beetle and aero

[pickstock]

Y Thought I’d put together a little article on aerodynamics of the beetle and specifically racing modifications for all sorts of racing applications. I am not an expert in this field and I doubt anyone will ever say they are, if they do its pretty easy to pick that they don’t really know what they’re talking about.

The orginal stock standard Volkswagen beetle, in its 1938 guise has a drag coefficient of 0.48, while most people here hopefully will understand what that means i will try and put in terms other people can understand (I.E me). The coefficient of drag (Cd) is based on how much energy must be used to move the object if it is out of any other forces such as road friction.


As the picture above shows the most areo dynamic 3D shape is a rain drop with a length 3.5 times its dimater (that one im pulling out of my head so i could be wrong but it seems to work)

Anyway more to Beetles
I found the following images on a website called Gerrelt’s garage and am unsure where the author got them from.



The pictures shows the comparison of the closest vehicular rain drop. The two biggest diversions from the best practise are the front window and the rear end. I’ve been talking to a few people who don’t really understand this about it, the biggest argument they had was that longer the vehicle the more air would be hitting the surface. The real problem is that the rear of the car is in a low pressure zone and this ‘pulls’ air behind it, more energy being used.

[Gerrelt]

The first picture I found somewhere on the internet, don't know where anymore. But it's a scan out of aerodynamics book.
The second picture (beetle with teardrop overlay) I made myself.
The third picture I got from somebody, but I think he found it on the internet too.

The complete story those pictures come from: Beetle Aerodynamics / Roofspoiler

The problem with the beetle is that the back is angled to steep. Compare the beetle rear end with a Toyota Prius rear end, for example.
See the article for more details.

[FJCamper]

RSR Duck Tail





RSR-style Ducktail



Air Dam (not a spoiler)





Hi Pickstock,

You don't want the teardop shape -- it is a wing. This is how land speed record cars were built back in the 1930's and they frequently tried to fly. The Bug is enough of a wing already, being flat-bottomed and hump-backed.



They tried the teardrop shape becaused it is efficient if you are a rain drop or a fish, but it creates lift.

All the aerodynamic designs we use today promote downforce which creates drag, ups the Cd, and this takes power to overcome. Porsche began using street car aerodynamics to its advantage in the early 1970's RSR's, managing to achive negative lift without adding much downforce that stole power. These were the now famous duck-tail cars. That's why we use it on our 53 car.

FJC

[yodogg]

We have been road racing our '73 Super beetle and have had good results at speeds up to about 80 mph with our own very rudimentary front spoiler and a Harold Helper copy rear spoiler. Additionally, we lowered the beetle by about 1.5" front and back and have not yet seen any issues with excessive lift. The bigger challange will be when the car will (hopefully) run Sebring this fall. Much longer straights and we will be aiming for top speeds closer to 120 mph!

Front spoiler:


rear spoiler:


At this point I would like to replace our rear spoiler with something more akin to a ducktail, as I think it will be more effective at higher speeds.

[pickstock]

The first picture I found somewhere on the internet, don't know where anymore. But it's a scan out of aerodynamics book.
The second picture (beetle with teardrop overlay) I made myself.
The third picture I got from somebody, but I think he found it on the internet too.

all credit goes to gerrelt

They tried the teardrop shape becaused it is efficient if you are a rain drop or a fish, but it creates lift.
FJC

i beg to differ in that statment. The shape is perfectly nuetral, it should if my thinking is correct create no lift or downforce.
However downforce is required on race cars hence the addition of wings and such to make the wing shape.


i will add another post about a consultation job we did on a fomula vee

[Piledriver]

Many aerodynamics books... are pure theory/aircraft centric and go to great lengths to ignore the whole "sitting 4 inches off the ground" thing.

If you had a true teardrop (in all 3 dimensions) and no flat bottom or tires, suspended in free space, your assumptions of neutrality would be true.
Simply having it in close proximity to an incompressible plane (or having any angle of attack <>zero) changes ~everything...

If it has a flat bottom, and especially if reasonably close to the incompressible plane (AKA the ground) your now imperfect teardrop becomes a very efficient lifting body in ground effect. (AKA wing)

There is also a good reason to have more cross section in the aft end, for stability.
(Probably closer to the porpoise cross section (used for transonic+ speeds on aircraft))

IIRC the most aerodynamically efficient practical passenger car design in my memory looked disturbingly ~similar to a banana on wheels.
It was higher in the middle, on the bottom, probably for some downforce,(EDIT--it was actually to maintain cross section) and had Cd about as good as you could theoretically get.
http://www.pininfarina.com/index/storia ... jj88khbd85

A Notchback has a Cd of ~.34, and even a square is ~.37... IIRC a Ghia and 914 are in the same ballpark, all very darn good for '50s/'60 production designs.
Here comes the weird...

The T3s(also probably T4s, haven't researched) Ghia and 914 all have a relatively small frontal area compared even to a Bug and have a lower Cd/frontal area and thus much lower total drag than most 2011 model vehicles, aside from the Prius and a few others.

[pickstock]

ahh yes that makes more sense with the flat bottom

[pickstock]

Now my first task as a 'consultant' was the redesigning a formula Vee/formula first car. Built for the state series that only runs on one track. The basic thing about formula Vee is that no aerodynamic aids are to be used, we took this and looked at the big problems and where we could fix them, we also looked at what other people were doing and evaluated it. The main of this exercise was to reduce as much drag as possible from the overall setup.

As you may have heard we don’t have all the good bits so we used the best things we had, our eyes, wool tuft testing (ill explain this later) and practise on the track.

Starting at the front of the car, the big overall picture of frontal area, the front beam suspension, the wheels and brake package, engine cowling and finally the rear end to get good flow separation.

I will try and get some photos to show this but I didn’t think to take any = (
[*] front beam- I’ve seen 2 ways to deal with the front beam aero, whether they work I highly doubt. One was to cover the top beam with an extension of the body work. The other was shaving the beam down to give it less frontal area. Our solution was to make covers that go over each beam roughly 100mm long and in the middle be just a gnat’s whisker wider than the beam.
We have been the stewards with this design as it was protested for being an aero devise, except that by having the same length top and bottom it offers no aero gain and we got away with it.

[*] front nose cone- the original was an older style with a wide angular nose; we changed that to a sharp point type front end, less frontal area.

[*] wheels and brakes- you’d think there arent many things that you can do with wheels and brakes but we took a little bit of inspiration from formula one cars on 2009. We had planned to make full custom new wheels with all the weight (minimum 5 kgs each) closer to the centre to reduce rotating mass, instead we went with pretty rashed up alloy wheels, we welded a ring to the outer diameter for mounting a 'hubcap' using multiple tiny counter sunk screws. The inside of the wheel had the same idea with a bit of propriety wizardry that we are still evaluating its worth.

[*] engine cowling- the original engine cowling had multiple vents and apertures to get air in for combustion and for cooling, the old cowling ran as tightly with the engine as possible. We tested by closing up all of the vents with tape and running some hard laps, intake temperature went up marginally but engine temperature stayed relatively consistent and in a range we were happy with. The new cowling followed from the top of the roll hoop to the rear of the car, incorporating a cold air intake from just below the roll hoop.


this car finished in the top 5 for the state series in 2009, with an average driver and engine/trans combo. It was then sold interstate and we havent see much of it since, however its design has flow on effects that are slowly entering the rest of the fields.

[Gerrelt]

I strongly advise you all to read this chapter of this book
The Automotive Chassis: System design, chapter AN OVERVIEW OF VEHICLE AERODYNAMICS
Skip over the formulas!


I've read it and it was an eye opener, it's really great.

[Humble]

I've taken a lot of beetle aero theory and applied it to my car where I could but it's still incomplete. I have a 73 super hillclimb & track special with a few mods for high speed stability.





I've lowered the bug quite a bit in the front, too much actually since the airdam/splitter assembly bottom out (which is why it's not on the car). I've cut windows in the front fenders to let air escape and it helps fill the void behind the fenders reducing lift. I've trimmed about 5" off the back fenders to keep them out of the air flow and to keep them from flapping about. I've also got a very large wing which makes the biggest difference of all. It doesn't generate down force so much as it scrapes the air off that follows the body, significantly reducing lift. The wing is about 6 feet wide and 12" deep and I tried to put it even with the top of the back window.

I've driven with just the airdam and no splitter or spoiler and it felt more stable at 80mph than stock. With the wing and no splitter or airdam, it's rock solid stable up to 140mph but turn in suffers above 75-ish. Without the airdam at high speed it understeers noticeably more and adds to the pucker factor for sure.

[FJCamper]

Hi Humble,

That is some wing! You are truly deeds, not words, and your experience should convince those who are still in theoretical mode. A Bug body is hard to push through the air and stay on the ground at high speed.

We still hold the Talladega Speedweeks record of a stock-body VW on a closed course with our old mid-1970's Super Bug, and it was a handful on the banks. Talladega is where records are made or broken. You may recall that was where the Porsche 917 set it's closed course speed record on 09 Aug 1975. Mark Donohue posted a 212.12mph average in a 917-30 Spyder. He ran 250mph on the straights.

We ran in the high 130's on the banking, but went on record as almost 100mph average.

You really have a beautiful and functional Bug.

FJC

[pickstock]

the wing placement looks good
size is about right (the bigger the better usually)
but the shape...... it has no fence on the rear to keep air on, the bow in it is dubious aswell
maybe somthing to look at?

[Humble]

No need for a gurney flap just yet since I don't have my front aero in place. That would just make the understeer worse at speed. The wing design was originally meant for a corvette where the air flow at the sides and back hit the wing at different speeds. I used the same design since, much like on the corvette, the down turns where the wing attack angle is more aggressive is in open air. I've been looking at several different wing designs and a properly designed wing can cost 2500-5000 so it's not a light hearted decision. My current setup will be replaced with a large dual element wing once I get the front aero worked out.

[FJCamper]

Be careful -- you don't want to ground loop at idle.

FJC

[yodogg]

When it comes to effective Beetle aero mods the Fun Cup guys have it worked out quite well: