Hatchback Vs. Sedan (why does a sedan have a better Cd?)
 

Why does the sedan get a better Cd?
if we look at a contemporary model, say the 2014 mazda 3.
Cd: 0.275 for hatch, 0.255 for sedan.
I would argue that from the B pillar forward the cars are identical. with the same cross section. So where does the difference come from?

http://i2.photobucket.com/albums/y24...ps6dbd3a93.jpg
http://i2.photobucket.com/albums/y24...psf4354e43.jpg

A few ideas i'm toying with. with the sedan, quicker plan taper in the green house area allowing more air to fill in the wake, reducing pressure drag.

the other idea relates to the rear window and the reflex camber curve of the truck. As i see it, the air from the roof accelerates over the rear glass because it slopes faster than the ideal AST. I assume it stays attached because the transition is smooth and the angle is less than 22 degrees (22 deg triangle shown as reference) but a low pressure zone is created with a horizontal component leading to more drag(red arrow). But when the air turns the other direction at the trunk and slows down a high pressure area is created. Does the high pressure area cancel out the low? is the net effect less pressure drag than by just following the AST (as the hatchback does)? Is there anything else i can be taught about this situation.
http://i2.photobucket.com/albums/y24...psaa1e8bc7.jpg

on an aside. I've noticed many many contemporary 3 box vehicles all seem to have 22degree rear windows. are they all using the same effect?

The hatchback that you showed above is slightly shorter than the sedan, so it better follows the sedan before termination, whatever difference that makes.

It's important to be sure all other elements of the sedan & hatch are identical (eg. engine size/power, tire size, ride height), since these things will affect aero/Cd.

Sedans may not get the tidy flow separation that hatches do, but modern sedans aren't terrible in that respect. If you look at smoke tests in the wind tunnel, you'll see the trailing wake (filled with smoke) makes up a much smaller cross section than the hatch's wake.

I believe that the Hyundai Sonata hybrid sedan has a lower Cd than the Prius.

The longer trunk may also have an effect as a diffuser, read a diffuser with lower drag.

:-P

Seriously though, I had no idea that the Cd was so good on this car. Thanks for the post.
I never would have guessed the flow was so good with such a steep rake to the back window.

Aerodynamics is such a tricky thing.

The reason is because the template is an optimum and or maximum air adhesion concept. Whereas wind tunnels and CFD models will illustrate (some) air flow over truncated or scaled down shapes and slopes.

Just because it doesn't match the template it don't mean it's a complete fail.

I would argue that the hatchback may perform better in real life with such things as cross winds over it's sedan counterpart. The air should be flowing over the hatchback with a little more stickiness than over the sedan (my current theory).

My other theory is the template works scaled down, but in a percent of efficiency scaled down. This particular example would put a big crack in this theory, and seem to indicate that scaling down the template has no adverse affects and in fact could be an improvement. Hard to draw wide conclusion off just one example, must use many examples to see a pattern.

Aerodynamics Photos by kach22i | Photobucket
http://i184.photobucket.com/albums/x...ps888660d7.jpg

EDIT:
2013 not 2014 model below
Jim Ellis Mazda Marietta | New Mazda dealership in Marietta, GA 30060 serving the greater Atlanta area including Alpharetta, Roswell, Buford and beyond
http://pictures.dealer.com/j/jimelli...9e348f3ab9.jpg

My guess, just looking at the pictures, is that the sedan roof tapers inwards at the rear. You could probably see it if you looked down from above.

Cd factors in a cars cross sectional area, which in this case is the same at the front. The problem arises at the rear. The Hatch has way more area at the back, this ends up acting like a vacuum on the car because the air cannot remain in laminar flow as well on the hatch.
If Tractor trailers came to a point at the back, there wouldnt be much benefit drafting them, but because there is huge area at the back, they create a void of low pressure air behind them. In a similar way, the Mazda hatch is fighting this same fact.

Cross wind affect on a car also has to do with surface area, so its way more likely the Hatch will be more affected in a bad way, since it has more area for cross winds force to act on.

I'd say the hatch has slight more upper side area, and the sedan significantly more lower side area.

Aerodynamics Photos by kach22i | Photobucket
http://i184.photobucket.com/albums/x...pse03ea1d8.jpg

The original template overlays don't exactly line up, but I bet the high points on the roof are the same. Small differences, but perhaps they could be redone to line up for a more accurate picture.

