GT 40 Duck Tail Spoiler

OLD H2S · Nov 2, 2015

In one of my other threads I said I wanted to copy the GT 40's rear spoiler I liked. It is a very simple shape and I knew I could copy it when I saw the car up at Summit Point. (See my post "I beat a GT 40'). Back in the day, 1966, I remember as a 11 year old kid when the Ford GT 40 came out and all the hoopla about aero testing the car with a picture of pieces of string held down with a piece of tape and some one in a chase car taking pictures of the GT going around the track with some strings flapping and some sticking out straight, this was high tech stuff to a little kid making balsa wood air planes.
I got a 4' piece of aluminum 1/4" thick and bent it to fit the rear deck curve of the trunk. I used 1/4-20 rivet nuts sunk into the sheet metal to make the mounting flush with the curve. The support brackets are overkill, too big and thick but very strong. To get the angles right I did TIG over MIG welds, not pretty but very strong. I will cut down the fillets and coat the whole thing in matching Midnight Blue and it will fade into the car better and not look as "different" as it does now. Does it work???? I don't know, but the air blast with the window down is not hitting me in the face anymore and that is a big plus when you wear glasses like I do all the time to see. Damn kids and their Lasik surgery, grumble, grumble.

ttocs · Nov 2, 2015

not sure that would be labeled as a spoiler. I am thinking more like an air brake myself...

Mack · Nov 2, 2015

I would do some basic aerodynamic testing before installing it and finding out you can't go over 50mph because of the air resistance.

g36 monkey · Nov 2, 2015

I agree, it looks like it would just slow you down. Maybe the pictures don't show it, but here it looks like it's canted a little more towards the rear, so it's not just stopping air, rather using it to direct the car down.

ElrodKTPQ_89 · Nov 2, 2015

[MENTION=15115]hotmustang95[/MENTION] had a similar spoiler on his car at one time.

Ferocious · Nov 2, 2015

I'm with everyone else. It looks like it's missing some serious angle to be acting as a spoiler.
Wichers123 also built his own GR40 variant I believe. Maybe he can shed some insight.

ttocs · Nov 2, 2015

it looks like you have a good welder. Draw a line on the spoiler by simply dropping a pen on the top of the trunk lid and then dragging it all the way across. Cut it on that line and then you will need to do some trimming to get it to fit but adjust the angle so you do not have it so straight up and down and then weld it back in place. If you don't want to cut it you could try just scoring it by cutting it half way and then see if it would allow you to adjust it but I am afraid with the curve on the back that it would make it hard to make that bend.

Wichers123 · Nov 2, 2015

Actually being more vertical does not create a "air brake" effect. The stopping of air infront of the spoiler causes a low pressure spot and the in coming air then in turn goes over that pocket of air. The effect stops the air from wrapping around down under the car causing upward force to lift the back of the car.

96blak54 · Nov 2, 2015

Coanda

ttocs · Nov 2, 2015

I guess all the wings/spoilers I have seen before just had it all wrong if a wall is all we needed.

Wichers123 · Nov 3, 2015

Do alittle research before you knock a design of a spoiler. Btw wings and spoilers do two different functions. Educate yourself with nascars spoilers you might learn something like I did. NASCAR this year at Indy just added the more height to their spoilers to a total of 9".

http://oppositelock.kinja.com/wings-spoilers-youre-probably-doing-it-wrong-1665312667
^^^^ good read btw

ttocs · Nov 3, 2015

is it a misnomer then when high end cars like the SLR for example pitch theirs up under braking and call it an air brake? I would assume they only do it under braking to reduce drag when not under braking? I am not doubting that this will make a good amount of down force I am just wondering at what cost of drag its creating?

wmfateam · Nov 3, 2015

That article is not the one all for information, it is not 100% accurate. Yes the spoiler aids in reduced drag, but to a certain height. Like the perfect example given above, that spoiler and some other hyper cars, raise and have a stupid amount of rake to be an air brake. The height has to be tuned for a downforce/drag ratio that suits the track, power and shape of car. Just like NASCAR plays around with rear spoiler height at different tracks.

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hotmustang95 · Nov 3, 2015

OLD H2S said:

Looks very close to mine.

Currently mine is off the car only because I ruined it focking around. I plan to fab up an even better one next time out of aluminum like yourself. The next one will have some angle to it but only for looks. It will still do what my current one did. And that's reduce drag. It does not act like an air brake. I once thought that too before researching and educating myself. For example think about Nascar. They are the ones that use this type of technology. I'm no engineer, but I bet my ass the reason they use these types of spoilers is not to slow down their cars.

hotmustang95 · Nov 3, 2015

Wichers123 said:
Actually being more vertical does not create a "air brake" effect. The stopping of air infront of the spoiler causes a low pressure spot and the in coming air then in turn goes over that pocket of air. The effect stops the air from wrapping around down under the car causing upward force to lift the back of the car.

Almost got it. Your right on about the pocket of air that gets trapped in front of the spoiler. What's off is the low pressure caused by it. What really happens is without a spoiler, there's a low pressure area over the truck at high speeds causing lift. Adding a spoiler will cause that pocket of air to form in front of it and allow the air traveling over the roof to pass easily over the truck. Because there's no longer a low pressure area over the truck, lift is reduced to a neutral balance. This is why drag racers use an extended wing/whale tail type of spoiler. It too allows air to continue moving freely over the car and not cause any dips in the stream that would cause a low pressure area causing lift. Drag racers and Nascar do not want lift on the back of their cars and because they are not allowed to use a wing that would produce downforce, the next best thing is a spoiler.

The "AIR BRAKE" thing only comes into play if the spoiler is actually in the free flowing air and by design, it is not. Don't forget what's directly infant of a spoiler? The cabin. At 100mph, how will air ever hit a spoiler when it's directly behind the cockpit or cabin? Only a wing mounted high enough into the free flowing air which starts a few inches below the roof line will actually touch high speed winds. Many cars come from the factory with some type of spoiler because of their ability to reduce drag and therefore add to fuel milage. Think about it. All those factory cars with spoilers. They're put there for a reason.

hotmustang95 · Nov 3, 2015

Wichers123 said:
Do alittle research before you knock a design of a spoiler. Btw wings and spoilers do two different functions. Educate yourself with nascars spoilers you might learn something like I did. NASCAR this year at Indy just added the more height to their spoilers to a total of 9".

http://oppositelock.kinja.com/wings-spoilers-youre-probably-doing-it-wrong-1665312667
^^^^ good read btw

Exactly.....