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Old 11-21-2020, 05:03 PM   #1 (permalink)
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e-Golf rear belly pan advice

Hi,

I'm looking to finish off the mostly covered underbody of my 2019 e-Golf, as it seems like the void at the back of the car might be acting as a parachute. I found a German forum post that I'm using as inspiration (goingelectric.de/forum/viewtopic.php?t=60018&start=20), but I'm wondering if I should add some holes to let any accumulated air out as it seems the Nissan Leaf perhaps does (i.ebayimg.com/images/g/PFoAAOSwZVhWTM4N/s-l400.jpg). Another question would be if adding Airtabs (or something like them) would have any benefit, to maybe make the new belly pan act more like a diffuser.

Thanks and sorry for the perhaps naive questions.

P.S. Sorry for the lack of clickable links; first post!

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Old 11-21-2020, 05:13 PM   #2 (permalink)
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The aftermarket one looks pretty good - I assume the bottom of the rear suspension arms are flush with the new undertray when the vehicle is at normal ride height.

I wouldn't worry about any holes.

I've tested AirTabs on undertrays on two vehicles; on one they made a subjective difference (giving better stability) and on another they made no measured difference in terms of airspeed or pressures. But they're cheap enough to just tape some into place at various positions and do some testing.

If the new undertray can be curved upwards at the rear of the car, that would likely be beneficial from both lift and drag perspectives.

Addition: Actually, now I think about it, I've tested AirTabs on the undertrays of three vehicles. On one front undertray they caused a great increase in vibration, presumably as the vortices impacted the firewall!

Last edited by JulianEdgar; 11-21-2020 at 05:58 PM..
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Old 11-22-2020, 01:10 AM   #3 (permalink)
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With the pictured undertray, it might also be a good idea to:

- add flat plates to the suspension arms so that when they are in their normal position the undertray is more continuous

- put a rolled radius on the leading edge of the undertray behind the suspension arm openings so that flow re-attachment can more readily occur
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Old 11-22-2020, 05:40 PM   #4 (permalink)
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No Airtabs!

Here you have a items list with the Golf 8 new deflectors arms mounted on the Golf 7 (eGolf):

5Q0501733 - 1pcs
5Q0501734 - 1pcs
8E0825267 - 4pcs
91133501 - 4pcs
90920301 - 4pcs
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Old 11-22-2020, 05:55 PM   #5 (permalink)
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Quote:
Originally Posted by JulianEdgar View Post
If the new undertray can be curved upwards at the rear of the car, that would likely be beneficial from both lift and drag perspectives.
For this type of body, negative lift and drag? Nope.
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Old 11-22-2020, 07:14 PM   #6 (permalink)
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Quote:
Originally Posted by IRONICK View Post
For this type of body, negative lift and drag? Nope.
Do you have any evidence for that opinion? Mine would be it's more likely than not that lift and drag would be reduced with a slight diffuser.
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Old 11-23-2020, 08:07 AM   #7 (permalink)
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In this case you have to choose, either drag or negative lift.
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Old 11-23-2020, 03:54 PM   #8 (permalink)
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Quote:
Originally Posted by IRONICK View Post
In this case you have to choose, either drag or negative lift.
I am not sure why you say that. In many cars, reductions in lift are associated with reductions in drag. We're not talking about generating overall downforce but instead reducing lift.

Re the Golf's body shape, the Nissan Leaf is an example of a not dissimilar body shape that uses a standard rear diffuser.
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Old 11-23-2020, 04:37 PM   #9 (permalink)
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Yeahh, they have a 5-door body ...
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Old 11-23-2020, 06:48 PM   #10 (permalink)
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Quote:
Originally Posted by IRONICK View Post
Yeahh, they have a 5-door body ...
OK, now you've lost me entirely. I've never seen that the number of door closures has an impact on car aero. My point re the Leaf is that it's a squareback hatch, much like the Golf.

Basically car shapes with different aero behaviour can be divided into:

- notchbacks (eg sedans)
- squarebacks (most moden hatches and wagons)
- fastbacks (eg coupes).

I again suggest that if the rear undertray can be sloped upwards towards the rear, it's more likely than not to reduce drag and lift over having the rear undertray horizontal.

In fact, if you look under many of today's cars, you can see virtual diffusers often being formed by the shape of the spare wheel well, transverse rear muffler, and of course rear undertray (where fitted).

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