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Old 12-19-2018, 05:25 PM   #11 (permalink)
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Here's a thought - would one of these help until I can find a way to build a proper boat tail or kamm?

https://www.realtruck.com/bizon-truck-cab-spoiler/

I know it's designed for trucks, but it would definitely provide a cleaner separation on the airflow. And keep the snow off my back window - my rear wiper sucks. Doesn't matter what brand I put on, it just sucks. I get like a foot wide section that's clean and then it just smears stuff around through the rest of it's footprint.

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Old 12-19-2018, 07:24 PM   #12 (permalink)
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Quote:
Originally Posted by DeliveryGuy89 View Post
I like this idea, although it seems that it would reduce my rear visibility quite a bit. I'd have to play with it to get it to a point that I wasn't sacrificing safety.

Some quick research says that "To gain most of the effects described by Kamm, a car’s tail has to be tapered to about 40-50% of its largest cross-sectional area."

I have no idea what this means, could someone break down exactly what I should be measuring here?
Cross-sectional area (sometimes called frontal area) is the maximum area of the car as viewed from the front or rear. If you can find a blueprint online or a front photo taken from a couple hundred feet with a telephoto lens, you can run it through photo-editing software and get a pixel count and convert that to square feet or square meters. Or, for a rough estimate multiply the manufacturer's width by height by 0.85. For a '96 Outback, that's 68" x 62" x 0.85, or 24.9 square feet.

For a Kamm tail, you want a nicely tapering extension that terminates in a flat face approximately 50% of the cross-sectional area, or about 12.5 square feet. Wunibald Kamm discovered that lopping the end of a full tail off preserved most of the drag reduction but with reduced length. Here's one of his cars:



A box cavity is a short tail with perpendicular sides, top and/or bottom, inset from the rearmost surfaces of the car. Here's one from a formerly-active member here:



These can reduce drag by anywhere from 2-5% typically, and they're a lot easier to build than a full tail.
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Old 12-19-2018, 11:07 PM   #13 (permalink)
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Quote:
Originally Posted by Vman455 View Post
Cross-sectional area (sometimes called frontal area) is the maximum area of the car as viewed from the front or rear. If you can find a blueprint online or a front photo taken from a couple hundred feet with a telephoto lens, you can run it through photo-editing software and get a pixel count and convert that to square feet or square meters. Or, for a rough estimate multiply the manufacturer's width by height by 0.85. For a '96 Outback, that's 68" x 62" x 0.85, or 24.9 square feet.

For a Kamm tail, you want a nicely tapering extension that terminates in a flat face approximately 50% of the cross-sectional area, or about 12.5 square feet. Wunibald Kamm discovered that lopping the end of a full tail off preserved most of the drag reduction but with reduced length. Here's one of his cars:



A box cavity is a short tail with perpendicular sides, top and/or bottom, inset from the rearmost surfaces of the car. Here's one from a formerly-active member here:



These can reduce drag by anywhere from 2-5% typically, and they're a lot easier to build than a full tail.
So would it be possible to combine those two concepts? Say instead of rear wheel skirts, I extended them out past the back to taper together, creating a sort of boat tail/box cavity below the rear window?

Edit: ****. That would still leave me needing to redo the electrical array. Hm. Decisions, decisions...
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Old 12-20-2018, 01:46 AM   #14 (permalink)
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Delivery you are in good hands here, these people know their aero! Do you have a receiver and trailer wiring plug? Not that hard to install a plug if not. Then you can plug lights in I think a legacy would be tricky to boat tail and keep access to trunk. Might be able to go lightweight kammback stuck to back door, which lifts up and out of the way. A box cavity is a simple sided box behind the vehicle inside the draft. Not as good as an aero boat tail. But help.help
Vman455 the Prius looks awesome with the aero hitch box tail, very slick.
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Old 12-20-2018, 02:27 AM   #15 (permalink)
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Well, I'm doing a road trip to Florida tomorrow and I'm reading this before going to bed. Looks like it's time to go get a box cutter and some coroplast...
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Old 12-20-2018, 05:59 AM   #16 (permalink)
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As soon as he takes car of his tail drag, we gotta find this guy an aerodynamic bull bar for those animals on the road he was talking about in the opening post.
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Old 12-20-2018, 09:29 AM   #17 (permalink)
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Quote:
Originally Posted by kach22i View Post
As soon as he takes car of his tail drag, we gotta find this guy an aerodynamic bull bar for those animals on the road he was talking about in the opening post.
Do the same thing that a bunch of us do on the back. Put a bullbar on the front and use it as a building point for an aerodynamic nose, add a belly pan and he could probably get some nice smooth flow underneath
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Old 12-20-2018, 09:36 AM   #18 (permalink)
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Quote:
Originally Posted by Vman455 View Post
A box cavity is a short tail with perpendicular sides, top and/or bottom, inset from the rearmost surfaces of the car. Here's one from a formerly-active member here:



These can reduce drag by anywhere from 2-5% typically, and they're a lot easier to build than a full tail.
Angie will be getting this prior to my drive out to Cali next summer. I've got a hitch basket I'll be using and this will fit nicely around it. I'll also be able to use it to keep road spray off the basket and to smooth out the outflow from underneath.

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