MetroMPG's Honda Insight boat tail extension (cardboard) tuft video; ABA test +9.7%

[Jyden]

Great project, and very well made. I would wheater proff it with marine grade epoxy on the tinside, and if needed som fiberglass cloth (woven) on the outside, but that will add some wheight.

[pascal28]

Originally Posted by MetroMPG

Well, after a good night's rest and a fresh look at the tuft testing results today, I've decided it's good enough as-is. I'm not going to change it.

EDIT: Neil, I might just test that wheel strake/boattail idea to reduce turbulence, too. If that doesn't help, when I make the permanent tail, I'll use a little less curvature in the sides, which will add about 20 cm / 4 in. to the width of the rear.

Next task: A-B-A test and weather-proof it. Weigh it, measure it, to answer those inevitable questions.

Many thanks for sharing the results. This is just what we need to help understanding.

As you mentioned the current curvature on the sides is not ideal. Do you have a picture of the boat-tail from above & (ideally) the angles used? This will help determine what angles to not exceed.

[MetroMPG]

I don't have any "from directly above" photos. But I will take measurements at some point and post them.

[HydroJim]

I wonder is what is causing the turbulence on the side? seems like it would be nice and smooth over there.

[freebeard]

It looks like you could relax the curve on the lower rear since the air is wrapping down over the edge. But what about shortening the overall length?

Don't forget you could [carbon]fiberglass the outside, dissolve the cardboard and hang the skin on an aluminum tubing version of your substructure. Kind of like a taxidermist.

[NeilBlanchard]

I think that 2" more width each side sounds just about right! The video makes it look like it is the angle of the sides that needs to change to solve the fluttering tufts on the sides, but wheel strakes will help on the underside.

[MetroMPG]

(The U.F.O. at a picnic pull-off on the 1000 Islands Parkway, along the shore of the mighty and lovely St Lawrence River. I parked here to remove/reinstall the tail for A-B-A testing. That's upper N.Y. State in the far distance on the opposite side.)

I just got back from testing on my usual route - jumped at the chance to go this afternoon because the conditions were near perfect.

I did A-A-A-B-B-B-A-A bi-directional runs, where A=tail, B=no tail.

Results show: 9.7% better MPG at 80 km/h (50 mph) with the tail.

Happy, happy!

[MetroMPG]

Oh, and I passed a police speed trap on my way to the test route with the tail ON the car. (I was going exactly the limit.) I was sure he was going to pull out behind me, but he didn't!

To make this legal-ish, I probably need side/rear reflectors/lights and a light above the licence plate.

[Daox]

Very cool.

How much money do you have into this?

[MetroMPG]

- Cardboard: free.
- Wood scraps and screws: on hand (so, not free, but close to it).
- half dozen ratchet straps: on hand
- Duct tape: on hand.
- Hot glue sticks: a couple of bucks.

9.7% fuel economy improvement at the posted speed limit on secondary highways: priceless!