Metro aerodynamics

[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:

Attachment 214

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

Attachment 215

(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:
Attachment 216

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.