Aerodynamic aids on Cobras

[750hp]

Bernie - there were heaps of photos of this radical car floating around 6 months ago, but all of the links failed when I tried them yesterday. There aren't any more photos that I can find.....

JM - would certainly prevent "snakebite" from those hot pipes !

Bevan - the thing that I like about the car is that, while being probably the most radically altered Cobra that I've ever seen, everything is done for a reason and looks good as a result. I'd love to see the car in the flesh. I bet there'd be dozens more little bits and pieces that the photos haven't shown !

[BevanWright]

750HP I agree. Have a look at the back end shot, there looks like there is a small snorkel in yellow in the middle probably a diff cooler inlet.

[coyled]

Bevan, going down in rear spring rate might help your traction problem. I went down in my(660hp) rear spring rates and it helped the hook up off the corner. I have spoken with a few of the shelby spec racer builders when I was thinking about the splitter in front. I had a high speed push above 80 mph corners. I was told to tape off most of the front radiator air hole instead. They saw more improvement from that then the splitter. It did seem to help a few months ago at road america. If you think about it, the tape was a cheap way to go with no down side as long as your motor does not overheat. Next time out, I am going to tape off more of the opening. I did about 10 inches of taping on each side last time. I have seen Bob Bacon's cobra at Harrison's auto dynamic s in Phoenix. It started as a hi-tech but as you can see is more of a can am car today. Chris Harrison did most of the work and is a great guy as well as a suspension and aero expert. I think he runs it in the gt1 or gt2 classes and does quite well. For you ford BB lovers, it has an aluminum 427. Scott

[Bill E]

This car is paned on the under side. And has a tilt front end. the reverse scoop vacuums air out of the engine bay. The windshield is a wing. The sister car to this one. Just had a wing installed and improved its lap times bye two seconds at Thunder Hill. Dry weight of the car is 1835. it is a small block all aluminum dry sumped just over 700 HP Bill E

[rdorman]

Interesting thread.............

A wing to give downforce at relatively low speeds would need to be rather large. Seems to me you would create a aerodynamic imbalance in the car that while may help corner exit could in fact slow your lap times due other factors as a result of the imbalance.

Where is Richard Hudgins?

Seems to me you have done some good things already. Additional items I would think about. Get the air out of the wheel wells. You need to let the air out in a low pressure area, ideally where it will not further interrupt the laminar air flow over the body (little Cobra joke there ). Stop the air from coming under the car. Front dam (as wide as possible) and side skirts. Plug up that BIG opening in the front. I know you have ducks to the brakes, but do the body openings really need to be that large? Even a extruded type mess over the radiator, oil and brake duct openings in the body should improve aerodynamics (I have heard 2-3 MPH on the top for this). Be SURE that the duct from leading to the coolers is sealed to the coolers and that the taper from the opening to the cooler is a gradual as you can make it (no more than about 15 degrees). No sharp edge on entry. It would be great to give this air some place to go. The 'Turd' was an attempt at this. Rear chin spoiler I am sure would help. Make it like the nascar boys do and play with height and angle.

The rear diffuser seems to be an interesting idea.

IF I ever get spare time I have daydreamed about a scale wind tunnel where I could play around with the aerodynamics of these cars. But, in the end, they are aero-bricks.

A manometer, some tape, yarn tufts, a video and still camera and a willing couple of friends with another vehicle can give you lots of valuable information.

Rick

[BevanWright]

Bill E - are you sure that the air flows out of that scoop? Why I ask is that a few years ago a friend had a saloon and had a scoop just like yours, and when he tried putting some strings of wool on the car, and driving down the road the wool sucked into the backward scoop. (showing that the flow worked opposite to what he thought). The reason for this is that the windscreen creates a low pressure zone, which the scoop sucked in and directed the air under the car. Some people also use backward scoops to feed air-intakes for carburettors.

Anyway - just a thought - might be worth testing.

[Excaliber]

Bevan sounds like your referring to the "cowl induction" effect. The air piles up at the base of the windshield and "feeds" the engine scoop more effectively than if the scoop was facing forward.

BUT, the windshield on this Bill E. example car is VERY laid back. I can see where the windshield may in fact have a "wing" effect.

I to would test this application using the "string theory", there may be more than "three dimensions" to this thing (pun intended).

Ernie

[750hp]

I've gone for the cowl-induction theory with my car. That's why I mounted the air filter at the base of the screen - theoretically to receive the coolest air possible (as it enters the engine bay).

I haven't used the wool tufts to prove it either way yet. Just another of those things that I'd planned to do and never got around to...

[Richard Hudgins]

BevanWright,

Ok, I will throw in a few simple things.

1st. Front splitter, proper design with ramp up into wheel wheels. Note: not an air dam. A splitter. Look at some of the current LeMans cars, old WSC stuff etc. for ideas. You wish to control the underbody airflow, not stop it.

