Ideal car rear roofline angle and shape for low drag?

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Fluido
Fluido
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Joined: 25 Mar 2022, 17:17

Ideal car rear roofline angle and shape for low drag?

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What is ideal car rear roofline(rear window and trunk) angle and shape(straight line, curve ..etc) for lower drag? Keep in mind if you increase angle, you reduce butt area / wake, but induce low pressure at rear window/trunk , if you decrease angle you increase butt/wake but increase pressure at rear window/trunk.

What car aerodynamics theory says about this?

Greg Locock
Greg Locock
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Joined: 30 Jun 2012, 00:48

Re: Ideal car rear roofline angle and shape for low drag?

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I suggest you get hold of a copy of Hucho "Aerodynamics of Road Vehicles" or start with https://en.wikipedia.org/wiki/Automobil ... oefficient

MadMax
MadMax
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Joined: 22 Oct 2022, 03:23

Re: Ideal car rear roofline angle and shape for low drag?

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Isn't this a classic "how long is a piece of string?" question? :shock:

feni_remmen
feni_remmen
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Joined: 26 Mar 2009, 15:43

Re: Ideal car rear roofline angle and shape for low drag?

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The answer is 11

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Stu
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Joined: 02 Nov 2019, 10:05
Location: Norfolk, UK

Re: Ideal car rear roofline angle and shape for low drag?

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A lot depends on what is ahead of and behind (and above & below) in terms of required airflow.

Race-cars tend to look like they do because of regulations, road cars tend to look like they do because of comfort/carrying requirements.
Perspective - Understanding that sometimes the truths we cling to depend greatly on our own point of view.

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Andres125sx
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Location: Madrid, Spain

Re: Ideal car rear roofline angle and shape for low drag?

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Fluido wrote:
11 Nov 2022, 16:30
What is ideal car rear roofline(rear window and trunk) angle and shape(straight line, curve ..etc) for lower drag? Keep in mind if you increase angle, you reduce butt area / wake, but induce low pressure at rear window/trunk , if you decrease angle you increase butt/wake but increase pressure at rear window/trunk.

What car aerodynamics theory says about this?
I think this is the only car designed to lower drag as much as possible even if it ruins practicability, comfort or aesthetics

Image
Image
Image

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Stu
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Joined: 02 Nov 2019, 10:05
Location: Norfolk, UK

Re: Ideal car rear roofline angle and shape for low drag?

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Looks A LOT like something I drew up when I was at Uni 30 years ago!!
Mine had three seats and was intended to be powered by a flat twin BMW motorcycle engine.
Perspective - Understanding that sometimes the truths we cling to depend greatly on our own point of view.

Rodak
Rodak
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Joined: 04 Oct 2017, 03:02

Re: Ideal car rear roofline angle and shape for low drag?

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Here in the US a three wheeled vehicle is classed as a motorcycle in most states.......

Greg Locock
Greg Locock
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Joined: 30 Jun 2012, 00:48

Re: Ideal car rear roofline angle and shape for low drag?

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That's the Aptera, their early estimates of Cd (0.06) were hopeless. The interference drag between the front wheel pods , the suspension arms, and the body were totally ignored. Judging by the wild claims, they'd assumed laminar to 60%

Current estimate is 0.15, which is almost down to solar car numbers, and better than GM Impact, so that's a pretty incredible outcome given that it has physical rear view mirrors.

Neilwade
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Joined: 22 Jul 2024, 01:48

Re: Ideal car rear roofline angle and shape for low drag?

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Fluido wrote:
11 Nov 2022, 16:30
What is ideal car rear roofline(rear window and trunk) angle and shape(straight line, curve ..etc) for lower drag? Keep in mind if you increase angle, you reduce butt area / wake, but induce low pressure at rear window/trunk , if you decrease angle you increase butt/wake but increase pressure at rear window/trunk.

What car aerodynamics theory says about this?
For optimal aerodynamic efficiency, the ideal rear roofline angle for a car is typically around 15 to 20 degrees. This angle strikes a balance between reducing the wake area and minimizing low-pressure regions at the rear window and trunk. A smooth, gently curved shape rather than a straight line helps maintain laminar flow and reduces drag according to automotive aerodynamics theory.