I am working on my grassroots motorsport challenge car and want to take the windshield out of my car and want to do the mini windscreen. How do you make it???

you can just buy it from most vendors of fibreglass bodyparts

grassroots has a VERY small budget for building cars....you must make or adapt junk yard parts....$2006 total!!

2006$ HOLY COW!!!

If you can't use fiberglass, then you could use lexan and a heat gun to shape it. Otherwise you could make a fiberglass form out of styrofoam.

Why remove the windshield? My friends GT can run either way, and he claims it makes no difference. Most of the guys he runs against cannot remove the windshield, and would bitterly complain of an unfair advantage if he did. So he doesn't and still beats them. Saves a lot of cost and effort.

Dave is right. To an extent. You are removing the windshield ro reduce the drag coefficient. GT cars have the aerodynamic equivalent of a trash can hanging from each quarter panel.

A GT car can fit really aggressive tires, but it has to deal with a worse drag number. Meanwhile, the drag coefficient won't really come into play unless you plan on getting to a pretty aggressive speed.

Base your decision on track layout.
Big track that favors top speed (roundy round and such)=get rid of windshield.
Small track that favors light, balanced cars, and handling= don't worry about it too much.

QUOTE (alpha434 @ Feb 16 2006, 07:00 PM)

GT cars have the aerodynamic equivalent of a trash can hanging from each quarter panel.

dude, get a clue.
where do you pull this dumb stuff from ???

the 914 has a slightly better CD than the 911 turbo ...

i told you that before ...
Andy

homemade

Thomas..

I love that car....

That sucker is CLEAN..!!!

And fuching loud.

Twystd1

QUOTE (SirAndy @ Feb 16 2006, 07:14 PM)

QUOTE (alpha434 @ Feb 16 2006, 07:00 PM) GT cars have the aerodynamic equivalent of a trash can hanging from each quarter panel. dude, get a clue. where do you pull this dumb stuff from ??? the 914 has a slightly better CD than the 911 turbo ... i told you that before ... Andy

You said better than a 935 by .02 points. Where did that come from? I'm not advising him on the drag difference between a 911 and a 914

Look at a slab-side 914 and then look at a GT. Which between these two have a better CD? It's nice to go by numbers, but sometimes you don't have a wind tunnel handy.

Yeah?

914= .034
935= .036

I think this is what Andy told me before. I don't know the number difference between a 914 with fender flares as opposed to one without. Nor do I know where to find that info.

So, a custom windscreen.

Either you make a foam template, or plug..and you can fiberglass that...

Or,,,cut the a-pill till its 6 inches tall and either cut a stock windshield, or make one from lexan....

Or, make a plug or a mold from plywood...a form to bend some lexan on.
You can use a heat gun, but 1/4 inch lexan takes a long time to heat up enough....

So, go find a cool car guy who works at a pizza place, or a restaraunt and has one of those big ovens...

Get all ready wit your form, get the lexan cut to the proper shape....
Do a couple of test pieces and use a timer....

You have to get it just right..hot enough to bend, but still stiff enough to lift out of the oven..

If you make a mold from metal or aluminum..you can lets it slump into the mold..so you can look through the window on the oven..when it finally slumps...your done, take it out.

Good Luck.

Rich

QUOTE (Twystd1 @ Feb 16 2006, 08:42 PM)

Thomas.. I love that car.... That sucker is CLEAN..!!! And fuching loud. Twystd1

mah ears be hurting thinking about it....

WOW
what a great response, it would have a taken a week for this much info on any other forum. The $2006 challenge is an autocross, drag race and councours for creativity and ingenuity. I want to remove the windshield for the fun of it. No class rules or anything to worry about. It's a budget build up competition.
I am thinking of doing a blog to keep a diary of my adventures.
Just joined and still looking into it.

thanks again.

You get free weblog space with membership.

I probably should add that I have 7 914's so don't think I am chopping up some possible restoration. I got a few given to me, a race car and my wife's, and some for later engine swaps. The challenge car is a turbo 2.0 with 1.7 heads. Oh yeah let me know if you need parts.

