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

[wagonman76]

Coming along nicely, looks great! Now if you want you can use the cardboard as temps for coroplast.

I also used that same "angle iron cardboard" for my original airdam on the wagon. It held up pretty well. The rain started to delaminate it but it still held together till the day I took it off.

[BamZipPow]

Don't fergit yer gonna need some side reflectors nearest the back end of yer boat tail...

[MetroMPG]

The OEM tail lights at the back of the tail have built-in side reflectors. But if I decide to go with aftermarket lights on the end, I'll have to add some.

Originally Posted by wagonman76

Now if you want you can use the cardboard as temps for coroplast.

I still haven't decided on construction technique for the permanent/removable version. Competing pros & cons of various methods have me waffling.

I was going to do tuft-testing videos today, but I got stuck with a sick computer that needs attention. So hopefully tomorrow. Has to be before Thursday, because it looks like winter's finally coming...

[MadisonMPG]

Originally Posted by Cd

As I pointed out to Mike, you could actually make some free money on the side if you wouldn't mind having vinyl adds on your car.
According to a guy I met that posts on PriusChat, there is a guy around Austin that has done it to his car. Supposedly, he has a "boat tail" on his car too.

My mom even thought about doing it to help with gas money. The only draw back is that you are required to drive a certain number of miles per day.

The only problem with this is liability.



The boat tail look great.

[Christ]

What liability is there for displaying advertisements on your ride? I don't see how there could be, as long as you're following traffic laws in whatever area you're driving in.

[wagonman76]

Maybe on a stock vehicle I see no liability. But ads on non DOT certified mods, if something happens like it comes off and hurts someone, it's the advertiser's name getting dragged thru the mud.

[Nerys]

irrelevant wagonman76. if something comes off and hurts someone they advertisers name WILL be dragged through the mud regardless if its dot certified off road or even OEM. its the way of our very litigious society.

its a matter of risk versus reward. a big name advertiser might not mess with it someone small who really has nothing to lose. What are they going to care?

[JackMcCornack]

In Oregon at least, there's no cert required for advertising mods. You might be better off if you <did> get an advertiser on board because then you could argue your boat tail is a sign, rather than a modification. Several states have silly rules re any modifications which might change (not necessarily increase, just change) your exhaust emissions, and a functional boat tail will indeed change what comes out the tailpipe. But those delivery guys with fried chicken and pizza signs strapped to their roofs, those realtors with magnetic door signs breaking up the boundary layers on the sides of their cars, they don't have any certification for them.

[MetroMPG]

I set up a 5 x 5 grid of tufts, minus 2 directly behind the exhaust. I got some video as well, which I'll post eventually.

First, below, here's how I dealt with the exhaust (2 layers heavy tin foil):



Below: 5 rows of 5 tufts, except in 2 rows behind the muffler:



Below: web cam set up, looking forward. (Not shown: I also attached it to the driver's side mud flap to capture video looking rearward.)



Observations:

• obvious separation behind the wheels (no surprise there!)

• turbulence everywhere else, but not reversed flow. It seems more turbulent toward the outsides (effect of the wheels and control arms), less worse toward the centerline

• Correction: alternating between "sometimes separated / sometimes turbulent" for the two tufts closest to the control arms

I lent my Hucho to Ben, so I'm having a little trouble remembering my definitions. I believe "separation" is characterized by reverse flow (eg. behind the wheels).

The flow down the middle 3 of 5 columns (parallel to centerline) appears turbulent, but there does not appear to be any reverse flow, so I don't think it's separated.

That's a good sign for 2 reasons: first, I believe it means the bottom angle isn't too steep. And next, it suggests that adding the full belly pan will help reduce turbulence & minimize wake off the rear. (If there had been separation due to the angle, the belly pan wouldn't have helped.)

The outermost tufts in the last 3 rows (inline with, but furthest away from the wheels) are also in turbulence, but not separated flow.

Next in the boat tail evaluation: I'm going to reinstall the full belly pan and re-tuft test the underside.

[MetroMPG]

Here's a screen capture / pretty picture of the tuft testing that may explain better what I was seeing: