Metro aerodynamics

[MetroMPG]

I'm not sure why I made that statement about the effect on less powerful cars though. Must have had some thought going on at the time...

[hotroddr]

Im curious about something. Could you repeat the test except do one with the hatch closed and another with the hatch completely off? I have had the thought that it might actually help the drag to take the hatch completely off(less surface area for the negative pressure to act against). I cant do back to back tests since my car has no electronic means of tracking mileage(84 rabbit diesel), so I was wondering if someone might consider trying this on their hatchback car.

[skewbe]

Originally Posted by hotroddr

Im curious about something. Could you repeat the test except do one with the hatch closed and another with the hatch completely off? I have had the thought that it might actually help the drag to take the hatch completely off(less surface area for the negative pressure to act against). I cant do back to back tests since my car has no electronic means of tracking mileage(84 rabbit diesel), so I was wondering if someone might consider trying this on their hatchback car.

You can do a coast down test however. We might not have wind tunnels, but we know how to make wind.

[Peakster]

Originally Posted by hotroddr

Could you repeat the test except do one with the hatch closed and another with the hatch completely off? I have had the thought that it might actually help the drag to take the hatch completely off(less surface area for the negative pressure to act against).

I wondered about that too. If the hatch comes off easily with a screwdriver I'll give it a try sometime!

[GasSavers_BluEyes]

Originally Posted by hotroddr

I have had the thought that it might actually help the drag to take the hatch completely off(less surface area for the negative pressure to act against)

How do you figure less surface area? The low pressure area will still exist, and it will act on the rearward facing areas of the interior which should add up to the rearward projected area of the hatch.

If anything, I predict that this will increase drag. If the edges of the hatch are rounded, you will lose those and be left with the abrupt corners at the edge of the sheetmetal. You will also lose any areas of attached (although turbulent) flow across the angles glass surface and be left with completley detached flow behind the vehicle.

[MetroMPG]

I vote for higher drag also.

[hotroddr]

I had a little time to think about this more. BluEyes you may have a point IF the interior of the car was sealed completely but there is a vent through the cowl where there is a high pressure area into the interior of the car. I imagine this could potentially cancel out the low pressure area behind the car, cracking the windows could help prevent the low pressure area from acting on the inside surfaces of the car as well.
I dont know that rounded corners necessarily produce less drag either. I have been noticing that most hatchbacks have a sharp abrupt edge on the top of the car and rounded corners on the edge of the car. I imagine this has to do with producing vortices on the sides of the car that swirl in behind the car narrowing the wake, the top spoiler with the abrupt edge is to minimize lift and create a cusp underneath it. I would guess that an open backed vehicle with the air vents open would get better gas mileage than a closed back vehicle but this would have to be proved through experimentation and at the moment I dont have the time. If someone else does I would be interested in seeing the results, otherwise I will eventually get to it when things slow down around here.
Look at some experiments for reducing drag on semis, they extend the sides and top and leave the ends sharp.

[jwxr7]

Sorry to pop in like this .
I just made an accidental aero mod to my geo this morning. My outer driver's side mirror was kinda rigged up after being broken before I owned it. I bumped it just right in sub freezing temps and it broke off badly. It even broke the little manual remote controller mechanism . I duct taped over the hole for now. I may look into an inner side mirror. I'll have to check michigan laws on that kinda thing.

[Peakster]

MetroMPG (or anyone else that's done an exterior mirror deletion), what type of material did you use to fill in the void where the side mirror attaches to the door? I'm in the process of removing my mirror(s) again.

[Peakster]

Originally Posted by theclencher

I used coroplast- ugly as heck but it matches my ugly skirts and cowl panel. People get a kick out of my car though!

Yeah, that's what I've used for the time being. Painted it black to match the rubber trim, but it just looks kind of hap-hazard. Next time I find some thin rigid black plastic, I'll cut some. Unless someone else has an easier, better-looking idea.

I'd love to see some pictures of your customized Tempo, theclencher. You should update your pictures in the garage with some.