QUOTE (alpha434 @ Feb 16 2006, 07:42 PM)

Look at a slab-side 914 and then look at a GT. Which between these two have a better CD? It's nice to go by numbers, but sometimes you don't have a wind tunnel handy.

so, without the numbers or a windtunnel handy, you just make up your own "truth" and tell a newbie he's going to drive a square brick around the track ...

ever thought that the new guy *might* just think you know what you're talking about and takes you seriously?
Andy

1969 Porsche 914/6 - Technical specifications
This record added before December 1995.
Last modified 2005-11-10.
bodywork
Type 2S FHC
Designer
Number of doors 2
dimensions & weights
Wheelbase 2450 mm 96.5 in
Track front 1343 mm 52.9 in
rear 1383 mm 54.4 in
Length 3985 mm 156.9 in
Width 1650 mm 65 in
Height 1220 mm 48 in
Length:wheelbase ratio 1.63
Ground clearance 128 mm 5 in
Kerb weight 940 kg 2072 lb
Weight distribution
(Front)
Fuel capacity 61.8
litres 13.6
UK Gal 16.3
US Gal
aerodynamics
Drag coefficient 0.450
Frontal Area
Cx
engine
Code
Manufacturer Porsche
Type F-6
Dry sumped
SOHC
12 valves total
2 valves per cylinder
Main bearings 8
Construction light alloy block & head
Bore × stroke 80.00mm × 66.00mm
3.15 in × 2.6 in
Bore/Stroke ratio 1.21
Displacement 1991 cc
(121.498 cu in)
Unitary capacity 331.83 cc/cylinder
Compression ratio 8.60:1
Fuel system 2 We carbs
Aspiration Normal
Compressor type N/A
Intercooler None
Catalytic Converter N
Max. output
(DIN) 111.5 PS (110.0 bhp) (82 kW)
@5800 rpm
Max. torque
(DIN) 157.0 Nm (116 lbft) (16 kgm)
@4200 rpm
Maximum rpm 6500 rpm
Coolant Air
Specific output 55.2 bhp/litre
0.91 bhp/cu in
Specific torque 78.85 Nm/litre

that goes against everything ive heard....

QUOTE

Drag coefficient 0.450

i always heard in the .3's....

QUOTE (Aaron Cox @ Feb 16 2006, 10:29 PM)

i always heard in the .3's....

and you are correct, sir ...


From "UP Fixen", Vol.5, Page 314:

1) Top on, Windows up & Headlights shut = CD 0.363

2) Headlights up = CD 0.380

3) Headlights up, Windows open = CD 0.381

4) same as above but Roof off = CD 0.484


Andy

Now that's where I'm confused.

Is that .34 WITH or WITHOUT the flares?

QUOTE (alpha434 @ Feb 16 2006, 11:53 PM)

Now that's where I'm confused. Is that .34 WITH or WITHOUT the flares?

without.....

My numbers were WITH.

That's what I've been saying.

Adding fender flares and increasing frontal profile would very rarely reduce drag.

I hope that I can shine some light on the aerodynamics as far as the CD is concerned between the Flaired wheel wells and the non flaired wheel wells....

(Given that both are painted steel with roughly equivilent surface roughnesses):

The flaired wheels will created a larger surface area perpendicular to the air stream and therefore create a larger wake of turbulence behind the car as the airstreams seperate from the car, thus creating drag. This is evident when one stands directly infront of the car and looks at it head on. If the car in this case were 2-dimensional one would say the the car with the wheel flairs takes up more space than the one without, thus it displaces more air (read: deals with more friction) as a result.

Also the wheel wells add surface area to the sides of the car. As air moves past the car one can assume that the air directly touching the surface of the car is stopped in its tracks and as you move away from the car the speed of the air gradually increases to the overall speed of the car (known as the freestream velocity). This is why the wind is always much faster at about 10 to 15 feet above the ground. Now, keep in mind that this all happens in the course of a few micro-meters on the car. None the less this creates a certain friction which the car must fight against - the fast the car goes the more friction this creates. An example of this would be like trying to run in water. If one moves slowly it is pretty easy, but if one tries to go faster it gets much, much harder.

Now, as far as the flairs go - from an aerodynamic standpoint the stock fenders are more desireable as they end up creating less drag. However, the boost in handling and traction for very high accelerations provided by the extra meat under the wheel wells well outweighs the aerodynamic cost of the flairs - especially if one applies other aerodynamic techniques to the front of the car (which is where most of the drag is coming from anyways).

Hope this wasn't too long ,

Peace and Hope,

Steven

QUOTE (alpha434 @ Feb 16 2006, 10:53 PM)

Now that's where I'm confused. Is that .34 WITH or WITHOUT the flares?

no, with the flares it's like strapping two trash cans onto your quarter panels ...

btw. your tech-info listing was for a '69 914-6.
and the 914-6 did *not* have flares. i'd be interested to know what source that info came from ...

so, you really think flares would add more than 0.1 to the CD?
first of all, have you *ever* looked at a GT from the front?
i don't think so ...

the front is just as wide as the rear, so the rear flares don't add *anything* to the frontal area (which is how the CD is measured)

as you can see, the headlights add a whopping 0.017 to the CD.
now, correct me if i'm wrong, but looking at the headlights up compared to the curving of the flares, i'd say the headlights have *at least* the same surface area (frontal, remember?).

