Rear Parachute

[NewBModder]

I drive an '06 Aveo hatchback, so naturally I've been checking out the redesigned '09. I noticed on the '09 rear bumper what appeared to be a 3-4 inch wide grill or vent spanning almost the entire length of the bumper. Now, I've never seen one in person but apparently that is fake, i.e., not a vent at all but a decorative stick-on.

But it got me thinking about the whole 'parachute' nature of my rear bumper. I looked under the bumper on my car and it looks like the bumper (cover) comes down at least 6 inches below the actual bumper (safety device). I was thinking cutting out vents (or even copying the look of the '09 Aveo with a full-length grill-type thingy) would reduce drag by quite a bit.

Am I dreaming? Think this would accomplish anything for FE?

[Bicycle Bob]

My own Scientific Wild-Ass Guess (hereinafter: SWAG) is that it would be a lot easier to stick some Coroplast underneath to keep air out of your parachute. Grill work is amazingly draggy. If you want to do some serious work, cut away as much of that parachute as you can with a reasonable angle on the Coroplast, and make a smooth little spoiler tip to trip the air up into a stable vortex. To restore the original lines, put a heavily cambered wing at the bottom to really put some air into the vortex.

[slipstreamer97]

I would tend to agree with BicycleBob, i think it would flow smoother around rather than through. i'm working on the same problem

BicycleBob, could you explain the little spoiler? i'm having difficulty understanding spoilers. i'm contemplating removing mine.

Jake

[Sean T.]

If you are willing to do it, I would chop the bumper all the way up to the actual bumper support, then cut a piece of coroplast that runs on an angle from the rear axle/gas tank/something to the freshly cut edge of the bumper.

[Bicycle Bob]

If you start with a classic elongated teardrop shape, at least five times as long as it is wide, you can probably keep the air moving smoothly along the whole shape for minimum drag. If you don't have room for that much length, it is better to just chop off the back end than to try for a much sharper taper. Otherwise, the air can't follow the shape and goes turbulent at the maximum cross section. With the cut-off tip, it is only turbulent behind the smaller flat section. That's the basic kamm-back shape, discovered by Herr Docktor Wunibald Kamm.

The Honda CRX has the classic refinement of that shape - a slightly larger flat, with a gentle rise just before the end. This has the effect of providing a better separation of the flow, and a single large, stable vortex behind the flat, which absorbs less energy than general chaotic flow.

On racing cars, that lip was often exaggerated into a flap to produce downforce before wings became popular for their greater effectiveness. The wings and spoilers on street cars are mostly styling affectations, but a little wing on a trunk lid is sometimes able to raise the pressure on a back window enough to generate a trapped vortex and delay general flow separation, producing a virtual fastback shape.

[metromizer]

Most drag racing car builders that use factory rear bumper facias, will trim the lower edge such that it's higher off the ground. There's a couple reasons, but the 'parachute' effect is one. Some dragcars rear panel will incorprate a top-hinged flapper door or two, that pop open releasing trapped high pressure while the car is at speed. While the car is at low speed or stopped, the flapper door rests closed, looking factory OEM. Only one moving part <a hinge> and no actuators of any kind.

Still, an under panel sounds like the better solution.

[NewBModder]

Originally Posted by metromizer

Most drag racing car builders that use factory rear bumper facias, will trim the lower edge such that it's higher off the ground. There's a couple reasons, but the 'parachute' effect is one. Some dragcars rear panel will incorprate a top-hinged flapper door or two, that pop open releasing trapped high pressure while the car is at speed. While the car is at low speed or stopped, the flapper door rests closed, looking factory OEM. Only one moving part <a hinge> and no actuators of any kind.

Still, an under panel sounds like the better solution.

So, how much of the aerodynamic drag on a car can be attributed to the bumper parachute effect? Or is that a separate drag and not really part of the 'cd'? (Yes, I'm displaying the depths of my lack of knowledge and/or understanding

[metromizer]

Originally Posted by NewBModder

So, how much of the aerodynamic drag on a car can be attributed to the bumper parachute effect?:

Only your friendly neighborhood windtunnel engineer could say for sure... I'd guess it's some small contributor. But the small things all add up, an mpg here, an mpg there, pretty soon you pull in for your weekly gas fillup, open the cap and fuel spills out! ok, that was just silly, but it's late in the day, working with spreadsheets and our contracts department all day has me loopy...

Originally Posted by NewBModder

Or is that a separate drag and not really part of the 'cd'? (Yes, I'm displaying the depths of my lack of knowledge and/or understanding

Remember that a car's total drag is cd x frontal area.

Just like a blunt nose or poorly designed roof section, by definition that rear facial's drag component is included in the vehicle's advertised cd number.

What is uneffected by shape is the 'frontal area' part of the equation.
Whats the difference? (paraphrasing a famous drag racer) "Drive a car into a mortorless brick wall, and count how many bricks fell out the otherside. Adding up the number of bricks x (brick length and width dimension), would roughly give you the car's frontal area. cd is much harder to figure out"

[aerohead]

If the bottom of the bumper is no lower than the bottom of the spare tire well or trunk floor,it may not be catching any air at all.If the bottom is level with the trunk floor,it is probably reducing drag as it is,as was discovered in the development of the Subaru XT,with Cd 0.29.