EcoModder Forum Kamm back speed at which it starts to work?

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 12-16-2008, 02:11 AM #1 (permalink) EcoModding Apprentice   Join Date: Sep 2008 Location: Australia Posts: 109 Thanks: 1 Thanked 5 Times in 5 Posts Kamm back speed at which it starts to work? Kamm back speed at which it starts to work? I used to have a book that gave a guide to the speed at which the airflow held its shape, but I have misplaced it. Any references online etc. Cheers
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speed

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 Originally Posted by Geebee Kamm back speed at which it starts to work? I used to have a book that gave a guide to the speed at which the airflow held its shape, but I have misplaced it. Any references online etc. Cheers
The coefficient of aerodynamic drag for your car is fixed,and remains constant,once up to about 20-mph ( 33 km/h ),and the experts now say that aero drag is significant by 35-mph ( 56 km/h ).
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Quote:
 Originally Posted by aerohead The coefficient of aerodynamic drag for your car is fixed,and remains constant,once up to about 20-mph ( 33 km/h ),and the experts now say that aero drag is significant by 35-mph ( 56 km/h ).
Since I think most cars spend the majority of their time above 30 MPH, would it be safe to say that aero-mods "always help", but that the return, the "bang for the buck", is better above 35 MPH? I keep thinking there's an "economy curve" that would slope up and apex around 55 MPH, and then start to slope down with diminishing returns. Orrrrrrrrr, would the apex of the curve change as a function of the Cd (aka basjoos)? Kind of a BSFC for each Cd.

CarloSW2
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apex

Quote:
 Originally Posted by cfg83 aerohead - Since I think most cars spend the majority of their time above 30 MPH, would it be safe to say that aero-mods "always help", but that the return, the "bang for the buck", is better above 35 MPH? I keep thinking there's an "economy curve" that would slope up and apex around 55 MPH, and then start to slope down with diminishing returns. Orrrrrrrrr, would the apex of the curve change as a function of the Cd (aka basjoos)? Kind of a BSFC for each Cd. CarloSW2
I think the low speeds are a"wash" for aero,and emphasize to people that I meet, that my mods are virtually useless around town.So yeah,I think they really begin to shine at the higher velocities.------------------ The thing about the "apex" is that as one continued to streamline towards the ultimate,say Cd0.11-12.that you could push that curve up,forestalling the effects of the air.---------- The whole premise of CAR and DRIVER's "Crisis-Fighter-Pinto" was,leave the 70-mph speed-limit alone,and streamline the cars so the achieve the same mpg at 70,they then achieved at 55.----------------------- With the T-100,I've got data from 45-mph(72km/h),to 80 mph(129km/h),and the speed/mpg plot is pretty linear.The best mpg is at 72 km/h (39-mpg),it falls to (36.5 mph) at 88 km/h,and by 120 km/h she's down to 32-mpg.The jump to 129km/h costs a whole mpg.It's not legal to test at higher speeds so I'm stuck.Best mpg should occur in the 35-40 mph range (56-64 km/h) but it's not safe for me to attempt steady-speed driving at those kinds of velocities to check mpg ( no scan-gauge!).These lower speeds will always remain the domain for best mpg but I can't imagine the highway motorist surrendering to such speeds.So in the meantime,the mods will help.
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Quote:
 Originally Posted by cfg83 aerohead - ...aero-mods "always help"... CarloSW2
Kammback creates drag. It doesn't reduce drag.

To make an object stable while in motion, the center of gravity must be forward of the center of pressure. This is true for high speed automobiles, airplanes, rockets or javelins.

Kammbacks are used in Top Fuel Funny Cars for two reasons. 1. to keep downforce or pressure on the rear tires. 2. To increase drag aft of the CG.

In the movie World's Fastest Indian Burt Munro did a stability demo with a pencil, the pencil became stable when the CG was moved forward of the CP. Burt wanted to add weight to the front end of his Indian MC to shift the CG forward of the center of pressure, thus make it more stable.

IMHO, dams, wings, spoilers, kammbacks are "thangs" that are used to overcome bad aero design. Anything and everything that has mass and moves in a fluid creates drag.

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Bsfc

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 Originally Posted by akashic Speaking of... In which direction does the BSFC curve shift/stretch as a result of a decrease in Cd?
I think at the eco-modding forum there are some BSFC maps which have been posted in the sticky.Hucho gets into this very well,shows the shifts,and how the gearing should be optimized to get the full-monty.Without the graphics we shouldn't even touch this,and we probably should take it over to the general eco-modding page.I don't have my text.From G.W.Carr,"The Development of a Low-Drag Body Shape for a Small Saloon Car",MIRA Report #2,1965, he shows that you can lose up to 45% of the streamlining benefit without gear-matching.------------ That's all I have with me.------------ The engine see's a lower load with streamlining,and will over-rev without the taller gearing,shortening it's life if not self-destructing.
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kammback

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 Originally Posted by CobraBall Kammback creates drag. It doesn't reduce drag. To make an object stable while in motion, the center of gravity must be forward of the center of pressure. This is true for high speed automobiles, airplanes, rockets or javelins. Kammbacks are used in Top Fuel Funny Cars for two reasons. 1. to keep downforce or pressure on the rear tires. 2. To increase drag aft of the CG. In the movie World's Fastest Indian Burt Munro did a stability demo with a pencil, the pencil became stable when the CG was moved forward of the CP. Burt wanted to add weight to the front end of his Indian MC to shift the CG forward of the center of pressure, thus make it more stable. IMHO, dams, wings, spoilers, kammbacks are "thangs" that are used to overcome bad aero design. Anything and everything that has mass and moves in a fluid creates drag.
The Kammback citing may have been used in the context of separation-free flow up to the point where the car body was chopped off.And with Cd0.37,the K-cars were remarkably "cleaner" than their contemporaries.And yes,the K-cars did have cross-wind stability problems.The up-shot is that Dr.Morrelli solved that one back in the 1980s,so we're better protected from those challenges.
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Quote:
 Originally Posted by akashic Speaking of... In which direction does the BSFC curve shift/stretch as a result of a decrease in Cd?
BSFC only has to do with the engine. Aerodynamics does not effect it whatsoever. It affects how much power it takes to cruise at a given speed, so it affects where on the BSFC map you fall at a given time, but it does not change the map in the slightest.

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aero/BSFC

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 Originally Posted by some_other_dave BSFC only has to do with the engine. Aerodynamics does not effect it whatsoever. It affects how much power it takes to cruise at a given speed, so it affects where on the BSFC map you fall at a given time, but it does not change the map in the slightest. -soD
Hucho claims in his text that should a person streamline their car,that the road load can shift so far,as to knock the engine out of it's island of highest BSFC.The car will also overspeed,and if the gearing is not changed such that the engine "see's" the same load as before the streamlining,then MPG potential from the drag reduction can suffer up to 60%.

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