Coefficient of drag

[Thunder26]

I'm wondering what other people would estimate the coefficient of drag is on our broncos. I know there are a lot of variables but I would like to know some values.

The reason being is I made a fairly accurate speed calculator and would like to estimate the power needed to push my bronco down the road at certain speeds.

If anyone would like a copy just tell me and I'll figure out where I can host it so people can download it.

 

[73azbronco]

I doubt anyone has that. One way to reverse engineer it is to go to a dyno, figure out how much HP/manifold vacuum you have at cruise and other settings, then go drive it to same speed/same vacuum settings and see what that delta is either in vacuum or MPH based on HP, then figure whatever that delta is caused by air drag, then figure your Cd.

 

[Dave]

How about this: Pull it down the road with a long rope and a pull scale. Record the force at different speeds. Disconnecting the rear driveshaft would eliminate drag from backdriving the drivetrain. Just a thought.

 

[73azbronco]

that would work as well.

 

[Thunder26]

Good advice keep it coming please random numbers out your ass are fine too!

 

[broncnaz]

There are so many other random factors that would affect drag coeiffiecnts. like tires, lifts, and other accesorries its just not the most precise way to estimate anything.
I'm not quite sure what your trying to figure out as you will probably pull a quite low power number with your figures. That power number would only indicate what it would take to move the bronco at X speed doesnt mean it would be very good and doesnt account for tires, gearing, weight or the engine RPM or any other factors.
Something like your doing is really only good for drag race applications where your looking to at least attain a minimum HP that it would take to attain a certain speed.
Basically I think you'd be better off asking a question that is related to your actual wants based upon your rig. Most of us already have real world experiance with different combos and can give you good feedback on the subject and explain some of the whys and how's of certain setups.
Math is good but it doesnt answer all the questions and is only based upon numbers in a given situation it doesnt accoutn for varibles.

 

[hpprose]

I was looking for mpg info last week and I can across a thread in here that talked about drag. Try doing a search for mpg or fuel economy (don’t remember what I searched under.)

 

[Thunder26]

There are so many other random factors that would affect drag coeiffiecnts. like tires, lifts, and other accesorries its just not the most precise way to estimate anything.
I'm not quite sure what your trying to figure out as you will probably pull a quite low power number with your figures. That power number would only indicate what it would take to move the bronco at X speed doesnt mean it would be very good and doesnt account for tires, gearing, weight or the engine RPM or any other factors.
Something like your doing is really only good for drag race applications where your looking to at least attain a minimum HP that it would take to attain a certain speed.
Basically I think you'd be better off asking a question that is related to your actual wants based upon your rig. Most of us already have real world experiance with different combos and can give you good feedback on the subject and explain some of the whys and how's of certain setups.
Math is good but it doesnt answer all the questions and is only based upon numbers in a given situation it doesnt accoutn for varibles.

No my calculator accounts for quite a bit actually (Engineering project) it estimates most cars to within 3 mph of top speed. And yes I understand there a lot of variables hence why I just wanna generic number or range.

You are correct though that yes just asking for peoples experiences would be easier its not what I want.

 

[Viperwolf1]

The words "efficient" and "drag" should never be used in the same sentence when talking EB's.

It might be easy to estimate based on the top speed of some of the 6 cylinder models.

 

[Chinook Pilot]

In my line of work the coefficient of drag is directly inverse to the coefficient of lift! Not that it helps you out. Just a question, but why do you want to figure that out?

 

[is_wiz]

if they give you a range of coefficient's just assume the bronco is the worse. Its basically a moving billboard.

 

[broncnaz]

Ok thats a different story then your just asking for a class project. Still numbers only represent knowns its the little unknown varibles that make the differance. And top speed is different than cruise/other speeds. Ok from what I remember of that other post that was mentioned the drag coeffiecent for a bronco was something like .68 or so.

 

[DirtDonk]

Many years ago, I think it was in Car and Driver magazine that they actually gave out the equations the manufacturers use for figuring out the drag coefficient of a vehicle.
Might look in their archives to see if it's listed.
Yes, there are variables, but I seem to remember that, as in much of mathematics, they were taken into account to a certain extent. I don't think rolling-resistance is part of the drag they were factoring though. It certainly effects mileage and horsepower needs, and can be considered "drag", but it's not part of the "aerodynamic" drag side of things.
If you need to take into account drivetrain losses and tires though, you could be in trouble!

Unless you already have the theory and equation, but are looking for something more specific.

Paul

 

[VT_Don]

In attempt to earn a 200 MPH license at Bonneville in my Bronco, has been necessary to achieve far greater horse power then initial Cd estimates dictated. To maintain stability, have added ground effects and tail fins off a '59 Eldo.

 

[Devin]

I broke out the old fluid mechanics book and they had a section on drag coefficient for automobiles. It gave a pretty generic equation for drag that consisted of a number of variables corresponding to the shape of various features such as front end, windshield, roof, rear end, windshield peak, cowl/fender cross section, etc. Based on that equation, I get a Cd of .464.

I think that is a bit generous, but seems to also fit the trendline that the book shows for the average Cd for cars over the past century. But, there are many factors to consider and I think that some of the variables in the book don't truly correspond to a Bronco's shape. I would still say it is in the range of .5 or so. Maybe higher. The Cd for a solid cube is 1.08 or something like that if I remember correctly.

 

[Thunder26]

The words "efficient" and "drag" should never be used in the same sentence when talking EB's.

It might be easy to estimate based on the top speed of some of the 6 cylinder models.

That is a great idea thank you.

I wanna know just for comparison, like everyone calls it a brick or billboard goin down the road but is it more of a brick than an H2 and if so to what degree. I'm not looking for "Oh its .56 definitely" just like "It's between .8 and 1"

 

[Thunder26]

I broke out the old fluid mechanics book and they had a section on drag coefficient for automobiles. It gave a pretty generic equation for drag that consisted of a number of variables corresponding to the shape of various features such as front end, windshield, roof, rear end, windshield peak, cowl/fender cross section, etc. Based on that equation, I get a Cd of .464.

I think that is a bit generous, but seems to also fit the trendline that the book shows for the average Cd for cars over the past century. But, there are many factors to consider and I think that some of the variables in the book don't truly correspond to a Bronco's shape. I would still say it is in the range of .5 or so. Maybe higher. The Cd for a solid cube is 1.08 or something like that if I remember correctly.

Thanks I would agree that number does sound very generous as I personally would estimate it at where bronconaz did based upon its shape compared to an H2.

If I may ask what kind of engineering did you study Devin?

 

[Devin]

Thanks I would agree that number does sound very generous as I personally would estimate it at where bronconaz did based upon its shape compared to an H2.

If I may ask what kind of engineering did you study Devin?

I'm a ME.

 

[Dave]

Here is a link to a discussion from a few weeks back.

http://classicbroncos.com/forums/showthread.php?p=1694534#post1694534

 

[Hauq]

Another option to determine the amount of hp that is required to push it through the air is to have it chassis dinoed then use a Gtech like device that determines hp from acceleration. The difference between the two would be the power required to push it through the air.

I would suspect that if you don't have a top on your bronco the Cd is much higher then the H2, and if you do have a top it is a little higher then the H2.