but even if we assume for a second that front flares are as "bad" as the open headlights, that would just add 0.017 to the base CD.

so, worst case, the GT would have a CD of 0.380. if that ...

unless of course, you were serious about those trash-cans ...
Andy

Andy - you are correct in saying that the frontal area of the car plays a big point in determining its CD, however....

the curvature of the body in the front plays a very large roll as well. Take for example a Jeep Cherokee vs. a Ferrari Testerosa. The Jeep has a very broad and flat frontal area wich creates a VERY large "stagnation point," meaning that one might as well be trying to run down a windy ravine holding a piece of plywood directly infront of them and perpendicular to the wind. The Testerosa on the other hand takes that plywood and tilts it back so that the airstream from the wind is not directly stopped on account of the plywood, but rather only slowed down. One can test this out on their own by buying a piece of plywood and standing with it in the wind at different orientations (BTW: even if the tilted piece of plywood is longer to make up for the decreased frontal surface area it should still create less drag than the directly perpendicualr piece). This is why the pop-up headlights on the 914 could potentially create more drag than the fender-flairs

Peace,

Steven

i think an open cockpit would creat more drag than roof on. hence reduced windscreen without addressing air flow thru the cockpit would not be the way to go.

who has the light weight car that just did this? just painted it orange.

With no back window, the vortex shit is all different... the drag is reduced when you chop the front down, and toneau it.. big time...

If you don't believe it, just remove the glass after a top speed run, and hit the same road for another. Easy test.

Wear goggles, oh, and don't get caught.



M

QUOTE (skeates @ Feb 16 2006, 11:15 PM)

Andy - you are correct in saying that the frontal area of the car plays a big point in determining its CD, however....      the curvature of the body in the front plays a very large roll as well. Take for example a Jeep Cherokee vs. a Ferrari Testerosa. The Jeep has a very broad and flat frontal area wich creates a VERY large "stagnation point," meaning that one might as well be trying to run down a windy ravine holding a piece of plywood directly infront of them and perpendicular to the wind. The Testerosa on the other hand takes that plywood and tilts it back so that the airstream from the wind is not directly stopped on account of the plywood, but rather only slowed down. One can test this out on their own by buying a piece of plywood and standing with it in the wind at different orientations (BTW: even if the tilted piece of plywood is longer to make up for the decreased frontal surface area it should still create less drag than the directly perpendicualr piece). This is why the pop-up headlights on the 914 could potentially create more drag than the fender-flairs Peace, Steven

Actually, the frontal area of the vehicle doesn't factor into the CD, Though it has everything to do with the overall drag of the vehicle. Steven's second paragraph is a good description of what the CD means.

The CD is a measure of the aerodynamic eficiency (or lack thereof) of a given shape. The CD of a car (or any other shape, for that matter) doesn't change with the size of the object. The CD of the 1973 914 in your driveway is about the same as the CD of the 1/43 914 on your desk. A lower number indicates higher aerodynamic eficiency. Frontal area is factored into the equation seperately. The basic eqation for aerodynamic drag, ignoring compressability, Reynolds effects, etc. is:

Air density x CD x Forntal area x velocity, squared = overall drag

So to reduce drag you can reduce air density (move to the mountains), lower the CD of the shape ("streamlining"), reduce frontal area (sawzall), or slow down (blasphemy...).

Since no-one on this list ever wants to slow down we can eliminate that one...

So if you assume a given speed and air density (it can be tough to change), you are left with two factors that can affect the overall drag of the vehicle. It's shape and it's size.

Though the CD change with the shape of a 914 (.363 all closed up vs. .484 with everything hangin' out. A difference of .121) is pretty significant as far a CD figure goes, if you look at the equation you'll see that even a change that great will not have a huge effect on the overall drag. Lowering the CD usually means a lot of work for very small improvements. As an example, a clear "fastback" rear window over a 914's rear decklid would probably net a reduction of about .015 or so to the CD (that's just a guess. if anyone has seen actual numbers for such a thing let me know!).

That leaves frontal area. For any given change this will have the greatest effect on the overall drag. Besides, its much easier to reduce frontal area than CD! So while removing the windscreen and rear window would increase the CD by as much as a point, the loss of around 5 - 7 square feet of frontal area would mean that the overall drag had been reduced. In the same way the GT flares will add very slightly to the CD of the car, but their major drag penalty comes from an extra two or so square feet of frontal area.

Sometimes it works the other way as well. When Prosche was running the original 908 coupes they installed a raised rooftop on one car to give extra helmet clearance for a tall driver. It turned out that the (very) slight increase in frontal area resulted in a lower CD and a very, very slight decrease in drag.

