Aerodynamics and handling

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FJCamper
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Aerodynamics and handling

Post by FJCamper » Mon Dec 31, 2007 10:59 pm

Having had the experience of racing our 1973 Herbie look-a-like IMSA Super Beetle at Daytona and Talladega back in the 70's, and our 1970 Ghia at Road Atlanta and the other major southeastern tracks, I've learned some things about Bug and Ghia aerodynamics.

Our Herbie set and still holds the U.S. land speed record for a 1600cc stock-bodied VW sedan on a closed course at Talladega. That's 120+mph on the straights and 90+mph in the banked turns, for an overall average of just under 100mph overall. It takes more power to stay up on the banking.

That might not sound like a lot, but in a Bug trying to fly off the ground, it is a handful.

No "aerodynamic aids" were allowed, but we cheated somewhat by rigging sheet metal air deflectors on the belly we said were for brake cooling, but really acted like a spoiler.

At top speed, even on the banking, Herbie wanted to lift his rear end, and the rear tires tracked a little higher on the banking than the front tires, the car actually skipping along the pavement at times like a rocks across water.

Our only compensation was holes cut at critical points inside the body and on the rear sides of the fenders to let high pressure pockets of air escape.

On other tracks, we did try a special box-shaped front spoiler under the front bumper sold via JC Whitney and shaped like a top and bottom wing connected by vanes. It did cut nose lift a little.

A short, sheet metal rear wing that clipped to the air intake vents below the rear window never seemed to have any effect at all.

The trouble was the Bug became too much of a brick above 100mph, tried to lift alternately both front and rear, and was all around too twitchy as it got light on the tires.

I know that smoothing off a Super Beetle as much as you can, mounting airdams and wings and lowering it right to the deck helps, and we see the German Look people doing wonders with this, with power (remember we were just 1600cc) to push to 120mph and maybe more, but they don't have to contend with other race traffic.

By the way, the B210 Datsuns of our day managed 130mph on the same power. What a difference a shape makes.

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DORIGTT
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Post by DORIGTT » Tue Jan 01, 2008 12:32 am

What about the Ghia?

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Steve C
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Post by Steve C » Tue Jan 01, 2008 7:08 am

Hi

Interesting read. Was that a flat screen bug or a curved screen bug?

Steve

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FJCamper
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Aero Ghias

Post by FJCamper » Tue Jan 01, 2008 12:06 pm

Our Herbie was a '73, the first of the curved windshield Supers. We bought and built it in '75, when it was still a "new" car. We wanted that curved windshield for aerodynamics.

Our oil cooler was in the nose (Porsche 911 style), under the bumper, behind the factory grille, with long half-inch inside diameter oil lines all the way to the front and back again. Just the trip cooled the oil. Running a night race at Daytona, we had to tape off part of the grille just to stay at 180 degrees.

By the way, an oil cooler in the nose (by afterthought) is not the best idea. One bump into the tail of another car that might otherwise just bent sheet metal can take out your engine.

About our Ghia, or Ghia's in general. Different story. A Ghia has approximately the same drag coefficient as a 356 Porsche, about 0.39Cd, which is in the "good" range. The "great" range is from about 0.32 and lower. By comparison, a 1965 911 is about 0.38...while my family 2001 Chrysler Concorde was 0.28!

The Ghia's drag makes it capable of high speeds with low power output, just like it's first cousin Porsche 356. 90 to 100hp will return 120+ mph on the straight and level. It's longer wheelbase and side profile, resisting sidewinds because of a better center of pressure vs center of mass than the 356 coupe, makes high speed runs more stable.

If you take a stock-body Ghia (as we did) at standard ride height but give it some power (as we did) and run it on a racetrack with reasonably long straights (as we did), right away you find that the Ghia has considerable nose lift.

Our video tapes showed our car looking like a speed boat, bow up, just before braking at the end of even a quarter-mile straight.

Our first aerodynamic mod was to mount a Puma axle and drop the nose 2" in relation to the rear ride height. We had no room to drop in the rear, what with our deep sump and the fact the Ghia has no, repeat NO, anti-squat in the rear suspension.

