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

[euromodder]

Originally Posted by aerohead

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.

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.

[kach22i]

Originally Posted by botsapper

...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

Not bad for such a short article.

Cars with a near-vertical rear hatch have airflow separation that occurs at the end of the roof. This means that the wake is as large as the frontal cross sectional area of the car. People who locate spoilers half way down the rear hatch should realise that they are achieving nothing with this placement!

A large wake is also present on most station wagon designs. For example, the Cd of the AU Ford Falcon wagon is sedan is 0.341, versus the sedan’s 0.295.

[aerohead]

Originally Posted by euromodder

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.

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.

[NeilBlanchard]

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



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.

[Allch Chcar]

Originally Posted by spacemanspif

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...

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.

Originally Posted by botsapper

...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

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.

[aerohead]

Originally Posted by NeilBlanchard

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



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.

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.
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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'.

[Thenorm]

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

[euromodder]

Originally Posted by NeilBlanchard

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

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

[Cd]

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.

[aerohead]

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.