Sorry to geek out on you guys, didn't mean to go on for pages. Aero is one of my favorite subjects. It's easy for me to ramble on and on about such things....

Sounds like you guys work for NASA or something

I bet that a flared car would only add .03 to the CD number. But who the hell cares? To get back on topic this car will not be going over 90 mph anyway.

I thought the cars for the GR Challage had to be street legal. And to be street legal you need a windshield.
I would hate to see you win this thing than have the #2 finisher protest your car.

Keep the w/s. Making a cowling is a big PITA (ask me how I know) and won't get you anything for your effort.

Paul

Thanks Mike,

Yea, I got my CD mixed up with the equation for overall drag .
Good description by the way.

Peace and Hope,

Steven

QUOTE (skeates @ Feb 16 2006, 11:15 PM)

This is why the pop-up headlights on the 914 could potentially create more drag than the fender-flairs

I enjoyed the posts (such as Mike's) with good science behind them. To change the topic though, I think a GRC car like the one in question will better benefit from reduced weight than it will from better aerodynamics. By the way, losing the windshield is a great way to dump some weight.
-Ben M.

Here is how you make a cut down windshield:
The best way to cut tempered or laminated glass is to tape off both sides of a cut and sandblast through it. Just leave the old windshield in place, cut the pillars and sandblast a slit from the windshield.
Mark

if anyone would be willing to part with thier cut down windshield, I can have them replicated pretty cheap. I used to be the lead designer for the largest aquarium manufactor in the world. They get lexan very cheap and have a huge oven (forklift can drive in). I can easily do this for you guys!

pm me if interested...

Has anyone here actually been in an automotive wind tunnel during a test?
Mark

QUOTE (marks914 @ Feb 17 2006, 06:48 PM)

Has anyone here actually been in an automotive wind tunnel during a test? Mark

No, but I stayed at a Holiday Inn Express last night.

QUOTE (marks914 @ Feb 17 2006, 06:48 PM)

Has anyone here actually been in an automotive wind tunnel during a test?

have *you* actually been on the moon?

Andy

No, but I've been in a aeronautical wind tunnel.....not running at the time, thank God.

I'm with Ben, loose the weight up high. Piss on the aero.

QUOTE (SirAndy @ Feb 17 2006, 07:21 PM)

QUOTE (marks914 @ Feb 17 2006, 06:48 PM) Has anyone here actually been in an automotive wind tunnel during a test? have *you* actually been on the moon? Andy

Never been to the moon, but have spent plenty of time in wind tunnels. 3/8 scale and full size.
Fun

Mark

QUOTE (marks914 @ Feb 17 2006, 06:48 PM)

Has anyone here actually been in an automotive wind tunnel during a test? Mark

No, but when I was a kid me and my friend Steve used to stick our hands out the car window and 'fly' them up and down while doing an impersonation of Mario from that 1980's Ford Thunderbird TV commercial... "Oym Mario Andretti, and theese is Aero-doynameeks."

-Ben M.

a guy ran a 914 with 2.3 turbo ford motor last year and it didn't have a windshield. The rules didn't say completely street legal, it has many provisions for open cockpit cars. soo..... I want to drop the windshield and back glass for weight reduction, I don't expect more than 140hp out of the turbo. I have gutted the rest of the car so hp to weight is my angle. I have an old imsa gt flair kit ( fiberglass) that allows me to run 12's in the back and 9-10's up front. this thing should be quite wild when done. I am wanting to fiberglass a hood without headlights, and already have a fiberglass trunk. any suggestions on the quick cheap way to do the hood would be appreciated.

QUOTE (usa4cheer @ Feb 18 2006, 12:43 PM)

I am wanting to fiberglass a hood without headlights, and already have a fiberglass trunk. any suggestions on the quick cheap way to do the hood would be appreciated.

I know how to do it, but it ain't cheep
Cheep would be get a FG hood & do the covers your self outta somethin'.

is it possible to cut the hood off its frame and use it as a template for laying fiberglass on to make a hood. I could fill in the light areas and lay glass on that too. Then I could rebolt to the frame, might not lose a lot of weight, but this is just for fun and the experience.

I have a full f/g hood without headlights & would consider an even trade for one with (since I need the openings anyway).

usa4cheer, I'd love to see you document this project here. You can start your own topic here if you want.

I've always thought that a 914-v8 (or maybe subie these days) would be a winning combo.

looking to do blog if i can (new to this )
I travel a lot and will get into the project full force in April, I have been trying to do this for years, but the date for the challenge works with my schedule finally so I can make it.