Aerodynamics and handling

For road racing, autocrossing, or just taking that curve in style. Oh yea, and stopping!
kdf
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Post by kdf » Thu Jul 16, 2009 10:16 am

You're right about taping the seams. Many times it's not only done because of aerodynamic drag, but also to have something more to secure the body panels in case fasteners fail. I'm impressed with the gains you saw.


I had a couple pages scanned from a french book on vw beetle history. It had a very interesting section about testing the prototypes where aerodynamic testing was included.

The model with the Cd 0.32 didn't have any mirrors, drip rails, bumpers or mudflaps. The rear wheels also had wheel covers.


I'm really bad with numbers, and now when thinking about it, Cd 0.44 for the newest aircooled beetles isn't right, I'm sure it was as bad as 0.48. :roll:

It's sad that I cant find the pages at the moment to see if the book had pictures of the wind tunnel prototypes. I'm also not 100% sure about the drag numbers, please correct me if I'm wrong.

John_S
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Post by John_S » Sat Sep 19, 2009 4:18 pm

i like all of these body mods but i want to keep it looks as 'cal look ' as possible

64 bug (king and link ) dropped spindles disks up front running 145's (yes i know smart car tyres , probably too small )
back end is dropped so floor to wing in about 25 3/4 inch (spline and a bit with adjustable spring plates + IRS )
oh engine is 2276 turbo with megasquirt (225bhp :-) 10psi )
my problem is when i approach 100mph it just gets very light and twitchy.
what alignment do u all run. as in toe in toe out back end square ?
i have just rebuilt the backend with polyeurothane bushes and i havent got it 4-wheel aligned so wondered what u all have there cars set to ?
can u all help at all

thanks

John

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FJCamper
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Stability

Post by FJCamper » Sat Sep 19, 2009 8:36 pm

Image

Dear John,

The Bug body was made for top speeds of about 75mph, and becomes a wing after that. Lots of guys push theirs up to 100mph, but (suspension setup being equal) everybody has the same problem.

The center of gravity (just behind the driver's seat) and the center of pressure (just ahead of the driver's seat) are not together, and the Bug tries to rotate around its center of gravity when buffeted by side winds.

Without aerodynamic aids, keep the car low and level, and avoid 100mph.

FJC

John_S
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Post by John_S » Sun Sep 20, 2009 11:37 am

i've had the car 4 - wheel alignend and i had both rear wheels toeing waaayyyy out , where no just about parrallel after elongating the spring plate holes.. had it round to the oil light and felt ok ... so i'll see , thanks for the info ...

Gerrelt
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Re:

Post by Gerrelt » Tue Apr 27, 2010 1:33 pm

This is a great thread, a very good read! :)

I like that center of gravity vs center of pressure drawing.
66vw wrote: also what do you think about blocking rain gutter - part of it wchich is next to windshield to make it smooth and improve Cod? saw it in old 911, talked to owner and he said it helps a little at high speeds (like 200+ km/h), but biggest advantage is wind noise reduction. just an ideas...
I made rain gutter fillers for my 1973 curved windshield super beetle. And the main reason was noise reduction.

Image

Image

It's not hard to make, it just takes time. See this part of my homepage on how I did it: Link to raingutter fillers

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FJCamper
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Re: Aerodynamics and handling

Post by FJCamper » Thu Apr 29, 2010 9:42 pm

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Hi Gerrelt,

We also use drip rail air deflectors, and ours are hand made. Versions of these are sold for 911's, which was our inspiration.

And they are very effective.

FJC

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petew
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Re: Aerodynamics and handling

Post by petew » Sat May 01, 2010 2:49 am

http://www.oceanstreetvideo.com/download1.html

check out the clip of Chris Bubetz doing a flat 9sec time in his pro-turbo car. Listen to the narration. The aerodynamics no doubt have a lot to do with the way the car behaves. Scary stuff.

Gerrelt
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Re: Aerodynamics and handling

Post by Gerrelt » Sat May 01, 2010 10:34 am

I just found (on the internet) this windtunnel test-document of a kamei front spoiler:

Image

The scan isn't too great, but the results are:

decrease in aerodynamic drag coefficient: minus 4 %
theoretically possible increase in top speed: plus 2.3 %
reduction of fuel consumption ... combined driving conditions: minus 1.5 %
decrease in lift coefficient on front axle : minus 64 %

The graph shows that the front axle lift at 125 km/h is reduced from 2 centimeters to aprox. 0.5 centimeters!

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petew
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Re: Aerodynamics and handling

Post by petew » Sat May 01, 2010 5:37 pm

course the other part of the problem is the roof of the car. Look at what VWoG did with the body kit for the NBeetle and you can see them trying to address it.

Image

drmiller101
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Re: Aerodynamics and handling

Post by drmiller101 » Sun May 23, 2010 12:43 pm

Frontal area is more important then Coefficient of Drag. Anything you can do to narrow or shorten (height) the car will make a substantial difference.

And all else being equal, width at the back of the car takes more horsepower then width at the front of the car.

Narrower fenders. cheating rear sheet metal smaller. Shortening the car.

One thing a buddy does with his 914 is not run a windshield or rear window in club racing. The other guys laugh at him, but he is there to win, not get style points, and he says 6 mph on the ends of the straights.

A HUGE problem for bugs is the roof. If you look at the curve of the roof, and the curve of an airplane wing, they are identical. there is HUGE life on the center of the roof of a bug.

If you want to kill that lift, go read about frost on top of airplane wings. If you get just a little turbulence, you can break the upwards suction.

Properly applied stickers could do it. The gaps in body lines for a a sunroof would help.

blacktruck
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Re: Aerodynamics and handling

Post by blacktruck » Thu May 27, 2010 10:53 am

drmiller101 wrote:
If you want to kill that lift, go read about frost on top of airplane wings. If you get just a little turbulence, you can break the upwards suction.

Properly applied stickers could do it. The gaps in body lines for a a sunroof would help.
like those little sharkfin lookin things?.....really texturized rhinoliner areas?

Gerrelt
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Re: Aerodynamics and handling

Post by Gerrelt » Fri May 28, 2010 12:17 pm

blacktruck wrote: like those little sharkfin lookin things?
I think these things are called vortex generators?

On autospeed.com they did some testing. Their end conclusion (in part 4) was that it was only effective on the underside of the car.

See:

Blowing the vortex, part 1

Blowing the vortex, part 2

Blowing the vortex, part 3

Blowing the vortex, part 4

drmiller101
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Re: Aerodynamics and handling

Post by drmiller101 » Fri May 28, 2010 11:01 pm

vortex generators keep energized air pulling on the wing surface.

We want energy air pulling the car DOWN, so we put them on the bottom of the car.

On top of the car, we want things to screw up the lifting surface. Like rino liner or something with texture. Look at the ceiling in your living room. That ceiling texture would be good.

blacktruck
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Re: Aerodynamics and handling

Post by blacktruck » Thu Jun 03, 2010 9:03 am

so the hail damage on my bugs roof might not be such a bad thing after all....

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petew
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Re: Aerodynamics and handling

Post by petew » Tue Jun 15, 2010 4:51 am

you should read the background behind golf ball dimples. they have them for those sorts of reasons.

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