Metro aerodynamics

[Peakster]

My dad has gone to Las Vegas on vacation so that means I can finally use the garage for aerodynamic modifications without freezing outside.

I finally replaced my barely-working driver side windshield wiper with a "Reflex" blade which doesn't have hinges. Not only should it be good for aero, but it keeps the window streak free:

IM001207small.JPG

Next was the removal of the mirrors. Man, If I knew that the mirrors were this easy to remove I would've done it long ago! Just 3 screws for each door and they're off. I stuffed the holes with paper towel and covered the void with plastic:

IM001209small.JPG

IM001210small.JPG

To compensate for the loss of the side mirrors, I clipped on a large panoramic mirror to the existing interior mirror:

IM001208small.JPG

Up next is the wheels skirts and Kammback.

[MetroMPG]

Good stuff. There's about a 2.3% increase in FE at 90 km/h in mild temps (more in winter).

Don't crash into anyone without the mirrors - takes a while to adjust your driving to compensate. (You WILL find yourself automatically looking to where they used to be, for a while.)

[Peakster]

Originally Posted by MetroMPG

Don't crash into anyone without the mirrors - takes a while to adjust your driving to compensate. (You WILL find yourself automatically looking to where they used to be, for a while.)

Yeah, I know what you mean. I drove to Wal-mart just before they closed tonight so I could get a panoramic mirror (after removing the side mirrors), and I never knew just how much I used the mirrors until they were gone. Kept looking at the void space . The panoramic mirror covers everything except behind my head. I'm planning to mount a convex mirror on the inside of the driver's door like basjoos did.

A 2.3% inclease in FE is quite substantial for little cars like ours!

[MetroMPG]

That's a good point.

You won't see a definitive difference in tank to tank comparisons from removing 1 outside mirror. The change is too small (which isn't to say it's not effective, just that it's effect will be lost in normal FE variability).

[MetroMPG]

Ah! OK. All that should show up, I'd wager, comparing highway tanks.

[green swift]

Originally Posted by MetroMPG

not sure if these images will show up in this forum

these pics does anybody sell them????

[MetroMPG]

Nope. Think you've got to roll your own.

[MetroMPG]

Undertray

[Peakster]

My dear mother gave me her old kitchen table and chairs (can you tell that my parents are just itching for me to leave the house? ) and when I stuffed them into the Geo, only the table's legs could only fit in the hatch. The table top was sticking out with the hatch wide open, creating tons of drag.

It was late at night so I took the Trans Canada Highway back to my dad's house doing 35 mph. I couldn't believe it. I needed 0.7 GPH to keep the Geo in the 33-35mph range all the way home (around 50mpg at that speed) 0.7GPH got me into the high 45-48 mph range just hours before (around 65mpg).

Clearly aerodynamics play a big role even at low speeds since the table and chairs only weigh 100 pounds at most.

[Peakster]

Originally Posted by theclencher

Wind- how strong and what direction?

At 8:30pm is when I drove home and the temperature then was -11*C (12.2*F) with winds coming from the south-east @ 20km/h (12.4mph).

Earlier that day, (let's take 1:00pm for instance) 0.7 GPH got me into the 45-48mph range and the weather at that time was around -9*C (15.8*F) with north-west winds @ 7km/h (4.3mph).

[MetroMPG]

Originally Posted by Peakster

Clearly aerodynamics play a big role even at low speeds since the table and chairs only weigh 100 pounds at most.

I get the feeling that many people mistakenly assume that

[Silveredwings]

Originally Posted by MetroMPG

Even at just 20 mph, 1/3 of the total power (about 1.2 hp) required to move a Metro goes to overcoming aero losses. 1/5 of the power at 15 mph.

Is that from a source of empirical data on the metro? I'd really like to see more of that curve.

[Peakster]

I'm going to do a little test run today. I'll drive west to Moose Jaw, Saskatchewan on the Trans Canada Highway (very straight an flat road). First run with hatch up:

IM001260.JPG

I'll record MPG at different speeds (let's go with 35, 45, 55, and 65mph) and the second run with hatch closed:

IM001262.JPG

(again recording MPG at those same speeds).

I'm interested to learn what percentage MPG loss/increase there is to each speed. (this has made me wonder for a while since MetroMPG got a 2.3% MPG increase when removing mirrors @ 55mph in mild temperatures, while I got a 2.23% increase when driving 70mph in really cold temperatures, without mirrors).

[MetroMPG]

Wow. Ambitious, Peakster. I'm guessing it's not -25C today.

I'm also guessing you're going to see a dramatic drop in FE. My WAG = -20%. (Better put a stick under the hatch to hold it open!)

SW: it's

[Peakster]

Originally Posted by MetroMPG

Wow. Ambitious, Peakster. I'm guessing it's not -25C today.

I'm also guessing you're going to see a dramatic drop in FE. My WAG = -20%. (Better put a stick under the hatch to hold it open!)

SW: it's theoretical data: http://www.metrompg.com/tool-aero-rr.htm

EDIT: Did you put the mirror back on? Or is that an older pic?

20% was a good guess... for 40mph . Take a look at the video and see the difference for yourself.

BTW, I've had the mirrors back on the Geo for weeks now. The plastic covers for the holes in the door froze and fell off and it looked dumb. I'll make better plastic covers when the weather gets a bit nicer.

Edit: *gasp* I forgot to put the gassavers link on the video

[MetroMPG]

Can't! I'm on dial-up at the moment

[Peakster]

Here's the little test in a nutshell:

Temperature: -4*C
Winds: south @ 11km/h (I was driving west)
Cruise control was used to maintain speeds
Each mpg reading was taken after 2 miles of travel
Route:
geo exp 2.1.gif

Hatch closed:
73.6 mpg @ 35 mph
62.9 mpg @ 45 mph
49.2 mpg @ 55 mph

Hatch open:
62.4 mpg @ 35 mph (15.22% mileage loss)
48.2 mpg @ 45 mph (23.37% mileage loss)
34.4 mpg @ 55 mph (30.08% mileage loss)

Those are huge numbers for something as simple as opening up the rear hatch. What do you think my Cd was when it was open? 0.44? I'm now beginning to believe that small aero mods to these little cars (such as mirror removal, wheel skirts, etc) would help FE in more than just highway travel.

[The Toecutter]

At 25-30 mph or so is when aero losses become as large as rolling losses on flat ground. So even at those speeds, aero mods should in theory have significance. But this is steady speed driving as opposed to stop and go driving. Stop and godriving will make weight and rolling losses a much larger factor than they are in steady speed driving.

[MetroMPG]

Originally Posted by Peakster

Those are huge numbers for something as simple as opening up the rear hatch. What do you think my Cd was when it was open? 0.44?

(I think it's in my "favourite GS links in my sig.)

[Peakster]

Originally Posted by MetroMPG

@ 88 km/h (54.7 mph) ...

• Just adding (empty) roof racks changed my mileage by -7.0 mpg / -12.7%.
  .
• Putting my mountain bike up there on the rack absolutely destroyed the mileage: -15.1 mpg / -27.3%

And unfortunately, it's likely the negative effect of roof-top carrying is more pronounced on smaller, less powerful cars.

Holy smokes. These results make me want to find another SG compatible metro, chop the top off and see what crazy MPG numbers show up.

[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!

[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.