Track Roadster Cd or Coefficient of Drag

I know there are A LOT of variables involved here, and I know this sort of thing wasn't/isn't "common knowledge" in our hobby, but does anyone have any idea on what the Cd of say, an open wheeled '23-'25 T roadster body with track nose and full bellypan would be (or any open wheeled hot rod for that matter)? Maybe someone who's ran something similar at Bonneville has an estimate? Maybe you have ideas on how to estimate drag before the car is built? Without wind tunnel access, I know nothing will be exact, and I know you can do the "coast down" method, but I was just wondering if there are any tricks/info any of you might be willing to share. Any input is appreciated, and thank you in advance.​

 

Too many variables to venture a guess.For a good read,"Race Car Aerodynamics:designing for Speed,by Joseph Katz,Phd. Bruce Bowling use to have a web site to chart out cd based on frontal dimensions etc. But again, too many variables.Belly pans, Louvers to release trapped air, under the car, all contribute.It was said I forget ? trying to eke out those last few mph, on his streamliner,met with an aero engineer,who suggested.streamline axle,paint the car Black(for a thermal heatbarrier,)and Raise the car 6"(cleaner air).Did it. Got it.

 

Interesting point. All my car books are either chassis, carburetor, or engine related. I guess I'll be dipping into aero. I was (I guess) just hoping if Person A had X track roadster and happened to read this thread and be like, "I have this, this, and this on my roadster; I estimate my Cd to be ___". Then person B would come along and say, "I did the coast down method in my roadster and estimate mine to be ___". I guess I was hoping for a real world comparison on parts and geometry. Thank you for the book suggestion. I'll have to pick that up.

 

I did find that the late model Plymouth Prowler Cd to be 0.49, which is to be expected.

However, Wikipedia says this about a very similar shaped Volvo T6 concept: "It is powered by a twin-turbocharged 2.9-liter inline 6-cylinder engine from Volvo S80 giving 300 bhp (220 kW) and a top speed of 205 mph (330 km/h).<SUP id=cite_ref-edmunds_1-1 class=reference jQuery183007723780554445686="45">[1]</SUP> The engine is located behind the driver. Its design is obviously inspired by hot rod cars. It was named the 2004 Hot Rod of the Year by Hot Rod Magazine."

OBVIOUSLY, this thing would have to have an extremely low Cd to get 205mph out of 300hp (or it has way more than 300hp).

 

I have a short description by Bruce Bowling"Calculation of Frontal Area. Frontal area represents the frontal projection area of the vehicle.If one takes a picture of a vehicle, it is the area included in the outline.use the following to calculate:1. calculate the area of a rectangle which would encompass the front of the vehicle( multiply the width by the heighth)" then he gets into mc handlebars etc."2.Adjust the figure obtained for areas not included such as top rounded corners etc."..... Look for a web site Maybe (accelerator slide rule).They say a teardrop is the perfect aero design.I've heard that both a Porsche 911,and a T are better backwards.Seriously.If you ever get a chance check out the books from Carroll Smith.Engineer to Win, Tune to Win.. Warning... Now this is my hack Job of parphrasing so take none of this to heart

 

Caterham .7, Legends car .74,

 

The Wiki chart is here... Gary

http://en.wikipedia.org/wiki/Automobile_drag_coefficient

 

This is probably off-topic, but here goes: There was a feature article in Hot Rod Magazine a couple of years ago addressing horsepower vs speed. Car owner took a Bonneville street roadster to England for some wind tunnel time. (basically no fenders, no windshield, headlights installed, no major body mods) Results were that it takes 500 HP to overcome wind resistance @ 200 MPH with this particular car. Main factor affecting horsepower required seemed to be frontal area. No driveline or rolling resistance factored in. This info may be used to estimate, speculate, or extrapolate the CD of your car. Beer may help the process.

 

If my memory is correct the FAMOUS AK MILLER did turn a 1927 T roadster body around on the chassis and ran it at Bonneville !!

 

Do you have any pictures of this? Sounds like a picture would be in order to fully grasp what this thing looked like

 

Thank you for the suggestion! I knew if I posted a thread on this I'd get some good info! I found a few related HRM links:

http://www.hotrod.com/<WBR>techarticles/body/hdrp_0609_<WBR>aero_tricks_tips/
http://www.hotrod.com/<WBR>roadtests/hrdp_0604_thacker_<WBR>shine_a_v8_roadster_testing/<WBR>viewall.html
http://www.hotrod.com/<WBR>techarticles/body/113_0703_<WBR>car_aerodynamics/

The middle link is the Thacker/Shine roadster, which is the article you described.
What's really cool (to me anyway) is what HRM did with the Camaro. Those are crazy reductions in Cd!
I looked up that AeroDyn wind tunnel facility. If only I wasn't poor and a little closer haha! $1800/hr for testing!!! But- very cool that that sort of thing is available. For a school related field trip, I got to see Daimler's wind tunnel in Portland where they test Freightliner's trucks. That was VERY cool. I wonder if enough beer would buy me a couple hours with their wind tunnel people lol...

