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Old 07-04-2022, 12:51 AM   #1 (permalink)
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Effects of rake on CD

Has anyone ever experimented with adjusting their rake to see how it affects MPG. My car naturally has .5 inches of rake (measured at the weld seams at the bottom) and I'm curious if i can squeeze some efficiency out of it.

Insights have a flat floor and can more effectively do these tests as apples to apples. Building my flat floor soon and i'm curious if by adding a touch of rake, i can actually help reduce the negative pressure behind the car

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Old 07-04-2022, 01:07 AM   #2 (permalink)
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I did CFD 3 runs on the A2 body. these are degree angle.
0,0 -- >0,2762
0,5 -->0,2792
1,0 -->0,2904

So in that car level was better, differences in drag were big so should be able to notice the difference. If car is not high, lower it with racthes on springs etc to find the sweet spot front or back. It also changes roof and diffusor angles so results are not same for each car.
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Old 07-04-2022, 09:06 AM   #3 (permalink)
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I wonder if a roof form was just shy of having air attachment if raising the rear or lowering the front would cause improved attachment and lower the Cd.

Rake angle affects the top surface as well as the undercarriage plane is all I'm saying.

If the Audi A2 has complete roof attachment then its results may be for that car only?

Complete roof attachment is sadly rare, just say'n.

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Old 07-04-2022, 01:06 PM   #4 (permalink)
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Quote:
My car naturally has .5 inches of rake
Oh, 0.5. I thought it was excessive at first.

0.5 may be within the margin for error. Was it on a build plate when you measured it? Laden or unladen?
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Old 07-05-2022, 12:46 PM   #5 (permalink)
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just shy of attachment

The air suspension of the Ford Probe-IV concept was a critical element in its low drag. Nose down tail up.
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Old 07-07-2022, 03:37 PM   #6 (permalink)
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roof attachment

Quote:
Originally Posted by kach22i View Post
I wonder if a roof form was just shy of having air attachment if raising the rear or lowering the front would cause improved attachment and lower the Cd.

Rake angle affects the top surface as well as the undercarriage plane is all I'm saying.

If the Audi A2 has complete roof attachment then its results may be for that car only?

Complete roof attachment is sadly rare, just say'n.

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It's hard to tell. The camera vantage point is 'low' and 'forwards' alongside the body. One can't actually observe the lowest streamline smoke filament in relation to the spoiler.
I have a blueprint, and the roofline IS a bit aggressive in its contour.
The Euro-spec A2 was Cd 0.25 according to Robert Cumberford, designer, and contributor to AUTOMOBILE Magazine.
Smoke on BIONIC Boxfish looks like it's trying to take the paint off the car. It happens to fit the AST, so that would not be surprising.
If A2 was a bit too steep, a weak counter-rotating vortex pair could be inducing a touch of downwash which would otherwise appear like 'attached' flow.
The EPA says ' Local declination should be within 3-5 degrees of streamline locally... no slant perturbation beyond 4-degrees ... smooth, continuous.'
A2 probably satisfies that.

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