because as the speed increases we need to increase fuel economy with it.

The manufacturers are more concerned about looks than aerodynamics. It's looks that sell the car, not aerodynamics.
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Old 06-11-2006, 04:47 AM   #3
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Quote:
Originally Posted by Compaq888
The manufacturers are more concerned about looks than aerodynamics. It's looks that sell the car, not aerodynamics.
True. Although the higher gas prices go as a percentage of disposable income, the more interest people will have in the subject and the more cars it will sell.
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Old 06-11-2006, 08:06 PM   #4
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Quote:
Originally Posted by Compaq888
the problem is not really aerodynamics. The problem is speed. Aerodynamics isn't going to help a lot for fuel economy when the car is driving 100mph. Right now freeways are getting built for 100mph speeds because in the future that is what's going to happen. We need more gears. 4 speeds and 5 speeds are not cutting it anymore.
why would more gears help though? you still need the power AND aerodynamics to get up there. the amount of gears allows a better blend of acceleration and top speed. you can still have a higher speed with less gears.
Quote:
Originally Posted by Compaq888
Toyota is already working on 7 and 8 speed automatics because as the speed increases we need to increase fuel economy with it.
bmw already had them, last year, and automatic trannys suck.

anyway the bottom line is that there is no need to try to change the subject of this thread. aerodynamics are necisary and it is very to consider the fact that a cars coefficient of drag is in fact a speed dependant variable. good food for thought, try chewing.

to mira: kudos on the intreguing topic. it makes me feel that the number one concern in aerodynamics at any speed is reducing the frontal area.
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Old 06-11-2006, 08:10 PM   #5
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Mercedes is shipping their ML (or whatev) SUVs with 7 speeds now, WOAH, watch out 5speed autos...

That's all I'm clever enough to add.
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Old 06-11-2006, 09:32 PM   #6
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Quote:
Originally Posted by thisisntjared
why would more gears help though? you still need the power AND aerodynamics to get up there. the amount of gears allows a better blend of acceleration and top speed. you can still have a higher speed with less gears.bmw already has them and automatic trannys suck.

anyway the bottom line is that there is no need to try to change the subject of this thread. aerodynamics are necisary and it is very to consider the fact that a cars coefficient of drag is in fact a speed dependant variable. good food for thought, try chewing.

to mira: kudos on the intreguing topic. it makes me feel that the number one concern in aerodynamics at any speed is reducing the frontal area.
Essentially that's what it comes down to.

Really, the greatest length that would be realistic in a car is the length of a Ford Expedition, roughly 5.2m. I don't see a workable solution to extend that during travel, although perhaps it is possible if the car was constructed properly. There would have to be an extensible shell, coupled with indicators to do it properly. It would also require a portion of the sides to be windowless to pull it off correctly. I've done a very rough plan view to give an idea. Think of it unfolding much like one of those Mercedes convertables do, at speed like the whale tail of a Porsche does.



Such a contraption could make the effective length of a car longer, perhaps bringing the 5.2m of the Expedition up to perhaps a maximum highway 7m.

The other constraint is that it needs to be wide enough at the base to enable adequate cornering.

So, I suppose we eventually get something that looks more and more like an elongated (and probably taller) horseshoe crab:



Essentially this shape is where we are heading. After the wheels are covered, the underside is covered, the grille is basically just a tiny horizontal mouth, the wheels are as narrow and tall as possible with LRR tires, this should ultimately be our destination.



I suppose the only thing is that in most of the world, the econobubble shape will still predominate because older cars cannot easily be legislated off the road.

I think that one way the government can effectively legislatively provide incentives is to target mpg AND emissions.

Although now there are increasing market forces to bias us in this direction anyway. More if Bush (or his handlers) decide to invade Iran. He might ironically be the best president for the environment the US has ever produced, for all the wrong reasons!
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Old 06-11-2006, 09:37 PM   #7
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Another thing we will need to see is a more reclined, sportscar seating type of arrangement. Which should not be a problem because a longer car will enable a lower drag coefficient.
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Old 06-11-2006, 10:03 PM   #8
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Quote:
Originally Posted by thisisntjared
why would more gears help though? you still need the power AND aerodynamics to get up there. the amount of gears allows a better blend of acceleration and top speed. you can still have a higher speed with less gears.bmw already had them, last year, and automatic trannys suck.

anyway the bottom line is that there is no need to try to change the subject of this thread. aerodynamics are necisary and it is very to consider the fact that a cars coefficient of drag is in fact a speed dependant variable. good food for thought, try chewing.

to mira: kudos on the intreguing topic. it makes me feel that the number one concern in aerodynamics at any speed is reducing the frontal area.
More gears help with fuel economy too. If i want to drive like everybody else I would be in 3rd most of the time because there is no power in 4th till about 75mph. The average person pushes the pedal harder if they want more accelalration because some cars like mine the gears a very long. With more gears you'd waste less gas because you'd have to press the gas less. Plus you make final drive higher with more gears and the gears longer. You don't understand this concept because you don't drive a 4 speed automatic everyday. More gears is good.
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Old 06-13-2006, 09:58 PM   #9
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i understand more gears is good but thats not what we are talking about.

remember we are not talking about acceleration. we are talking about a maintained speed. the only thing that would matter is the one gear ratio. dont tell me i wouldnt understand a concept. i understand both physics and statistics as well as the topic of this thread.

back on topic: ive heard that its important to reduce the surface area as well as the frontal area. if a shape become too drawn out, it will not help much for the cd so its really difficult to strike the right compromise... man i wish i had a wind tunnel...
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Old 06-14-2006, 03:04 AM   #10
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Quote:
Originally Posted by thisisntjared
man i wish i had a wind tunnel...
you do - stick your hand out the window while driving down the highway!

But seriously get a blower fan and setup a small tunnel.
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