 

gnichols: That Wiki link is the very thing that got me wondering what the Cd of a roadster would be. Cd's have really gone down in the last few years, especially if you consider the concept vehicles. The creative juices are flowing!

 

I found the Caterham car and info; can you expand on the second: "Legends car .74"?

 

Thanks again for the info. I know I can take a photo of the front of a car and import the image file into a CAD program, and calculate frontal area. I just found out today that you can simulate wind tunnel testing using SolidWorks, so I think I'll play around with that in the future.

 

Here's a pretty neat link I found from the "Swoopy Cars" thread:
http://ecomodder.com/forum/tool-aero-template.php
You can import images of your car and scale/overlay this template to get an idea of what the airflow is gonna be doing. It's pretty fun to screw around with.

 

Think brick, or something like a small dead tree with each and every one of the bare branches catching the wind. At best, compare it to a Lotus 7 / Caterham, cars that have just about everything sticking out in the wind - tiny fenders if any, suspension bits, open wheels and tires, upright windshield. Those little cuties are duller than your average truck. I read another figure quoted once, that the Lotus 7 offers more wind resistance than your average Greyhound bus. Open wheel hot rods can't be much better than those 7s. But it would be nice to know the numbers of one from an actual wind tunnel test.

I'm no aero engineer, but I've always thought putting laid back / boat style windscreens on T/A/32s just looks silly. I'd hazard a guess that an aero / track nose, removing everything above the cowl or a severe chop with laid back front glass would make the most dramatic improvements but those changes probably just get you down to the level of the insanely poor Lotus 7 and a Hummer! After all, that's just what you see in the vintage Bonneville pix. Throw in a HUGE V8 and what do you create? A Sprint car! Wasn't it Ferrari who said something like - Aero is for guys who can't build engines? Gary

 

This is interesting stuff and something that I have thought about when driving my Model A sedan on the Pa. Turnpike. It's like a barn door going down the road.
I'd be interested to know what the Cd is for the typical hot rod. In this regard it seems we're not very efficient.
According to the chart, I'd guess that the average hot rod has more wind resistance than a Humber. I wonder what the impact of a streamlined body would have on trap speed in the quarter mile
It surprises me that Colin Chapman, who seemed to care a lot about vehicle weight, didn't pay more attention to aerodynamics when developing the Lotus 7, although, granted, this was over 50 years ago.

 

It's been a long time since I've used that grey matter.Many holes in the surface. In my mind,the cheapest cool car for Bonneville is Still the Studebaker 1953.Killer numbers.I go to Bonneville every year (except last).You can hear when a Stude rolls by at speed.Big block, small block,Hemi... they just sound different.Gives me Chills...

 

What you really want to know is CdA or Cd * Area. I found some calculations on a 32-Roadster derived from coastdown measurements at .42 * 18 = 7.56. This gives a Top Speed of about 160 with 375RWHP.

This was a rod they had been working on to get the drag down for Bonneville so it might be close. I an thinking a typical street rod would be more like 10-12.

http://ecomodder.com/wiki/index.php/Vehicle_Coefficient_of_Drag_List

 

How about this...
You can figure your horsepower by drag racing and using the et and mph to come up with a pretty close estimate of wheel hp. And you can do this on a road for a 1/4 of a mile.

Then you can go do a high speed run and see where it tops out at.

Now you know the horsepower and top speed where the areo drag = the HP.

From that you can figure Cd, right? And know how much more hp you need to get to the speed you want to get to.

 

Try coastdown testing.

http://www.iwilltry.org/b/wp-content/uploads/2010/05/Drag_Coefficient.xls

 

Yes, Enzo Ferrari is credited with that quote. That's a great quote by the way, especially considering the mouth it came from!

 

The easiest way to estimate drag if you have already built the car and it is aready running and driving is what's called the "coast down method", like hoop98 is saying. It would be neat to do a HAMB "coast down method" test. Post a pic of your car and coast down testing results. THAT WOULD BE SO INFORMATIVE!

 

Yes; you're right. I guess I should make the statement now that I'm not trying to build a 200+mph Bonneville car. I'm an engineering student, and I'm trying to get a few ideas and tricks up my sleeve for a future school project.

This day in age, the thing that matters most to people is fuel economy when they're talking cars. You could have all these innovations on your car to make it go faster and make more power, but because it gets 12mpg, most people will look down on you and not give those innovations a second look.

On the opposite side of the spectrum, if it looks like complete garbage, has no power, and is an environmental disaster from a manufacturing stand point, BUT it happens to get good mpg, people eat it up and all of the sudden they're everywhere (Prius).

Before I got into cars, I was/still am a skateboarder. In skateboarding, style is EVERYTHING. It doesn't matter how technical of a trick you can pull off-if it looks like complete filth, you still suck. So, people in this hobby spend so many hours refining and smoothing out their lines to make things "flow". Streamlining if you will. This mentality has been drilled into me, so when I see the cars of today which LACK STYLE ENTIRELY, I can in no way get excited about them.