The rake worked, only allowing the nose to lift a little, but still keeping it down. The only effect on handling was the "raised" rear roll center made our rear swaybar unnecessary, in the way a fire is unnecessary.

When we upgraded from a 1.6 to a 2.2 engine, and a new transaxle geared for higher speeds, we cut ten seconds off our lap times at Road Atlanta, but the ability to go faster made our lack of aerodynamics apparent.

Porsche has already done all the research on what works on cars of its size and weight distribution. I decided to copy the 1972 911 RSR, the first of the "aero" racing Porsches. It came with a medium height front spoiler and an odd-looking "ducktail" on the engine lid.

I chose to copy the RSR aerodynamics because they were minimal. Porsche had discovered that the bigger and more efficient spoilers, wings, airdams, etc., are, the more drag they themselves can create, and the more power it takes to go fast.

The RSR's aerodynamic aids were painstakingly designed to create stability, not necessarily improve drag coefficient. For instance, the ducktail does not increase downforce, but helps kill lift. That's not the same thing.

I made a hybrid front airdam, not a spoiler, for our car. The 934-35 Porsches proved the benefit of the airdam. It smooths the airflow rather than trying to break it up like a spoiler. And I took the airdam as low as possible to keep air out from under the car.

The airdam is made of painted sheet metal, mounts to the bumper with L-brackets, follows the contour of the bumper, and goes full width. It has two 3" holes with aluminum flex tubing running back to the front brakes.

The ducktail was fabricated of sheet metal to be the same height and slant of the RSR piece, and permanently pop-riveted to our engine lid so it sits right behind the air intake. The RSR and our Ghia ducktail has to be lust low enough so it does not block the driver's rear view.

I painted the ducktail to match our car's silver with wide red racing stripe
scheme.

The aerodynamics work, of course. Our Ghia is more stable, and looks more purposeful. Going through tech at Savannah last year, we were right behind a newer 911, whose driver did a double-take when he realized he was looking at a Ghia.

"Don't see that every day," he said.

Bruce2
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Post by Bruce2 » Wed Jan 02, 2008 3:22 am

Great read!
Got any pics to post?

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FJCamper
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Photos

Post by FJCamper » Sun Jan 06, 2008 11:40 am

Dear Bruce2,

Image

Image

Also, to see our (pre-Aero) '70 Ghia in action, go to Youtube.com at this address:

http://www.youtube.com/watch?v=yowKsYDy4UU

...we're knocking off BMW's at the Barber track outside Birmingham AL. The one who gives us the most trouble was a set-up six-cylinder driven by a BMW club driving instructor. We nailed him anyway.

On Youtube, our search words are retroracing and ghia. They'll bring up a couple of our music videos.
Last edited by FJCamper on Fri May 21, 2010 12:34 pm, edited 1 time in total.

66vw
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Post by 66vw » Sun Jan 06, 2008 3:59 pm

FJCamper, what do you think about drilling some 2in holes in rear part of beetle front fenders to make pressure way out, same thing in the rear fenders and apron?? 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...
thanks

edit. just realized you already answered my 1st question :)

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FJCamper
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Air Holes Work

Post by FJCamper » Sun Jan 06, 2008 8:10 pm

Hey 66VW,

Yes, air holes on the backsides of fenders and across the apron over the exhaust works.

Good luck,

FJC

volkdent
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Bug Aero

Post by volkdent » Thu Jul 09, 2009 1:10 pm

Starting fresh, and for posterity, what are thoughts about aero help on bugs? Reducing lift, increasing downforce, decreasing resistance?

How well do the roof lip spoilers work?

Image

What about cutting slots in this area of the fender to release trapped air in the fender/reduce lift over the fender?

Image

Is this window spoiler effective for downforce, or just reducing lift?

Image

What about this gadget, would it work? Is it worth it for how ugly it is!?( The clear hood lip, not the front spoiler)

Image

Jason

Feel free to pull pictures into MS Paint and draw away!

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Post by volkdent » Thu Jul 09, 2009 1:23 pm

Here's how I think of bug aero:

Red: Lift /or resistance vectors

Green: Downforce

Yellow: Lift /or resistance zones

Image

And here's some sort of CAD aero view:

Image

Here's a blank one to draw on in Paint:

Image

Jason

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FJCamper
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Post by FJCamper » Thu Jul 09, 2009 3:20 pm

Image

Jason,

The aero hood acts as a deflector and keeps the really high pressure area above the windshield. The slotted fenders, air dams, and roof spoilers are all well known.