Looks like you're right on the length

Mazda reports length of 5-door at 175.6 but I couldnt find there length for the sedan. Motor trend has these dims:

LENGTH X WIDTH X HEIGHT 175.6-180.3 x 70.7 x 57.3 in

So it appears the sedan is 180.3" long, a whole 5" longer, which to me makes no sense. Why cut down the interior room on a hatch?

better
 

I suppose you need to decide where you're going with the car.
If you're looking for limited length,choose the sedan.It will have great rear vision.Easy access to the trunk.Easy parking.
If you're thinking about remarkable drag reduction,go with the hatch for long term benefit.It is the better candidate for low drag through elongation.
In the long run,it's better idealized for the sectional density necessary for truly low drag.
There are no truly low drag notchback sedans.There never will be.

do you mind if I summarize this a little clearer.

1. for good Cd with no modification, get the sedan
2. if you plan to add a kammback/boat tail for remarkable drag reduction get the hatch.

I don't think that's the case, the vehicles are different configurations. The sedan is probably longer to get some more trunk space, and or meet the design criteria.

As far as I know, only Christofoo and 2000Neon have made sedan kamm-backs, and Christofoo's is an ongoing project.

Length in this particular case is not an indicator of headroom - look at the pictures.

What I find as an interesting surprise, is that the hatchback has more rear headroom than the sedan.

I don't think this is typical unless comparing a sport coupe to a hatchback.

Should we be calling the version with a trunk a sport sedan?

Marketing, we just cannot escape it.:cool:

summarize
 

*The sedan has been optimized as a sedan with no thought of modifications.
*The hatch was also not designed with mods in mind,but happens to have the perfect fundamental architecture if one wanted to do the ideal elongation.
*All the really low-drag vehicles which have been constructed were 'fastbacks',which followed the profile of a proper half-streamline body.
*If you continue the roofline and sides of the hatch,you're right on target for low drag.It's exactly like Kamm & Koenig Fachsenfeld's Cd 0.12 Kamm-back in it's embryonic stage.
*The Cd will just be a function of how far back you push the tail.
*K&K cut their tail off at 50% frontal area cross-section and got as low as Cd 0.23 with a 'practical' length in the late 1930s,early 40s.
Then Koenig-Fachsenfeld came up with the inflatable boat tail extension to push the Cd back down towards 0.12,for only open highway travel where length wasn't an issue.

sport
 

I think Mitsubishi is using 'Sport' with their Lancer which got the relaxed roofline compared to the original notchback roof folks were putting VGs on.
I think its become part of the vernacular now.

The VW XL1 and the GM EV1 are both "sedans," and have the lowest drag figures of any production car with Sub 0.19cd. The Mercedes CLA sedan (0.23-0.22cd) has a lower drag coefficient than the Prius (0.26-0.25cd) and Insight (0.25cd). Honestly with whats being done by automakers, I'm not all that impressed by the 1st gen Insight aerodynamics. 0.25cd will become the new common standard of an "average" cars drag coefficient. Anything below that number should be the only thing considered aerodynamic.

Sedan vs Hatchback? I think it's due to trunk height. A trunk can be lower to the ground so you can place things in it, and a lower decklid facilitates aerodynamics. A hatchbacks' roof needs to be higher so your not crouching. The spoiler on the Prius is higher than most trunk lids you'll find in a parking lot.

I've been thinking about putting a kammback on the Karen-mobile...

http://ecomodder.com/forum/attachmen...1&d=1381278626

I worked at pepboys about a year ago and we had 2 feistas for delivery cars, 1 hatch and 1 sedan. And these cars were the first time I noticed what car companies were doing with these small cars. The sedan definitely had a larger trunk than the hatch and I always thought that the hatchback had a completely useless cargo area.

Best I can tell is that Americans don't like station wagons because of the stigma of being "old lady cars" or whatever. So instead of extending the roof over a sedans trunk, they chop off the trunk and add a door and just like that the magazines call it a "hot hatch" so people want to buy it. In the mean time, people who want a useful vehicle like a wagon are left with nothing but what the Germans have to offer....

To me its stupid to cut a car down but I see tons of hatches running around town all the time. Best I can figure is the salesman says "the sedan has 50cubic ft of cargo space, but the hatch has 62, so its got more space for when you go on vacation with the family" and people eat it up...except the hatch has vertical space that is useless unless you want your luggage falling on top of your children...


