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

[Thenorm]

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?




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.


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?

[Xist]

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

[MetroMPG]

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.

[Xist]

Originally Posted by MetroMPG

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.

[RedDevil]

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

[Cd]

:-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.

[kach22i]

Originally Posted by Thenorm

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.

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


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

[jamesqf]

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.

[E.Roy]

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.

[kach22i]

Originally Posted by E.Roy

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


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.