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Old 10-20-2009, 10:35 PM   This thread is in the EcoModder Project Library | #1 (permalink)
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Join Date: Nov 2007
Location: 1000 Islands, Ontario, Canada
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Blackfly - '98 Geo Metro
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Project: Geo Metro boat tail prototype - 15% MPG improvement @ 90 kph / 56 mph

Above image clockwise from top left: aluminum frame, cardboard construction; finished
prototype; removed during A-B-A fuel economy testing; still frame from video of tuft-testing

Overview ...

This discussion thread documents the construction & testing of a temporary, proof-of-concept aerodynamic "boat tail" on a 1998 Pontiac Firefly (Geo Metro).

Some facts about the prototype ...
  • Weight: 5.5 kg (~12 lbs)
  • Length: it adds 1.4 m (~4.5 feet) to the car
  • Hard to park? No. The car + tail is only ~30 cm (12 inches) longer than a 2008 Toyota Camry (the most popular car in the U.S.), and shorter than a Ford F-150 (most popular truck). So it would easily fit in a normal parking spot.
  • Sturdy? Yes. I could shake the entire car up & down and side to side from the end of the tail.
  • Handling: I drove with the boat tail on the car for about 300 km (186 miles) at highway speeds up to 100 km/h (62 mph) and saw no change in handling. (Had to keep an eye on rubberneckers in other cars, though.)
  • Rearward visibility: obviously there's no rear window; the next version will have one
  • Tail lights: the car's original rear lights (brake, reverse, parking & side lights) were relocated to the end of the tail, as well as the licence plate
  • Ugly? Duh! It's a prototype made of cardboard and duct tape! But the idea is beautiful in my eyes. Most production vehicles are aerodynamic disasters on wheels, and I think that's ugly.

Testing the prototype ...

1) First road test: read my impressions of the first drive with the boat tail on the car. Skip down to Nov. 21, post #100

2) Tuft testing: the tail spent a lot of time in the poor man's wind tunnel (ie. being tuft tested). This means observing the motion of yarn tufts taped to the car. The tufts reveal the direction & nature of air flow, and indicate whether the design works or not. Tuft testing results can be seen in the YouTube video, below.

3) Fuel economy testing: I used a ScanGauge to do an A-B-A test (before/after/before) that revealed a 15.1% fuel economy improvement at 90 km/h (56 mph). For testing details: A-B-A testing results posted, Dec. 5th

The original discussion started here ...

These are the goals for the project:
  • design a removable boat tail under 5 feet in length
  • the disassembled pieces of which will fit inside the back of the car
  • lightweight, stiff construction
  • minimally "invasive" to the car (discreet mounting points, etc)

I'm imagining 6 separate panels that will somehow snap or wingnut on to a light frame that can be easily dis/assembled.

I've done lots of waffling on materials, but I think I'm going to go with aluminum for the frame and coroplast for the skin. Both for their lightness, and ease of workability. Assuming I can find black coroplast (Home Depot here only has white - $20 per 4x8 - and the sign making shop wanted an obscene amount for a 4x8 black sheet of it).

Runner up idea was a fabric skin - UV resistant boat top canvas (which I can get in black). But thinking about hurricane force wind that will rarely be coming from directly ahead of the vehicle made me think coroplast is probably better.

One task I've actually completed so far: I added 6 feet to the wiring harness of each of my tail lights so I can move them between the back of the boat tail and their stock position as needed.

Twice now I've pulled the car into the garage and fussed around with various building materails, from cardboard to styrofoam to wood to alumimum. Nothing built yet. I've spent a silly amount of time standing around looking at the back of the car.

I think I need to just start building!

Also see my second boat tail project:


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
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