Metro was right, I did a coast-down test.

The winglets on the sides suppress corner vortices, which helps lower drag.

Also, there is a small gap between the wing and the body. The positive pressure on the top of the wing pushes separation bubbles through the gap which helps maintain flow attachment.

There is positive pressure on top of the wing because the flow curvature is concave upward there-- this lowers drag and gives downforce.

Ernie Rogers

Thanks for the clarification on the winglets, Ernie.

Metro - Upon closer examination of the Volvo C30 photos you posted, I noticed a tighter wheel gap on the "Efficiency" model. Couldn't the lower cD be more attributed to the car having a lower stance rather than the change in the rear spoiler?

Good eye. The Efficiency engineers achieved the cD reduction through a combination of improvements, and the roof extension was just one of them:

I'm not sure if they're listed in order of largest contribution to the improvement, but I wouldn't be surprised.

I agree. Could be highly effective, and is a lot of work. On a Volvo 240/740/940, it's even more work than on the Metro. I had one on my 240 this past summer and finally took it off. It was a duct tape + coroplast project and I was never able to keep the forward edge of the extension smoothly attached to the roof plane.

One tough part is that the side "cuts" where the tailgate separates from the body RE on the car's rear face. And on the top surface, the cut is in the roofline. This creates all kinds of issues in achieving a smooth edge where the extension meets the side panels, and also the roof. I never suceeded so I removed it. I think it possibly could be done using semi-rigid thin panels and sheet metal screws - if you dare go there.

Something else worth mentioning as a sidenote:
The car dealership that I work for is located next to a major 4 lane highway. It was raining yesterday and I took some time during my lunch break to watch traffic pass by on the wet roads. I found it interesting how well the water mist illustrated the wake of each passing vehicle. It was easy to see the low pressure kick-back swirl behind every semi & box truck. But what I found most interesting was the height of the wake behind even the smallest cars. I watched a Chevy Aveo 5dr(no OE spoiler) pass by and the water mist peaked at least 20ft in the air, about 50ft behind the car. Once again, not really a scientific observation...but interesting nontheless.

It would have been neat to see basjoos go by just then, wouldn't it?

Hahaha....I was thinking exactly the same thing! :thumbup:

But I wonder if he has windshield wipers on that large false windshield.:rolleyes:

You can get a relative idea of wake height (comparing one car to another).... But keep in mind that water has more mass than air and aero drag plays a role on each water droplet (as opposed to a uniform fluid - air).

It's cool though -- I like watching cars drive by in wet conditions for that very reason :p It's interesting to watch the interactions at an intersection where wakes in opposite directions collide :p

Another interesting note:
It was raining again this morning as I traveled on a major highway on my daily commute. Traffic, of coarse, was very heavy. I was behind a full-size Chevy van, following @ a safe distance while travelling ~55mph...when I glanced in my rearview mirror, my "wall" of upward flowing water mist was gone! Naturally, I was a lil' miffed.:confused:
When I finally hit some open highway with no one in front of me, I looked back and the "wall" was there again. That's when it hit me....You commonly hear during a NASCAR race how a driver can "loose the air on his/her spoiler" while racing in traffic. I think this what happened to me here.

Anyone agree?

When I drive down a dirt road, I leave behind a light dust trail that's about 2 to 3 feet high (the height of the tip of my boattail). When I drive down a leaf covered road, the leaves just move a little bit on the road and an occasional leaf pops up to the height of the tip of my boattail. In the pre-boattail days I could see many leaves dancing behind the rear window and it would be covered with dust after driving on a dirt road. Of course I can't see it when I'm driving, but I don't think I am leaving much water mist in the air behind me.