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

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.

http://images.autotrader.com/scaler/..._A.562x421.jpg

http://images.autotrader.com/scaler/..._A.565x377.jpg

roof racks improve Cd?! :rolleyes:

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.

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 !

http://img2.netcarshow.com/Honda-Civ...llpaper_04.jpg
http://pdfmanualfile.info/images/man...ack-manual.jpg

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.

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

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.

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?)

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.

http://assets.clickmotive.com/ail/co...1_CX.jpg?w=590

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.

http://2.bp.blogspot.com/-47VG_VC2IV...us+ST+Rear.jpg
http://upload.wikimedia.org/wikipedi...12-31-2009.jpg

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.

Here's a side profile on the new HB.
Attachment 11281
It's a thumbnail so click for the full image.

wagon/hatch
 

If the pressure is different around the perimeter of the separation line,flow into a lower pressure area will introduce shear which can spin up into vortices which will absorb significant power.
The separation line also dictates the base pressure of the wake.
The station wagon would have a nice uniform pressure whereas is it is questionable for the other car.
The 2013 Focus of Cd 0.29 pushes the roof back like a wagon.It is PURE K-form!

I think it is the crisp trailing edge next to the tail lights that help lower the drag more than anything else we can see in that picture. Good on Ford -- that is about a 20% reduction in drag.

Just think of all the thousands of gallons of oil that were burned because Ford decided not to put a simple creased edge on all the thousands of Focus cars they produced over the years ! :mad:

To the OP : looks like its time to break out some clear plexi pieces over those lights !

And we would all love to see a tuft test of the area !

Good thing, I've got a wagon :D

Every car maker can get a 20% reduction is drag, if they tried.

Yes. If people only didnt buy cars based on the looks.

Low aero drag looks great to me.

How did you find the cD figures for the Focus lineup? I've tried unsuccessfully to find the cD of various vehicles and have finally quit trying......

I actually performed a brief tuff test on the rear taillight/window area of my ZX3 last night using my stickypod on the back window. Airflow along the back window is not exactly what I had expected. This was a really quick setup... I didn't get the best angle for the shot, and only a brief stint at 65MPH, but I'll post up some screenshots later tonight.

Google. I found them on half a dozen car websites. Someone had re-posted them on this site as well. The numbers vary but I saw .30 for the wagon consistently, .31-.32 for the sedan and .35-.36 for the hatchback.

my zx3 shows some mean vortices rolling while driving in the rain. I have tried adding some trips to the rear to help out but havent settled on anything permanent. I really think a box on the rear of the vehicle that utilizes the bumper would do the most good...

screenshot of a small tuft test at 65mph. Not much movement, but it's interesting to see how the air is moving laterally across the back window and collides with the air coming off the side of the taillight. The tufts coming off the lip on the trailing edge of the roofline are also pointing straight at the taillight...

is that a box cavity? good job! do you also have a diffuser?

The screenshot is from a stock Ford Focus zx3. The taillights just stick out above the hatch to create what appears to be a box cavity in the picture.

But cars don't have to look "jelly beans" to be aerodynamic !

Just look at the Lexus LS430 with its' .25 drag coefficient !

http://static.ddmcdn.com/gif/vehicle...990001-480.jpg

I see a LOT of area where stylist can improve drag, and still have cars that look nothing like one another !

When the 2nd generation Insight came out, I read articles that gave the reasoning for the Prius like style as having to do with aero.

Hogwash.

BTW, here is a tuft test of the back of my hunchback Civic

http://www.youtube.com/watch?v=-LhKE5hLgwE

I just completed a shallow diffuser for the rear of my focus. Today will be the test drive to see the changes and whether it will hold on. I also filled the spaces on my rims with some chunks of an aluminum serving tray. I had some black spray paint that I wanted to paint them with but it clogged up so they went on brushed aluminum. I will make and test a cavity next.

I think that another reason for the better aero of the wagon is the sides, you can clearly see that the wagon is narrower on the back.

I know this is a massive thread bump, but I bought a 01 ZX3 and while driving it noticed a couple things. The 2 liter engine seems nearly too powerful for such a light body, and when driving at less than highway speeds she's a spritely little car. But at highway speed, she seems ungodly slow to accelerate such as when passing. The wife and I could figure it out until I researched the drag coefficient and boom, a forum I haven't visited in years!

I now own a Volvo v70 that I love as well as the focus and my still running van. The maxima died sadly a few years ago.

If anyone wants to chime in on suggestions or improving the aero on this car it would be great. I noticed a member with a kammback on his focus that netted much improved economy over the 30 mpg we've gotten on longer runs. But we generally only drive the focus on short runs so losing the practicality of the hatch is undesired.

Oh how I miss my 83 Sentra station wagon. That 1.6 liter car gave me 35 mpg when I hammered it mercilessly with the ac on and I'm betting it was due to its very aero rear end. The air flow was such that the rain didn't touch the back glass at highway speed and snow would miss the windshield at moderate speed. It was quite a remarkable first car