Like it or not, the hot rod community is extremely environmentally and economically friendly. We recycle and repurpose more than any other auto industry out there. We are buying used parts off each other and new parts from small manufacturers and having things welded by "Joe Blow" and parts machined by "Billy Bob" and helping each other out through the spread of knowledge and lending a hand. We spend all this time refining and improving the mechanicals of an existing design to go faster and make more power. These are the things that we're good at and things we can be very proud of.

So, the thing that "holds us back", if you will, is our imposed necessity to preserve the past. The machines and vehicles of 60-80 years ago are so inefficient it's laughable, and that's what WE do. We laugh it off because we are able to laugh it off due to the fact we enjoy our hobby so much. We're not trying to save the future; we're trying to preserve the past. I think this is the major "flaw" (but also the core) of hot rodding, and I'm willing to accept that. In fact, I have accepted that a long time ago. I guess I'm trying to work an angle from an argumentative point for the school project.

So....the school project. I'm a senior in Mechanical Engineering this year. My current project is to do a temporary flex fuel conversion on my T bucket. This is a great learning opportunity because I get to work on my car for school, get credit for it, and learn about alternative fuels. Without getting into debate on the alternative fuel stuff, I'm very happy and thankful things worked out this way.

I'm considering just staying in school to get my Master's. Which led to THIS discussion. What do I want to do my Master's thesis on? That's when I got all excited about this idea I've had developing in the back of my mind for awhile now. I think building a track roadster which could compete with the high mpg (like 40+) crap cars of today is in order. I know it will not be easy to do this, but I don't think Master's thesis stuff is generally easy lol. I also know this would not be 100% traditional, so I'm trying to just limit the discussion to minimizing drag on a roadster.

It seems to me like the biggest way to do improve this will be to minimize frontal area, then work on smoothing out the shape of the body (belly pan, Hallock or similar styled windshield, removable hardtop, track nose, possibly integrated headlights, Moon/aero disc wheel covers inside and outside the wheel, and pinch the front/rear axles as much as possible). Anyway, enough rambling for one post. Let's hear more ideas! Thanks for the contributions so far!

 

Tim, glad you shared why you want this info. Like C.A.F.E standards, the specific performance requirements of your 40 MPG car will dictate where you will spend your efforts." A problem defined is 90% solved". As you know,the affect of drag increases exponentially with speed. Aerodynamic drag on a car that requires 50 MPH max for 30 % of the drive cycle, and 70% at some lower speed, will not have near the effect on mileage as a car required to go 85 MPH for 90 % of the cycle. You may spend a whole bunch of time to prove that drag is much less important on an urban commuter car than on an Interstate cruiser. If you want to explore the effects of other factors-tire contact, tire pressure, rolling resistance, brake resistance driveline loss, driving technique, etc, a lot has already been done. There used to be (or still is) a yearly contest to see how far a car could travel on one gallon of gas. If you can find info on this, you might get some good ideas on what others have leaned about drag vs mileage.

 

An example of an "aero" roadster:



40 MPG let's see what that means?



That should have said Gallons Burned, Pounds were 10.7

Those are demanding but realistic goals for a vehicle styled as above.

The problem is having an engine of say 150 HP which would give reasonable performance yet achieve a BSFC of .500.

I think, in my opinion, we could do some things to make our hobby more sustainable yet still enjoyable. It would be up to admin to say that's a valid topic and where it would belong.

The roadster pictured above with an EFI Mercruiser 4 would be clean and get 30ish.

 

Excellent points. I appreciate the thought input. You're right about considering all factors. Wikipedia displays the formula for the coefficient of drag (Cd), and later in the page it shows what it's equivalent to. drag coefficient (Cd) = pressure coefficient (Cp) + friction coefficient (Cf). Every little bit helps, and all systems of the car should be considered in the design process (new car or old car).

 

VERY good choice for an example. I've studied photos of that car many times. Where did you get that table from? It looks very useful. And yes, I agree there are many improvements that could be made to our hobby, but they (like you said) do not fit the template of this site (non-traditional parts). I was particularly intrigued with the new DFI turbo diesel engines GM is putting in the Cruze. Apparently they've been using them in Europe for a few years now....Those little engines are totally rad. Tons of style IMO. Anyway, I was just cruising through the "Vintage Bonneville car pictures" thread. There's a ton of inspiration on that thread, that's for sure...

 

I just made it up to make it easier. I had the Aero Power stuff from before. I can get you the Calcs. Yep a Diesel is your best bet. Otherwise you need to look up Atkinson cycles.

Hoop

 

I'd greatly appreciate any calc's you're willing to share. Thank you in advance. I'm familiar with the Atkinson cycle as well. It' brings me back to the days of Thermodynamics classes...Funny how I went from a "B" in the first term when talking about generic heat transfer and refrigeration concepts to an "A" in the second term when we started talking about engine cycles. Guess that sort of thing is more intriguing to me...Who would have thought lol.