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Old 09-02-2020, 06:30 AM   #1 (permalink)
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//// Level out car for better aero ? /////

Hi All,

I drive a 2003 Jetta wagon with TDI engine and got to thinking the other day that if I level out my car (the rear currently sits higher than the front) I actually may get better aerodynamics ( I say this because if you extend your arm out the window and make your hand into a "wing" then when it held flat it takes little effort to hold it but if you angle it up or down you have to use extra effort to hold it).

I realize it may not be much drag but thinking every bit helps... what do you think ? has anybody leveled out their car found found that it helps aero ? ( I realize lifting is generally viewed as hurting aero but in this case I think a neutral "wing" may offset any extra bits hanging in the wind ? .... I would only be increasing the front an inch or two with commercially available kits.

Thanks in advance for any and all help.

Andrew

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Old 09-02-2020, 10:58 AM   #2 (permalink)
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My thought: put $3.00 sandbags in the back until it's level and test to see if your coast down improves. Shouldn't take more than 3-4 to level it and weight isn't supposed to be known to affect steady state mileage.
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Old 09-02-2020, 11:50 AM   #3 (permalink)
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In this paper:
https://www.sae.org/publications/tec.../2016-01-1613/
Evaluation of the Aerodynamics of Drag Reduction Technologies for Light-duty Vehicles: a Comprehensive Wind Tunnel Study 2016-01-1613

They tested a range of cars model year 2013-2015 and found a 2-3% reduction in drag by lowering the nose by 20mm and a 5-6% decrease with a 40mm front and rear drop.

Based on this I would say definitely test first. Sandbags or otherwise. If you ever load the car down the rear will sag and have a reverse rake if you level it out. If that is something you do often it would be something to consider as well.

Stock image of OP's car:
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Old 09-02-2020, 04:30 PM   #4 (permalink)
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Quote:
Originally Posted by Piotrsko View Post
My thought: put $3.00 sandbags in the back until it's level and test to see if your coast down improves. Shouldn't take more than 3-4 to level it and weight isn't supposed to be known to affect steady state mileage.
I think road coast downs are a completely ineffectual way of testing drag. At minimum, first do a windows up / windows down coastdown comparison and ensure you can see a major difference in coastdown times.

But I do agree that you should so some testing - I would suggest throttle stop (constant engine power) testing.
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Old 09-02-2020, 05:08 PM   #5 (permalink)
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Quote:
Originally Posted by Piotrsko View Post
My thought: put $3.00 sandbags in the back until it's level and test to see if your coast down improves. Shouldn't take more than 3-4 to level it and weight isn't supposed to be known to affect steady state mileage.
Adding weight to a car will almost always cause it to coast further, as more kinetic energy has to be put into the car to reach the same speed.
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Old 09-03-2020, 03:44 AM   #6 (permalink)
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Thank you for starting this thread, cause I was already thinking about this. I also start to notice that when I load the car (or just wife and kids, not that they are so heavy but still ) and make a long highway trip, that I'm roughly getting the same consumption figures than with an empty (and stripped) car. Difference is often +500 lbs, but also a 2 cm drop in the front and 6 cm or more in the rear.

That is to say, the roof line with an empty car is quite horizontal, but when it's loaded it gets that typical streamlined downward slope.

It will coast better due to the extra weight, but there might also be the improved air restistance thanks to a better shape. And it's not only the air that passes over the car, but also less under the car and in the wheel arches ...
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Old 09-03-2020, 05:35 PM   #7 (permalink)
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Put bigger tires on the back. (example is lower profile on the front, stock on the back)

I'd be concerned that raising the front would increase front lift. Generally, you want minimal air intrusion at the front tires with a central jet feeding a rear diffuser.

Nose-high increases frontal area and makes the underbody a converging duct, tail-high exposes the underbody to the wake.
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Old 09-04-2020, 12:16 PM   #8 (permalink)
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body inclination

Quote:
Originally Posted by Andyinchville1 View Post
Hi All,

I drive a 2003 Jetta wagon with TDI engine and got to thinking the other day that if I level out my car (the rear currently sits higher than the front) I actually may get better aerodynamics ( I say this because if you extend your arm out the window and make your hand into a "wing" then when it held flat it takes little effort to hold it but if you angle it up or down you have to use extra effort to hold it).

I realize it may not be much drag but thinking every bit helps... what do you think ? has anybody leveled out their car found found that it helps aero ? ( I realize lifting is generally viewed as hurting aero but in this case I think a neutral "wing" may offset any extra bits hanging in the wind ? .... I would only be increasing the front an inch or two with commercially available kits.

Thanks in advance for any and all help.

Andrew
Since Hucho ran VW's wind tunnel for a decade, and has published about drag sensitivity to body inclination, I'd be comfortable presuming that this aspect of your Jetta has already been addressed during development.

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