Cd and blunt front ends

[TheDon]

Originally Posted by NoCO2

What is the "coast down" test you are referring to? I would like to see what my .Cd is as well.

http://www.instructables.com/id/Meas...t-of-your-car/

[Shawn D.]

Originally Posted by ttoyoda

.What would you estimate the height of the front of the car to be? About one and one-half front rim diameters?

So with a 15 inch rim (just guessing, I have no idea) that would be
22.5 inches.

Nope, I'd say about twice the wheel diameter, or about 30."

[Shawn D.]

Originally Posted by .Cd

How does wake size effect .Cd ?

It's already factored in to the Cd. Remember that the "C" in Cd is "coefficient" -- it's multiplied by the dynamic pressure and cross-sectional area to get the drag.

[ttoyoda]

Nope, I'd say about twice the wheel diameter, or about 30."

In gimp I measure the height of the moon hubcap (which is all I can see clearly) as 75 pixels high.
The front of the car I measure as 118 pixels high from the shadow on the ground. So that is a ratio of 1.573 to 1.
I still dont know the rim size or hubcap size though.

[Shawn D.]

Originally Posted by ttoyoda

In gimp I measure the height of the moon hubcap (which is all I can see clearly) as 75 pixels high.
The front of the car I measure as 118 pixels high from the shadow on the ground. So that is a ratio of 1.573 to 1.
I still dont know the rim size or hubcap size though.

Well, what you're looking at is the hubcap, which is a larger diameter than the specified wheel size (which is at the bead, not the external dimensions) by about 1". Wait a minute, what part of the car are you talking about? I'm talking about the top of the fender over the tire.

[ttoyoda]

Wait a minute, what part of the car are you talking about? I'm talking about the top of the fender over the tire.

Ah sorry I should have made a pic before.

Here:

[ttoyoda]

So if the hubcap is 17 inches in diameter the front of the car is 26.74 high...
(unless I messed up the math)
It might be easier to find one of these cars and measure the side window glass..

[aerohead]

Originally Posted by .Cd

So Phil, can I bring my .Cd down to under point 20 without installing a boat tail ?
( I drive a '93 Civic hatchback )

I have read that the new Tahoe hybrid has a .35 drag coefficient.
Ignoring .CdA, this would mean that the SUV would have a better .Cd rating than a Ford Focus hatch at point 36, despite having a much larger wake.

How does wake size effect .Cd ?

Thanks ! ( And Phil ...are you ever going to make it down to the Texas Mile ? It's a lot closer than Bonneville ! )

.Cd ,It's kinda questionable.I'm putting an installment together on tails,it will be the biggest,as I think it still has the most significance,if only 'cause all the big dogs still say so.The only surviving K-car,built on the Mercedes 170 chassis was tested by Hucho's buddies at VW's big tunnel and it scored only Cd0.37,kinda mediocre by today's standards.When VW was designing the VW 2000 concept vehicle in 1980 ( today's car ),they started with the Jaray/Klemperer "brick" as their starting point for aero.The brick would be like a minivan today,and it had a Cd 0.22,in 1922.If you haven't seen it yet,I'll eventually get it posted ( sorry! ).Anyway,even the brick had a truncated boat-tail,much like Mair published about in his work.Your CIVIC is much like the Koenig/Kamm K-car form,with early- on roof slope,and plan-taper at the sides.And like Koenig and Kamm did,HONDA just chopped-off the roofline where they liked it,leaving a large wake vehicle.The good news is that you won't have attached longitudinal vortices that I and other CRX owners have to sweat about,if we get too aggressive with our fast-back angles.The down side is that you have that big wake.I don't have my stuff with me but I can tell you that all the stuff you read about "phantom-tails" created behind a K-car,don't seem to hold up when examined.Comparisons of Jaray's fast-back,the Klemperer brick,Ley's research at University of Michigan,Mair's own boat-tail publication,Daimler-Benz' report on the C-111,the aero-modded Chrysler Airflow,EV-1 LSR car,Oldsmobile AEROTECH Long-tail LSR car,and others,GM/Aerovironment Sunraycer LSR car,etc.,all point to aftbody mods as the source of phenomenal performance.All the super-mpg concept cars pay particular attention to the aftbody.Since HONDA has hit Cd 0.10 with the Dream-2 Solar car,it may be possible to hit Cd 0.20 with a partial tail,but I think you'll need some tail extension.It's very much a wing-section and Dornier demonstrated that chopped-off wings and fuselage didn't suffer that much of a hit on drag.I don't have his stuff so I can't say anything on that one.Oh yeah,the thing about the Tahoe and Focus ( sorry,early senility ),a vehicles drag signiture is defined by it's "drag-factor" a product of Drag coefficient and frontal area.Drag varies arithmetically with frontal area,so for example,a two-fold increase in area would result in a two-fold increase in drag.The Focus,at Cd0.36 and say 19-square-feet frontal area would have a drag-factor,CdA 6.84-square feet.The Tahoe,at Cd 0.35,but maybe 38-square-feet frontal area ( Don't know real area ) would have a drag-factor of CdA 13.3 square feet,94-percent higher than the Focus and would lose 47-percent fuel economy right there.So yes,"Size Matters!".As far as the Texas Mile goes,I'm thinking it's just not long enough.Charles Farmer is a local Bonneville streamliner guy and goes down there.Perhaps I can crew for him and tag along one time to see what it's like.With low-drag,low-power vehicles,they seem to continue to accelerate forever.If I'm gonna do good with the T-100,I may need all that extra distance.We'll see.

[ttoyoda]

aerohead,
I am not trying to be a smartass, and I mean this in the nicest possible way, but do you think you could add some paragraph breaks to your posts? Say every 5 lines or so? You have lots of good info, but that solid mass of text makes human eyes bounce off of it.

[aerohead]

Originally Posted by ttoyoda

aerohead,
I am not trying to be a smartass, and I mean this in the nicest possible way, but do you think you could add some paragraph breaks to your posts? Say every 5 lines or so? You have lots of good info, but that solid mass of text makes human eyes bounce off of it.

Yeah they drive me crazy too! I even think that Darin sneaks in sometimes and tries to clean up some of my mess. When I get here to the copy center,where I use the computer,its kinda balls to the wall 'til the clock runs out,and I forget some of the finer points of communication. Your point is well taken and I'll be lookin' over my own shoulder next time I post.I appreciate the constructive criticism.