Off my soapbox and back topic: The wheel skirt threads all seem to say that skirts have more impact as the wheels get closer to the back of the car. Could the added Cd be from the turbulence caused by the hatch's wheels being closer to the back of the car?

Agreed ! That is why that the SUV replaced the minivan which replaced the station wagon. Stigmas !
SUVs are a cross between station wagons and pick ups, but people don't see it that way.

Even minivans are starting to have truck like front ends to lose the stigma that comes with driving a minivan.

I wonder what will replace the SUV. Hopefully something more efficient ! ( Already SUVs are labeled 'soccer mom vehicles' )

Back to the aerodynamics discussion, has anyone here ever tried to contact the manufacturer directly with a question like this ?

This site is filled with brilliant minds that i'm sure someone in the aero department would be more apt to speak with one of you.

Sure beats guessing !

And one more thing : If the manufactures wont give out information due to it being a " trade secret "* then perhaps one of you could contact the guys at the A2 wind tunnel and ask some advice there if they are willing.

* I contacted Lexus about the LS430 and asked some questions about the drag, but was given the "it's a trade secret" line.

A2 wind tunnel :

Address: 117 Godspeed Ln, Mooresville, NC 28115
Phone: (704) 799-1001
A2 Wind Tunnel

TIP -- You can contact EPA to get the "coast-down" data the manufacturers MUST supply to EPA as justification for the MPG values they provide to EPA for publication!

Then, assuming you're not math-adverse, you can simply backsolve the PWR-vs.-VELOCITY equation at 55 MPH(*) to find the vehicles CdA number.


(*) EPA assumes 55 mph for many tests, including coast-down.

The test results can be found here:

Test Car List Data Files | Cars and Light Trucks | US EPA

Karen-mobile
 

She'd be a good candidate.;)

contact
 

Once upon a time you could phone GM's Aerodynamics Lab and chat with Glen Scharpf (sp?)who ran things there.He was very helpful in sorting out the CRX's speed/mpg/drag data.And he was very open about discussing GM products.
That's all history now.
I have found that if you'll write to the public information officer for any respective automaker,that they'll be happy to mail you press kits from some of the concept cars which include a lot of technical data.
I've received packets from:GM,Ford,Volkswagen AG,Volvo,Honda,Renault,Chevrolet,Citroen,Adam Opel AG,Ford Werke AG,GMC/White Truck,Freightliner.
If more consumers talked to reps at auto shows about Cds and CdAs 'n frontal areas,they'd report back to headquarters and convey that there was actually consumer interest with such things.

I'm suffering from some 'déjà vu' today, going back to an old but not real old post.

http://ecomodder.com/forum/showthrea...tml#post374147
http://ecomodder.com/forum/showthrea...tml#post374147
NOTE: The following page in the link above (page 6) has some template overlays as a visual aid.

template
 

The thing about the 'Template' is that if you want to approach Cd 0.07-0.08 territory,as Hucho cites,that you need the profile of the streamline half-body,and you also need the effective fineness ratio which the 'Template' provides in order to protect the boundary layer.Remember,streamlining is all about reducing pressure drag and that means minimizing,or eliminating separation.The 'Template' cannot produce separation,nor can it produce longitudinal vortices.Notchbacks can and do both.
Compare the Ford Probe-IV to the Probe-V.Do you see what Ford does with the aft-body to go from Cd 0.15,to Cd 0.137?

All of Hucho's Cd 0.15 forms are 'fastbacks.'
If you want to go below Cd 0.15 you elongate the fastback.It's the only way you can get there.

Here's an older Edmunds article giving 0.25-0.26 Cd values for the Lexus LS430:

2001 Lexus LS 430 First Drive

...hatchbacks and wagons have a more abrupt transition and at a larger cross section of the vehicle, while three-box sedans have a stepped and smoother transition into the wake zones. AutoSpeed - Low Drag Car Aerodynamics

Or you go for the boxfish ... :)

And do it a bit better than Mercedes who left quite a bit of low hanging fruit to rot as they went from their original 0.095 model to 0.19 on the real car - which is also wider and higher than it needs to be, in addition to being overpowered.

Not bad for such a short article.

boxfish
 

Mercedes did a nice job with the Bionic car.And you'll notice it's a dead match with the 'Template.'
Cd 0.19 with a car this short is kind of a benchmark.
The Cd 0.095 was probably without wheels.The fish itself was probably Cd 0.0475.
Wheels can double the drag of a low drag body.On Jaray's 'pumpkin seed' they brought the Cd from 0.09,up to o.13.
The Pininfarina CNR 'banana' car jumped from Cd 0.161,to Cd 0.35.