FJC

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Post by gimmesomeshelter » Thu Jul 09, 2009 3:44 pm

Hello-

I've heard of panels that are installed on the bottom of a car that inhibit air flow. Does anyone know anything about these?

Thanks,

Paul

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Bellypans

Post by FJCamper » Thu Jul 09, 2009 7:35 pm

Hi Paul,

"Bellypans" were a racing innovation to decrease drag. They came into popular use in the 1950's with the Jaguar C and D models and the Mercedes 300SL's.

Porsche and VW had natural bellypans because of the chassis platforms.

But bellypans sometimes work too well, making the bottom of the car the flat part of the "wing" that the body becomes at high speed.

What you might be thinking of is more recent bellypan designs that have stamping patterns intended to reduce lift. These have been met with mixed success, because they go back to increasing drag.

The round track guys have been bellypanning for some time now, shaping the panels to direct airflow for downforce.

FJC

kdf
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Post by kdf » Tue Jul 14, 2009 5:12 pm

The beetle drag was as low as Cd 0.32 in prototype phase when tested in windtunnel in the 1930's. When it got into production I think it was 0.36, going to 0.38 in the 60's and further increased to 0.42 in the 70's.

According to the bosch blue book the average underbody drag of cars is 5-8% of the total drag. The beetle has a flat underbody, so I'd think that the beetle is in the lower region of this. The only sense that I see with flat bottom is if you want to improve rear diffuser efficency.

Adding aero widgets to the body removes unwanted lift. But they also add drag. Many cars spend countless hours in windtunnels and computer programs getting the shape just right. You can also tracktest the aero and coastdown testing is a quite quick and efficient way of testing the aero of your car. You just need all the expensive equipment to measure the gain in performance.

Here's a couple more ideas that I don't think were mentioned in this thread:
-No bumpers, bumpers add a significant amount of drag.
-Cut down the rain guard on the windscreen pillars and rear window pillars.
-No chrome trim.
-Aerodynamic mirrors.
-Cutting the lower rear edge of the rear fenders and valence panel.
-Plastic side and rear windows mounted flush with the body.

Going wild, you could add:
-New door hinges so they don't stick outside the car.
-Front diffuser
-Air exits at the rear of the front fenders to improve front diffuser efficency. Slotted fenders won't cut it, you need to get as much air out as possible.
-Rear diffuser (very limited space at the rear though)
-Flat bottom to smoothen the underbody airflow going to the rear diffuser.
-The engine cooling and intake air needs to rerouted.
-Dive plates at the front and rear (dtm car style)
-Exhaust routing. Best would be to use the exhaust gas to energize the rear diffuser. But space in the rear is quite limited, so I would carefully measure the space needed for exhaust. The diffuser needs as much space as possible, so I think that a good option would be to have the exhaust in front of the engine and the exit in front of one rear wheel.
-Aerodynamic wheels.


Aero isn't everything. In the greatest aero era of late 80's and early 90's Porsche never had the best aero package. Still they had an unbelievable winning record because their cars were very well engineered and were designed racing friendly. It's quite silly to add a diffuser that makes rear damper or spring changes impossible. I'm very impressed with the ability to change springs or shocks on some cars in less than 10 minutes. These cars have been so well prepared that the spring or damper change doesn't even change the corner weights or alignment and the car is 100% race ready right after the change.

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Aero tricks

Post by FJCamper » Thu Jul 16, 2009 9:29 am

Hi KDF,

Yes, every aero device can add drag. That's the tradeoff -- stay on the ground but go a little slower.

Back in my SCCA showroom stock days, we would tape over seams and bumper gaps to gain speed, and saw some cars go up by 5mph on top end ... until the rules people stopped the taping.

Any reduction of area (mirrors, door handles, etc.) helps. A favorite taping trick was the drip rails over the doors back when cars had drip rails.

Street cars get some fuel efficency help from good aerodynamics, as they do not go fast enough long enough to really get the best top-end results.

FJC

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