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  #16 (permalink)  
Old 04-09-2008, 11:49 PM
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Quote:
Originally Posted by OneMoreTime
Well here you are a fairly simple roll down test one can do with his own car..Might be useful if one is looking at how different mods affect airflow and drag..

http://physics.technion.ac.il/~rutma...own%20test.pdf

Opens as A PDF file..

Sam
This is actually a decent article, but it is missing a few items. Do you happen to have the authors contact info?

His rolling resistence for is only address the resistence from the tire to the concrete. He ignored the resistence force sin the driveline components. Not a huge deal but there are factors like the wheel bearings being new or old, the rotors being smooth or rough, e.t.c. I don't know how great of a deal this would affect your values. I would assume, not by much, only because there are so many estimated values

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  #17 (permalink)  
Old 04-10-2008, 12:31 AM
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Quote:
Originally Posted by Riot Racing
This is actually a decent article, but it is missing a few items. Do you happen to have the authors contact info?

His rolling resistence for is only address the resistence from the tire to the concrete. He ignored the resistence force sin the driveline components. Not a huge deal but there are factors like the wheel bearings being new or old, the rotors being smooth or rough, e.t.c. I don't know how great of a deal this would affect your values. I would assume, not by much, only because there are so many estimated values

If someone is being so exact in estimating the top speed of a car or the horsepower required to propel it by calculating all of the drag values and parasitic losses then all of those parts will be brand new.

Why don't you do it right. Take your car to a wind tunnel and pay for test time.
http://wind.tamu.edu/index.htm


I'm sure there are many other wind tunnels and aerodynamicists that will take your money..


Then, while you are in the tunnel, make modifications to your car that will reduce drag.

How do you know if you do find the COD of your specific car on some website that you can duplicate the results...What if the test car is a few inches lower? Or has the antenna removed? or has been waxed or the paint is oxidized?
Will the test subject car be using the same size tires as your car has? Or what about hubcaps? Fender skirts? Belly pan? Painted and waxed undercarraige? Hood ornament? Raked stance? All of that would make a difference.

LSR guys don't spend as much time worrying about some numbers they might find in a book or website...The proof is in the actual top speed of the car, at the track. ( I have worked on body construction of several record setting LSR cars, and can tell you that none of them did any wind tunnel testing.)

I saw several cars in the website that bentwings provided that would approximate the coefficient of drag for your specific car...Oh...that's right...you don't have a specific car....


If you want to go really fast, or perhaps you are trying to make a gas miser,.(I don't know, you never said what you are trying to accomplish with this information), either way, then a shoebox chevy ain't a good start .

Using the COD of a brick would be close.
Hahahahahahaha


later, mikey
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  #18 (permalink)  
Old 04-10-2008, 12:48 AM
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I disagree....there is no mention of removing the drag from all the bearings, drive shaft, u-joints, tires or transmission. All of these are included in the rolling resistance. You could very easily check tires by just changing them and repeating the test.

His purpose here is to separate the aero drag from all other drag.

You could also check the engine drag by repeating the tests with and without the transmission in neutral.
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  #19 (permalink)  
Old 04-10-2008, 12:52 AM
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I would take the rolling resistance as the summation of all the mechanical drag in the vehicle and let us assume that the vehicle has been adjusted to minimize as much as possible that sort of drag..that would leave the results of the roll down test in accordance with the aero drag of that car..A simple roll down test can be good to evaluate the result of mods to the car once a baseline is determined..To obtain the actual rolling resistance of the car may be beyond the scope of this test..

Now in order to totally eliminate rolling resistance in the calculations would require the use of a wind tunnel which is a whole different deal and beyond what we can do in a roll down test..And probably beyond the available resources to most people..

For most of us the usefulness of a roll down test would be in the area of a before and after the addition of a ground effects or aero package to an existing car or to answer the question to see if the money spent on that package was worth it or does the package live up to manufactures claims..

Hope I have not confused anyone here..

Sam
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  #20 (permalink)  
Old 04-10-2008, 01:03 AM
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One more time beat me to it. We are looking for baseline number the same as we do with a dyno.

Make a change and retest. Calculate % improvement/loss and add a calculation for dollars vs percent to see if it is worthwhile.

This is one of the things you can do to have fun with your ride.
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  #21 (permalink)  
Old 04-10-2008, 08:11 AM
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Quote:
Originally Posted by bullheimer
maybe because asking for this information is roughly akin to wanting to know what Paris Hilton's IQ is.

I mean, we all know she is really, really stupid right? isn't that enough??? Does one really need to know if she can be classified as a moron, an Imbecile, or an Idiot?? I think not. Brick wall, parachute, any way you slice it, you might as well find the drag coefficient of a school bus.

LOL, you make me raff all night rong.

Brian
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Old 04-10-2008, 08:13 AM
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Quote:
Originally Posted by Riot Racing
This is actually a decent article, but it is missing a few items. Do you happen to have the authors contact info?

His rolling resistence for is only address the resistence from the tire to the concrete. He ignored the resistence force sin the driveline components. Not a huge deal but there are factors like the wheel bearings being new or old, the rotors being smooth or rough, e.t.c. I don't know how great of a deal this would affect your values. I would assume, not by much, only because there are so many estimated values

You are talking variables that will blow the whole thing. How about retracting brake pads? The width of the tires, the air pressure in them, etc.

It really depends on what you are looking to accomplish. If it is a super high fuel economy in town car, the COD may not even be much of an issue and rolling and mechanical resistance would come into play more. It is a high speed LSR car, that is a whole different issue.

Check out some electric vehicle sites resistance is one of their biggest battles.

Brian

Last edited by MARTINSR; 04-10-2008 at 08:19 AM.
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  #23 (permalink)  
Old 04-10-2008, 12:03 PM
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""Aerodynamics are for people who cannot build engines"...... "

Enzo Ferrari..





Uh..he did change his mind on this somewhat later..
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  #24 (permalink)  
Old 04-10-2008, 06:16 PM
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Knowing the CD of a factory based car or a car that has been SLIGHTLY modified build suffice. I intentionally don't mention my plans or what car because some of these items are not real critical.

I was leaning towards the 53 and 54 chevrolet 2 door cars. The ford shoebox is a similar structure and body style, so the Cd for either would help me.

I just need a general number. are we talking over .34 or under?
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  #25 (permalink)  
Old 04-10-2008, 06:46 PM
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I guess you would be closer to .41-.42. Maybe even worse. I had an 89 T-Bird Super Coupe and it's listed cd was .30. very good for the day. Not many every day cars are under .30.

Some of the 40's vintage cars, 40 Ford had pretty good cd for the day. I think they were about .38-.40. hard to believe.

I don't know what the 32 and 33-34's had but when the fat fenders and 41 Willys came around they fell behind in the drag racing world.

As a side I just got to take a test ride in a customer roadster and it was like driving a sail. The coast down was pretty dramatic. It's overall drag was more than my big dually. About 10 mph over the same distance, same road.
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