View Single Post
Old 06-20-2009, 04:57 PM   #1 (permalink)
Batman Junior
MetroMPG's Avatar
Join Date: Nov 2007
Location: 1000 Islands, Ontario, Canada
Posts: 22,316

Blackfly - '98 Geo Metro
Team Metro
Last 3: 70.09 mpg (US)

MPGiata - '90 Mazda Miata
90 day: 52.07 mpg (US)

Winter beater Metro - '00 Chevrolet Metro
90 day: 61.98 mpg (US)

Fancy Metro - '14 Mitsubishi Mirage top spec
90 day: 58.72 mpg (US)

Even Fancier Metro - '14 Mitsubishi Mirage top spec
90 day: 70.75 mpg (US)
Thanks: 3,846
Thanked 6,723 Times in 3,486 Posts
Tested: what's the resolution of coastdown testing? Can it register an open window?

I was curious to know what kind of "resolution" coastdown testing has: in other words, what magnitude of aerodynamic change can be revealed by repeated, timed coasting between two speeds.

We know that large changes will show up, eg, here's Skylark's coastdown results of his Versa's Coroplast boat tail:

But what about the smaller things people are doing? Would they show up in coastdowns?

To try to answer the question, I chose one open window as the aero modification I would try to detect.

The speeds tested were 100 km/h (~60 mph) to 50 km/h (~30 mph)

Test was AA-BBB-AA-B

The results seem to indicate that, for my vehicle, at those speeds, a single open window is just at the edge of detectability - the average difference between the A & B results is just above the standard deviation in the control (A) set.

Any smaller aero "mod" would certainly disappear into the statistical noise / variation of the control runs.

Higher test speeds would presumably result in greater resolution.

Attached Thumbnails
Click image for larger version

Name:	coastdown-window-up-down.gif
Views:	2216
Size:	16.4 KB
ID:	3740   Click image for larger version

Name:	Mods.jpg
Views:	348
Size:	22.6 KB
ID:	3741   Click image for larger version

Name:	6.jpg
Views:	341
Size:	95.8 KB
ID:	3742  
Project MPGiata! Mods for getting 50+ MPG from a 1990 Miata
Honda mods: Ecomodding my $800 Honda Fit 5-speed beater
Mitsu mods: 70 MPG in my ecomodded, dirt cheap, 3-cylinder Mirage.
Ecodriving test: Manual vs. automatic transmission MPG showdown

has launched a forum for the efficient new Mitsubishi Mirage - fuel efficiency info for Geo Metro owners - electric car conversion on a beer budget
  Reply With Quote