Project: Geo Metro boat tail prototype - 15% MPG improvement @ 90 kph / 56 mph

[MetroMPG]

Conditions:

Grenadier Island Past 24 Hour Conditions
Date / Time (EST) ... Conditions ... Temp (°C) ... Humidity (%) ... Dew Point (°C) ... Wind (km/h) ... Pressure (kPa) ... Vis (km) ... Wind Chill
15:00 ... N/A ... 2 ... 59 ... -5 ... calm ... 101.8 ... N/A ... *
14:00 ... N/A ... 2 ... 57 ... -5 ... W 2 ... 101.7 ... N/A ... *

Speed: 90 km/h (56 mph)

Route: straight & practically level 1.6 km stretch of 2-lane highway, no other traffic ahead of me in my lane, infrequent oncoming vehicles in the other lane.

A = boat tail on
B = boat tail off

Readings in MPG (US):

    east    west    avg    
A    66.4    62.6    64.5    
A    65.2    62.8    64    
A    65    64.4    64.7    64.4 (avg these A runs)

B    56    56.6    56.3    
B    55.9    56.5    56.2    
B    54.3    56.3    55.3    55.93333333 (avg B runs)

A    65    64.3    64.65    
A    64.2    64    64.1    64.375 (avg these A runs)

64.39 A avg (all A runs)
55.93333333 B avg

15.1% improvement, A over B

[Cd]

Holy HELL !!!!

[theycallmeebryan]

Very nice improvement! I'd assume the "B" tests were done with the Kamm spoiler on the car, and the "A" tests were with the boat tail (And both had your belly pan). That means full boat tailing improves aero 15% over a kamm spoiler! Excellent!

[MetroMPG]

Originally Posted by Cd

Were you able to use the information from the test to find the cars new Cd, or will that have to wait ?

I only did constant speed testing, so it'll have to wait.

Originally Posted by Christ

Why cover up the sides of the tail lights? ... or you're using your OE tail lights with an extension harness?

Nerys was right: the lights weren't covered, the OE harness was extended to mount them on the end of the tail. EG: see pics in post 10.

Originally Posted by Cd

And you ( of coarse ) accounted for the weight difference right ?

What is the weight of the 'tail anyways ?

Nope! Which means ... the boat tail is slightly more effective than the ABA runs indicate. I figure it weighs in the 10-15 lbs range.

Originally Posted by thatguitarguy

If you're talking about Darin's experimental design, I think that in heavy crosswinds, it would tear itself free of the vehicle (creating another set of problems), or at least partially collapse.

It would have to be a seriously strong crosswind (talking storm force). This thing is remarkably secure on the car - I've demo'd this to several skeptical observers by grabbing hold at the rear and wrenching the whole car side to side & up and down using the tail as the lever.

Originally Posted by Christ

a well-aimed gust could be sufficient to force the tail down

You're describing downforce on the tail. I'd expect lift on the rear wheels and increased pressure on the fronts, if anything. Have a looksee at the cross-sectional shape a few posts up where you can see its outline detached from the car.

Originally Posted by theycallmeebryan

I'd assume the "B" tests were done with the Kamm spoiler on the car, and the "A" tests were with the boat tail (And both had your belly pan)

Right. The only thing changed between A & B runs was the boat tail. The full undertray was in place for the B runs.

FYI, the incomplete A-B testing (at the same speed) of the Kammback suggested it was good for a 4.9% improvement over stock.

[MetroMPG]

Originally Posted by Cd

Holy HELL !!!!

Yeah, I was pretty pleased too! It's not bad for a sub-optimal design.

[MadisonMPG]

Wow 15%. That with an alt delete would pretty much = MPG bliss.

[Christ]

So to figure out what it would do for a "normal" Metro, you'd add the kamm's original gain into the 15% for the boat tail, right?

That would make for some 25% improvement boat tail over OE design, IIRC?

44MPG rating + another 25% is 55 mippigs!!! That's huge!

[MetroMPG]

Originally Posted by MadisonMPG

Wow 15%. That with an alt delete would pretty much = MPG bliss.

Exactly! And the % gain from the alternator delete is going to be proportionately higher with the boat tail than without. Its load on the engine is more or less constant, but the total load has dropped. So I'd expect a better than 10% improvement combined with the boat tail.

[Christ]

Wow. Just wow.

And aerodynamically speaking, the tail could be somewhat more aggressive than yours, right? So it could end up shorter by some amount, which (technically) means that it could be slightly better (if optimal) even than what you've gotten already...

Does it get better than this?

So what were you thinking in your head for an estimate of performance while testing?

Did you think the first couple runs were glitches? I mean... a 15% improvement is huge to see...

[thatguitarguy]

I couldn't tell from the picture if this:

Originally Posted by MetroMPG

wheel tail is a part of the final tested structure.

Congratulations!! 15% over the modifications you already have is really significant. And as light as cardboard is, Coroplast is lighter and stronger. Electrical connections would be easy with a trailer type setup, and if you can figure out an attachment scheme that is easier than duct tape, you have a very successful removable boattail!

So cool!!