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Old 01-21-2023, 10:49 AM   #11 (permalink)
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The way I see it, without extending the sides, vortices cannot be captured in an attempt to reduce the pressure differential and reduce turbulent flow. Cd reduction will be minimal and hindered if the sides are not tapered.

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Old 01-21-2023, 02:16 PM   #12 (permalink)
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The way I see it, without extending the sides, vortices cannot be captured in an attempt to reduce the pressure differential and reduce turbulent flow. Cd reduction will be minimal and hindered if the sides are not tapered.
I would extend the sides. Iím saying the angle would be very minimal. Most of the low Cd cars out there just have a long roofline and the width in the rear isnít as narrow as how most ecomodders do their after market boat tails. I understand itís not as optimal, but in theory a proper boat tail would give me an average of 10 extra mpg, but Iím willing to add some styling personality and say, get only 8 mpg
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Old 01-21-2023, 03:23 PM   #13 (permalink)
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Alternatively, consider the Rumpler Tropfenwagen. When I designed a boat tail extension for the VW Beetle, I found that tapering the sides and not the top provided a better solution (interior volume, rearward visibility).
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Old 01-21-2023, 05:30 PM   #14 (permalink)
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Old 01-21-2023, 05:57 PM   #15 (permalink)
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Old 01-22-2023, 02:15 AM   #16 (permalink)
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Whatís the impact difference between tapering just the roofline versus tapering the sides too as a full boat tail?
Tapering the sides allows a larger reduction of the rear wake area than top-bottom only.

Also, since the boattail is far beyond the rear axle, it improves clearance when turning.

Among the reasons why production cars to not have as much side taper is to keep the internal volume (trunk space/dimensions) as useful as possible.
Clearance is not an issue as it is close to the rear axle. Speaking of which, they also do not taper to keep the rear wheels' tracking distance similar to the front (only a handful had much wider track in front than rear).
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Old 01-22-2023, 03:43 AM   #17 (permalink)
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Okay I’ll taper the sides. But I still need the boat tail to be stylish. Lots of eco modders just round the thing like a cone. I’m thing of having it more square. Should make it easier to build AND LOOK BETTER. Sure it may shave off a few Gained mpg, but it’ll still give a massive improvement versus no boat tail. Look at the vw xl1, a little of the lightyear one/zero, and my fav- the Mercedes EQXX. The rear rooflines are pretty flat then just cut at a right angle at the edges down, versus rounded and smooth. I really like the rest of the eqxx. The tail and the diffuser together. That’s mostly my inspiration. But in black!
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Old 01-22-2023, 04:15 AM   #18 (permalink)
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A cone only works on a torpedo.

Find the cross section at the rear axle line, and then reduce it. The amount depends on the length of the boat tail.

Then lay flexible slats faired off the original surface to find the curvature.
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Old 01-22-2023, 07:16 AM   #19 (permalink)
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Iím thing of having it more square.
Is this square enough for ya?
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Originally Posted by Weather Spotter View Post


From Weather Spotter's Full boat tail on a Matrix
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Old 01-22-2023, 02:03 PM   #20 (permalink)
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Quote:
Originally Posted by Phase View Post
Whatís the impact difference between tapering just the roofline versus tapering the sides too as a full boat tail? I noticed all the eco modders taper all directions for their boat tails. Makes sense. But what about if I just extended the roofline and didnít do too much to the sides? Think Mercedes EQXX or the light year zero. The sides of the rear only have a super small taper that isnít very noticible, the main thing is their rooflines going down and extending

How much CD drop would I see just be extending down my roofline while only lengthening out the sides like those cars?
If you wanted to have straight sides, but get the benefit of tapered sides, you could perhaps create ducts that channel air inwards to a second wall. You could then have the air exit out into your wake to make it even smaller .



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