Focus wagon (.30) vs hatchback (.35)

[LocoJason]

Please excuse me if this has been discussed before. I am trying to figure out the big difference that gives the focus wagon a lower coefficient of drag. The focus is the same from the B-pillar forward, so all of the difference is in the back of the car. The roofline is the same height, and the wagon has equal or as little taper as the hatchback. I wonder if their measurement also included the roof rack.

The things that boggles my mind is that the rear end is slightly longer, but the primary difference is only the rear roofline and hatch. You could count the squared off ledge in the bumper cover as well. In theory, you could duplicate the shape of the wagon easily on the ZX3/ZX5 and drop .05+ off your cD... ?

Sedan:
cD: 0.31
Frontal Area(est): 21.97 sq.ft.

ZX3
cD: 0.36
Frontal Area(est): 21.97 sq.ft

Wagon:
cD: 0.30
Frontal Area(est): 22.24 sq.ft.

[GRU]

roof racks improve Cd?!

[Allch Chcar]

Those numbers match what I've seen for the first gen. The ZX4 aero must be close enough for rounding because I've seen both .31 and .32 used on different sites. Or possibly some confusion with the second gen.

I don't know the exact angles but I believe the longer body on the wagon has better attached flow and the abrupt end actually help it compared to the ZX3.

[Cd]

I 1992-95 Civic sedan had a Cd of .32 , while the hatch had a .31.
The next years model hunchback had the Cd jump all the way up to .36 - the same as the Focus hunchback.

The focus and this hunchback both have a shorter roofline than previous, but surely not enough to add this much drag !!
Perhaps it is due to the rounded off taillights .
Speculating , I would guess that there are some mean vortexes forming here, but then why wouldn't the cars simply have a trip strip of some sort here ??
I'm puzzled too !

[NeilBlanchard]

It's the shape of the trailing edges, in large part, and the length. I owned a ZX-5 and I was disappointed with it's fuel efficiency. My current car has a stock Cd of 0.31, and I have improved it to about 0.27-0.26 or so.

[gone-ot]

...Kamm backs don't always work well, due to shedding "edge-effect" vortices, etc.

[PaleMelanesian]

The older civic has a slightly dropping roofline before it cuts off, while the new one drops off at the highest point. Bigger rear wake.

Did all the 96+ hatchbacks have that spoiler? If not, air is being dragged down the rear window at the absolute worst angle - about 30 degrees. Either one, a steeper cutoff or a shallower angle, would be better.

[PaleMelanesian]

Originally Posted by Old Tele man

...Kamm backs don't always work well, due to shedding "edge-effect" vortices, etc.

Ah, good point. Especially when the roof cuts off early. Side vortices from the remaining downward-sloped corners could interfere with a clean cutoff on top.

Also, the wagon is longer. That gives more time for the air pressure to even out before the end is cut off. (is this called reynolds number?)

[Cd]

Has anyone also noticed that the roofline is apparently longer on the Focus hunchback than the coupe ?
I find it strange that the hunchback has such a high drag compared to the coupe.

It seems that there is some major disturbance in the ...er...forces around the taillights of the hatch version.

[LocoJason]

I was contemplating a box cavity taking up the entire hatch area, allowing a section to extend off the rear roof at least 10 inches or so. Then I got to thinking about how much those curved corners must contribute. Thanks for pointing that out guys. I mean here's the evolution of the same type of design (essentially the 'new version' of the same body style), and it's .295 cD! Note the smaller mirrors, and the side profile is slightly different.




Just based on putting this information together, so far I think some small restyling tests on the rear of a 3/5 door 00-07 Focus could make a very large change in the cD. I think this warrants far more attention on testing vs the tire spats or front under panels I've made.