I think the Cd 0.095 was for the blue model with the covered wheels:

http://img.photobucket.com/albums/v7...ar-Body-lg.jpg

It is quite a bit different than the Boxfish diesel prototype. It has a protruding nose (similar to the fish itself) and the rear fascia is significantly smaller than the Boxfish. It has wheels, with very large strakes and very large skirts. It has no cooling tract, and the underside is entirely smooth and it rises toward the rear.

Here's a bunch of screen captures from a video that Mercedes had made, showing the transition between the blue model to the Boxfish (so the color is also morphed) - start here and then use the arrows on the left side:

Mercedes Bionic/Boxfish Clay Model Photo by NeilBlanchard | Photobucket

By the way, this blue model is the starting point I used when designing my CarBEN EV5.

Eh, the Bavarians make some good wagons. I did like the new Focus and Mazda3 wagons, but they're not coming here. It's kind of a shame that, for certain reasons, automakers are refusing to bring wagons to the US.

As for sedan vs hatchback, the added length goes to cargo room because the sedan would be woefully tiny without it. Hatchbacks do in fact have more cargo room because you can stack objects to the roof or rear glass. I grew up with minivans and there is a great deal of art to packing that rear cargo area. Occasionally something will fall onto the back passengers. But children and friends are pretty durable. ;)

This is another reasons why wagons are better than say a SUV or Crossover, the added length allows for smoother transition. In addition to lighter weight, better handling, and better MPG. :D

different
 

It's a beautiful car.And they've done so much of the right thing.I just can't get my head around the Cd.
Buchheim did some similar studies at Volkswagen in 1981 and couldn't get below Cd 0.14,even with a complete boat tail on the 'flow' body.
The transom area seems too large for which to achieve a 'phantom' tail.
The 1987 Renault Vesta II has a better windshield and scores only Cd 0.186.
If they're basing the streamlining on plan-view,then they'd be relying more on 2-D sections which cannot tolerate 'Kamm-form' truncation and maintain any semblance of their Cd.
The 1987 GM Sunraycer with full tail and complete wheel fairings got to Cd 0.089 and Cd 0.125 without them in actual race trim.
--------------------------------------------------------------------------
It would be nice to know the scale of the model and the tunnel wind speed.
GM models in the Cal Tech model tunnel always indicated Cds far lower than their full-scale counterparts.It would be nice to have more data.
My gut feeling is that,in production trim,with Hucho's 2% drag cooling system,and features drag,that she'd come in at around Cd 0.15-16.
We may never know.
Thanks for the Cornucopia of images! You've invested a fortune on 'Carben',I like to think you'll be delighted at the Watt-meter after 'top-offs' when she's rollin'.:)

speaking of boxfish. I've seen them around while snorkelling in Australia/Indonesia. they are very pretty fish.
sorry for low quality pics. they dont sit still and pose.
1 each, plan and profile
http://i2.photobucket.com/albums/y24...ps1876dc71.jpg
http://i2.photobucket.com/albums/y24...ps1f9db523.jpg

Which also happens to have far more planview taper aft, while the bionic concept remains wide.

http://upload.wikimedia.org/wikipedi...bionic_car.jpg

Does anyone know if the blue model posted above is on display alongside the rest of these models in this picture?

It sure would be great to get some useful images of the model. ( ex.: plan view )

Neil, have you contacted MB and asked about the model ? Since this was just a concept and it has been years ago, they might be open to sharing something like a blueprint.
After all, I see that there are models available of the final concept car, and surely the manufacturers of these models gained access to the blueprints of the final car.

If they are not willing to give out 'trade secrets', perhaps they can at least answer some of the questions regarding the Cd figure and if it was with wheels, if it was tested in water versus air, etc.

blueprint
 

They might have published an SAE Paper on the project which might have included the model data.
Mercedes would be sending some of their staff to each SAE/ISO congress and there's a chance that they shared the info.
Renault mailed me a press kit for the Vesta II and in it was a paper about their research.
They'd achieved Cd 0.135 with a development model at the time of printing and expected to be at Cd 0.10 by 2000.
Subaru was at Cd 0.088 in 1986 with there model 'G' wind tunnel model for the XT development.
So come to think of it,maybe Cd 0.095 isn't so implausible.
Neil,I may owe you dinner! Which would be a pleasure anyway.:D