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Old 08-11-2017, 11:08 AM   #11 (permalink)
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A kammback - which is basically a truncated boat tail - is probably most appropriate, as freebeard is suggesting.

Aerodynamics is a bit of a dark art, as far as I can tell. Although there are plenty of design guidelines that work reliably, you'll find that intuition often doesn't work with complex shapes. You can be pretty certain that, although the tiny kamm on the back may poke out of the profile, it almost certainly lowers drag overall, or Toyota wouldn't have put it there. The RAV is a very complicated shape, and the kamm could be correcting for something like the mirrors, or perhaps the windshield angle is too steep for there to be attached flow along the front of the roof, but it reattaches at the rear. Without doing extensive testing, you just can't know.

It's not a technical article, but I was amused by this recent post on Jalopnik:

deadspin-quote-carrot-aligned-w-bgr-2<\/title><path d="M10,3.5l3-3,3,3Z" style="fill:%23fff;stroke:%23fff"/><path d="M0,3.5H10l3-3,3,3H26" style="fill:none;stroke:%231b3a4d"/><\/svg>')}.f_branding_on.blog-group-deadspin .editor-inner.post-content .pu


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Old 08-11-2017, 04:49 PM   #12 (permalink)
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I saw the article, I read every thing that Torchinsky writes.

But I allowed Javascript for kinja.com and kinja-static.com and I still couldn't see the poll. I didn't know which of the other 16 scripts powers the poll, so I moved on.

Yay for the Type II!
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Old 08-12-2017, 01:33 PM   #13 (permalink)
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length

yes,the long length has always been the bugaboo of truly low-drag vehicles.It's why Kamm and Fachsenfeld spent so much time researching cost/benefit for varying lengths.
Kamm/Fachsenfeld settled for a car body which followed a streamlined pathway,then was chopped off at a point where the new wake area was 50% of the car's frontal area.
The Fachsenfeld went one better,creating an inflatable boat tail which would only deploy on the highway,far from parking garages,traffic,etc..
With the stinger,my truck will be 311.6-inches in length.
With only the boat-tail,269.6".
The longest vehicle on the road today is a 2012 Dodge RAM Laramie Longhorn DRW pickup,at 259.4".
At 30-mph a tail won't do you much good.I don't know what to recommend.
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Old 08-12-2017, 01:45 PM   #14 (permalink)
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E-Type Jag

Quote:
Originally Posted by Ecky View Post
A kammback - which is basically a truncated boat tail - is probably most appropriate, as freebeard is suggesting.

Aerodynamics is a bit of a dark art, as far as I can tell. Although there are plenty of design guidelines that work reliably, you'll find that intuition often doesn't work with complex shapes. You can be pretty certain that, although the tiny kamm on the back may poke out of the profile, it almost certainly lowers drag overall, or Toyota wouldn't have put it there. The RAV is a very complicated shape, and the kamm could be correcting for something like the mirrors, or perhaps the windshield angle is too steep for there to be attached flow along the front of the roof, but it reattaches at the rear. Without doing extensive testing, you just can't know.

It's not a technical article, but I was amused by this recent post on Jalopnik:

deadspin-quote-carrot-aligned-w-bgr-2<\/title><path d="M10,3.5l3-3,3,3Z" style="fill:%23fff;stroke:%23fff"/><path d="M0,3.5H10l3-3,3,3H26" style="fill:none;stroke:%231b3a4d"/><\/svg>')}.f_branding_on.blog-group-deadspin .editor-inner.post-content .pu

Ralph Gilles,who was responsible for the design of the SRT Viper refers to the E-Type shape as 'extreme cab-rearward'.
They can't produce low drag but stylists love 'em.
In the day,The E-type would have been low in drag,and with only 16-plus square-feet frontal area,the drag index wold be quite favorable.
Consider an MG TC,with Cd 0.75!
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Old 08-12-2017, 04:04 PM   #15 (permalink)
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Here's an example of a 'cab-forward XKE', with a Cd good for it's day or any other. A contemporary of the MG-TC as I recall.
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Old 08-26-2017, 10:58 PM   #16 (permalink)
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Yes, drag reductions can be noticed at 30 mph or below, the question is really whether you will notice/benefit from it in your CAR. Drafting on bicycles results in a significant reduction in drag at usual bicycle speeds of 13+ mph.


Quote:
Originally Posted by RAV4 EV View Post
So this beast of a boat tail should bring down my Rav4 EV's drag to Cd0.13 from Cd0.30?
Don't count on it. At that point, many other design details would probably keep your drag around the .18-.22 range.

It isn't necessary to make a ridiculously long boattail to realize noticeable drag improvements. I would suggest making a tail around the 1-2 feet long mark. That would give you plenty of drag reduction while keeping your length down. First things first, before you start building your boattail, you need to make some decisions. Are you going to move your taillights to the back of the tail? Install new taillights with oem in place? Keep oem with reflectors at end of tail? These things are important, especially when you catch the eye of the police. Mounting is among the other top consideration. I don't even know where to start with mounting, hence the reason my car doesn't have a boattail.
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Old 08-27-2017, 12:18 AM   #17 (permalink)
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0.13 CD is pretty close to the theoretical lowest Cd possible for a land wheeled vehicle - one that looks nothing like an SUV.
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Old 09-30-2017, 08:56 PM   #18 (permalink)
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Hey Guys,

I'm still around just got really busy and sick, mostly busy. Had to deal with some pending debt, but that is done with so I got some time on my hands. Over time I was thinking about maybe choosing a trailer, flat bed type, and creating a super long card board boat tail on top of it that I could tow with me everywhere I go. Then I found out my RAV4 EV is not meant to tow around trailers, which is weird because the gas version can and its the exact same chassis, plus it would seem that I would be totally liable if I were to get into a car accident .

