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Old 04-11-2022, 02:35 PM   #101 (permalink)
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Quote:
Originally Posted by aerohead View Post
https://ecomodder.com/forum/member-a...ap-fillers.jpghttps://ecomodder.com/forum/member-a...speed-aero.jpg
1) The most important part of the car is it's rear.
2) OEM longitudinal body camber, along the sides may 'hide' any rear skirt 'bulge', placing it in the 'shadow' of the body projection.
3) Even if there was an increase, the 'GOOD' the skirt would do for the tail extension would be orders of magnitude better than any frontal area penalty.
4) You'd want full rear skirts because of the onset flow issue. There are no really low-drag vehicles lacking full skirts.
5) If you had a bridge-fiducial, or perfectly-aligned laser, you could directly investigate any frontal area aggravation.
6) The boat-tail on my CRX was responsible for most of its speed at Bonneville. 'Bulging' rear skirts to get clean air onto the tail was a structural necessity.
7) Af penalty, if any, still netted an overall 26.5% drag reduction.
looks nice. thats about the same size bulge i was seeing when doing the quick cardboard test. i could avoid a bulge if i did the half skirt like light year one, but if im going to fabricate a rear wheel cover, i might as well go all out and do the full one.

the honda clarity has a mini one though. no idea how much of an impact it provides, but it kind of looks cool for a modern car

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Old 04-11-2022, 02:41 PM   #102 (permalink)
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skirt starts

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Originally Posted by freebeard View Post
Consider the modern hypercar. They all taper through the doors and then have swollen 'haunches' to clear the rear tires.

If the skirt starts at the door opening instead of the wheel well, or even has a fillet added to the door, it would smoothen the airflow even more.
On the IONIQ, the cut line for the rear door already intrudes into the wheel arch surround within a couple inches.
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Old 04-11-2022, 02:54 PM   #103 (permalink)
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bulge

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Originally Posted by Phase View Post
looks nice. thats about the same size bulge i was seeing when doing the quick cardboard test. i could avoid a bulge if i did the half skirt like light year one, but if im going to fabricate a rear wheel cover, i might as well go all out and do the full one.

the honda clarity has a mini one though. no idea how much of an impact it provides, but it kind of looks cool for a modern car
The leading edge of any bulge would be in a favorable pressure gradient, with the air 'held' against the panel by the oncoming flow.
Where the skirt started to contract span-wise, and beyond, is where we'd have to pay attention, providing for only very gradual cross-sectional area reduction, and subtle pressure rise, as with any streamlined profile; as this flow is otherwise, in an adverse pressure gradient, subject to separation.
I strongly advise using the full skirt.
If Lightyear One wanted lower drag, they'd need to alter their rear architecture.
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Old 04-11-2022, 04:45 PM   #104 (permalink)
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1978, D-B, M-B C-111 III elongation vs Cd

Please scroll down to # 15 (permalink ) to see delta-Cd as a function of rear elongation. Boat-tailing is the only modification being accomplished.
https://ecomodder.com/forum/showthre...c-27257-2.html
The 'short-tail', @ Cd 0.237 is similar to the IONIQ.
The 'mid-tail', @ Cd 0.195 is close to your target.
Looking from above, in plan-view, the tail introduces the side boat-tailing you need to complement changes to the roofline and diffuser area.
Here's another source, including images from above the C-111
https://www.car-revs-daily.com/2014/...benz-c111-iii/
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Last edited by aerohead; 04-11-2022 at 05:08 PM.. Reason: add photo data
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Old 04-11-2022, 05:29 PM   #105 (permalink)
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Quote:
Originally Posted by aerohead View Post
Please scroll down to # 15 (permalink ) to see delta-Cd as a function of rear elongation. Boat-tailing is the only modification being accomplished.
https://ecomodder.com/forum/showthre...c-27257-2.html
The 'short-tail', @ Cd 0.237 is similar to the IONIQ.
The 'mid-tail', @ Cd 0.195 is close to your target.
Looking from above, in plan-view, the tail introduces the side boat-tailing you need to complement changes to the roofline and diffuser area.
Here's another source, including images from above the C-111
https://www.car-revs-daily.com/2014/...benz-c111-iii/
ill for sure work on the boat tailing with the mid length. only difficult thing is going to be the side with the tail lights!

also since youre good with math, can you tell me what my highway mpg would increase to if i lowered the cd from 0.24 down to say just 0.20 without changing the engine or weight or anything system wise?
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Old 04-11-2022, 05:30 PM   #106 (permalink)
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Quote:
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Yeah driving by a friend on the ground is pretty hard to do in downtown Portland. I’ll have to do some gopro angle tests with the tufts. I’m envious of everyone having access to empty secluded roads for testing
20 years ago when I lived in Portland I used to do a lot of nighttime automotive "testing" at the Swan Island industrial area. Nice smooth multi-lane roads that you'll pretty much have to yourself on nights & weekends.

For zero dollars you can do coast down testing starting from a stop on the hill on going street (looking up hill):


You can then coast down onto Channel & around the slight bend onto Lagoon (but you have a stoplight that might mess up a run) or you can turn a little bit more sharply onto basin and coast unimpeded:



Start from the same spot on the hill each time (e.g. a certain lamppost) and see how far you coast (either to a stop or until you drop below a certain speed).
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Old 04-11-2022, 06:18 PM   #107 (permalink)
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ill for sure work on the boat tailing with the mid length. only difficult thing is going to be the side with the tail lights!
You're not the first to face this. It's why I suggested boxing below the lights and adding a wickerbill to teh spoiler.

You could ask aerohead what he thinks for me.
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Old 04-11-2022, 08:00 PM   #108 (permalink)
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Quote:
Originally Posted by Drifter View Post
20 years ago when I lived in Portland I used to do a lot of nighttime automotive "testing" at the Swan Island industrial area. Nice smooth multi-lane roads that you'll pretty much have to yourself on nights & weekends.

For zero dollars you can do coast down testing starting from a stop on the hill on going street (looking up hill):


You can then coast down onto Channel & around the slight bend onto Lagoon (but you have a stoplight that might mess up a run) or you can turn a little bit more sharply onto basin and coast unimpeded:



Start from the same spot on the hill each time (e.g. a certain lamppost) and see how far you coast (either to a stop or until you drop below a certain speed).
thats a good idea. whats the speed limit? i know aero matters more at higher speeds. going slower probably wont show much of a diff like it does with low rolling resistance tires tested
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Old 04-11-2022, 08:19 PM   #109 (permalink)
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From Google streetview it looks like the posted limit on the hill is 40mph. The coefficient of drag should be nearly identical at 40mph and 80mph, but you're right that it might be more difficult to suss out the impact of wind vs rolling resistance from coast down testing.

But with care, you should be able to make crude improvements which would allow you to more productively focus subsequent high speed testing later.
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Old 04-12-2022, 10:22 AM   #110 (permalink)
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Total drag should increase as a result of speed increases. If it didn't, everybody here would drive like everyone else@100 mph and there would be no reason to add aero

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