2nd.. a rear diffuser. The one shown on the black car made of alloy has too high of a divergence slope. It will stall early and not produce the downforce that a shallower curve will. You will need to extend the aft edge past the body at least 10 inches. (This is due to the rear body shape of the Cobra.)

3rd.. If you put on a rear wing, it does not have to be as large as you are speaking of. It needs to mount behind the rear of the car and about 8>10 inches above the rear deck level. This will be primarily used as a trim device and to help create low pressure above the diffuser.

4th.. Front radiator opening. The best treatment is an alloy or Lexan plate that completely blocks the opening. This should be mounted on the inside of the opening flush with the rolled edge. (Note: use a very strong mounting method as the pressure on the plate will be fairly high.) You will need to add air slots or holes to this plate to provide flow to the radiator. I have found that the area required is quite dependent on average track speed. It is best to test various opening areas to find what works for the assorted tracks that you run.

Just a little hint here: You need much less flow area then you might think for cooling.

5th. Install louvers in the bonnet on either side of the scoop. The more area the better. Run them as far forward as possible.

6th. Vent the front wheel area to the sides. The undertray should come as close to the rear of the tire as possible to keep the underbody flow from mixing with the wheel well flow. Once again look to modern Lemans type cars for examples. Louvers on the upper surface of the fenders starting ahead of the front axle center line and carry back to the rear of the tire are also good.

7th. Seal up the wheels wells around the rear tires. Get these seals as close to the tire as possible. It is much better to stop the air from entering than it is to try to evacuate it.

Also, aero work is not going to help low speed traction out of corners. (50>80 mph) This is mechanical grip area. Look to your spring and damper rates to help in this area.

That’s all I have for to offer for free. Anything else makes the clock start running. (I do after all get paid for consulting on this type of stuff.) If I say too much, my customers will cease to pay me. My mortgage folks would not be happy. Not to mention the wife.

[BevanWright]

Thankyou Richard, that is excellent advice, obviously from someone who knows his stuff.

Just one last question then, should there be a curve upward to exit the diffuser, or is a flat exit ok?

Thanks again

Bevan

[Richard Hudgins]

Bevan,

A flat top to the diffuser is fine. Actually better in that it will not be as sensitive to pitch changes.

A curved diffuser will produce greater low pressure, but only with a wing that is designed in conjunction with it.

They are very critical in shape and the only folks using them are the F1 guys. And they have wings and diffusers as matched sets. One does not work without the other.

[FSAEracer03]

Richard, do you think the FFR spec racer's front splitter (air dam really, i guess) would apply to his situation at all? If it can withstand the much higher speeds then what it was intended for (~140mph) would that in conjuction with a shallow diffuser out back give him the ground effects needed for 200mph stability?

Kevin

[Richard Hudgins]

Kevin,

The front air device as used on the FFR car (From the photos that I have seen) acts more as a dive plane then as a air dam. It does not act as a splitter at all.

I am sure that it reduces front lift on the FFR car. But it does not attempt to control underbody flow.

Therefore, it would not be the correct item to use in conjuncton with a rear diffuser.

I would think that it would actually hinder the rear diffuser flow and reduce any negative pressure effects therein.

But the FFR design is more in keeping with the "Period" look and many folks would prefer this over a visual effects of a front splitter.

[BevanWright]

So am i correct in thinking that the splitter I need to make then has a shape to stop air getting to the front wheels, but a gap in the middle to allow air to flow under the front of the car?

[Richard Hudgins]

Originally posted by BevanWright


So am i correct in thinking that the splitter I need to make then has a shape to stop air getting to the front wheels, but a gap in the middle to allow air to flow under the front of the car?

Bevin,

I cannot give a simple answer. The subject is quite a bit more complex then a "Gap in the middle" sort of thing.

You might wish to look at Mulsannes corner.

They have quite a few technical discussions on aero and many photos of past and current cars.

This should give you some ideas on how to proceed with your car.

[RaceDeck]

I run out on the Salt Flats and currently old 3 records in my 32' Street Rod , powered by a Dodge SRT-4 Motor ( 144ci)- with lots of tweaking in both the motor and the car, we were able to make passes at over 200 mph.

I would love to do a Cobra for the "salt". I think that there is a lot that can be done under the car to help the aero as well as setting up the weight distribution. I am pretty confident that we could build a Cobra to run iver the 200 mph mark.

Website: RaceDeckRacing.com

[FSAEracer03]

Hey Rich,

I noticed you drive a P-car... do you happen to be a Rennlist member?? It's easily the best Porsche forum out there. I'm hoping to join the Porsche family in March and buy my first 951 (944 Turbo for all you non-Porschephiles).