So, after wasting much time with that I ended up back where I started. So, I think that for the boat tail I could install a trailer hitch and add a cargo trailer from harbor freight. Use that as a base to mount the boat tail and also have a place to hold extra cargo; which I actually need because I usually carry around a lot of cargo stuff in my rav4 ev.

For the tail lights I would love to be able to move them to the back end of the boat tail and be able to have a full contour boat tail shape.

Firstly though I still need to figure out the boat tail length. Like someone suggested I will likely truncate the boat tail at the end specially since @ 80% boat tail length you get about the same aero benefit like 100% boat tail length.

My original template was calling for about an 11 feet long boat tail, but it looked like I had not adjusted it right. I did post a second set of pictures where I try to adjust it, but I never did get any feedback on if I had the bottom part of the template aligned properly.

The main confusion was if I had to align the bottom part of the template with the bottom part of the rav4 at the template bottom wheel fin or the template under body:

UNDER BODY



BOTTOM WHEEL FIN



I think that the top part of the body is now right after I did the correction for that. The kamback does get cut out some after the adjustment, but someone did mention that the Kamback could be reattaching flow from the mirrors and that I should not remove it. If you look at it carefully the template that aligns the bottom wheel fin instead of the template under body cuts more into the kamback.

So which template do you guys think would better suggest the right full boat tail length for the rav4 ev?
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Old 10-01-2017, 01:38 AM   #19 (permalink)
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Aerodynamic Streamlining Template: Part-C

The instruction is ground-plane to ground-plane, top of vehicle to top of template.

I wouldn't over-think it. Have you looked at the fore-aft profile of The Template?

At any usable truncation, the best you can do is produce the existing contour rearward and inward with a boxed cavity or concave bulkhead. I would limit vehicle length to 18 1/2ft based on the length of a parking space.

There are examples of hitch mounts on Ecomodder. Leaving a gap between the box and car body is similar to the separate bed on a pickup truck — a drag inducing feature. It might be better to use a non-reciever hitch and gain additional attachment points at the old taillight locations.
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Old 10-02-2017, 08:35 PM   #20 (permalink)
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Quote:
Originally Posted by freebeard View Post
Aerodynamic Streamlining Template: Part-C

The instruction is ground-plane to ground-plane, top of vehicle to top of template.
I think when I tried to do ground with ground and top with top I got the first image at the start of the thread, and that didn't seem to work . Unless I am missing something in which case I apologize!

Quote:
Originally Posted by freebeard View Post
I wouldn't over-think it. Have you looked at the fore-aft profile of The Template?
I'm not entirely sure I'm so new to aero moding that even that makes me confused lol.

Quote:
Originally Posted by freebeard View Post
At any usable truncation, the best you can do is produce the existing contour rearward and inward with a boxed cavity or concave bulkhead. I would limit vehicle length to 18 1/2ft based on the length of a parking space.
That sounds great, but it also is much more difficult to do . Others had suggested just tapering off the boat tail quicker which would be easier but much less ideal I'd guess?

The chevrolet express I used to own was 224.112=18.675 Feet long, and I could park it just about anywhere so your suggestion on length is spot on.

Quote:
Originally Posted by freebeard View Post
There are examples of hitch mounts on Ecomodder. Leaving a gap between the box and car body is similar to the separate bed on a pickup truck — a drag inducing feature. It might be better to use a non-reciever hitch and gain additional attachment points at the old taillight locations.
I was really hoping to get a smooth transition of the boat tail from the end of the rav4 onto the boat tail body. I will have to think about how to tackle this. I really would like to have some reliable storage space in the back. My Rav4 EV is usually pretty loaded. If I could load about 3 good sized boxes in the back I would be quite happy. Though I feel like I'd need for the storage are to reliably hold up to 100 pounds. Not sure if I could design that right which makes the cargo carrier interesting. There is an aluminum one in harbor freight that weighs 30lb. Doesn't seem too heavy?

Anyway, since I am so confused on which template out of the 2 I linked earlier is right I settled to check the dimensions on the 1st which has the longer boat tail.

Pixel Check:

65 390 front bumper
635 390 rear bumper

570 pixel difference

rav4 ev is 180" long

570180=3.166666667 (1" equals about 3.166666667 Pixels)

Boat Tail Length?:

635 395 tip of rear bumper
980 395 tip of boat tail

345 Pixel Difference

3453.166666667=108.94736841"Inches/12"=9.078947368 feet long boat tail

9.078947368−20% (Fluid Tail Reduction)=7.263157894 Feet long Boat tail 80% of full size.

And if I want to stay within your suggest length, which is about the length of a Chevrolet Express non-extended:

7.2631578942=3.631578947
180(rav4 length)/12"=15(Feet)+3.631578947=18.631578947 Total Feet

So I would have to cut the boat tail in half and design a slide out cone tail thing of sorts I can deploy before going into the highway. Half version for city driving and 80% of full boat tail for highway.

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