Any aerodynamic guru's on here? need your opinions..pics inside

[Scottyk]

I know this is a tough question without a wind tunnel, but obviously I dont have access to one. So heres what Im pondering, I have a asphalt modifed I race locally and its a IMCA class, which basically means the rules are made for dirt cars and we are the only track that races them on asphalt. Long story short, I can run a windshield on my car but no one has tried it "that I know of" because you wouldnt run them on dirt. Heres some pics of it, I want as much downforce as possible, but with a windshield in place would it create turbulent wind off the back of the roof and render the rear spoiler useless?? Im not too concerned about drag, more down force is what I want. Whats your thoughts, windshield or not? and why, how about a rear window?

 

[oilwell1415]

I doubt a windshield by itself would help much, but it might. If you could get rid of the sheet metal in the interior and add side windows with no back window it would probably help a lot because the air under the car would get sucked out the back window. With the back of the car open that might not even matter.

Can you profile the underside of the roof? Back when I worked on a dirt car there was a guy running ministock with a Pinto that had basically turned the whole roof into a wing. From outside you could barely see it, but if you got close and looked inside you could see that the roof was an airfoil. He won nearly everyweek by a landslide. I don't think he lifted all the way around the track.

 

[James Snover]

Keep in mind I have never worked on a car like this. But I would think:

Add a windshield, add a rear window, that, if possible, matches those side wings you have towards the rear spoiler. That ought to "feed" spoiler and keep the car nailed down.

I'd like to see one of these races!

Jim Snover

 

[Scottyk]

Keep in mind I have never worked on a car like this. But I would think:

Add a windshield, add a rear window, that, if possible, matches those side wings you have towards the rear spoiler. That ought to "feed" spoiler and keep the car nailed down.

I'd like to see one of these races!

Jim Snover

Thats what I was leaning toward, I was thining of only building a partial rear window and leave the bottom open "where the side panels switch direction toward front of car" to help extract the air brought in from the side window. No where in the rules does it say you CANT have a rear window or side window, it does say you CAN run a full windshield though.

They are a blast to race, we switched to basically slick tires this year so the overall speed should be way up. Like any home town racing it gets a little dramatic some nights but it usually calms down after a couple days LOL. Nothing better than rubbing tire to tire around a corner at 80+mph with open header cars and a wall right next to you!!

 

[oilwell1415]

No where in the rules does it say you CANT have a rear window or side window, it does say you CAN run a full windshield though.

"If the rules don't specifically says it's not legal, then I have to assume it's legal"--Smokey Yunick

That's from the guy who once built a 7/8 scale replica of a Chevelle and raced it because there wasn't a rule against it. There was a rule against it before long. The only problem is that if you do something that works too well there will soon be a rule against it or everyone will be doing it.

 

[key]

Can you profile the underside of the roof? Back when I worked on a dirt car there was a guy running ministock with a Pinto that had basically turned the whole roof into a wing.

I was thinking the exact same thing. Only issue would be that you'd have to cover the rollbars. They would inhibit the air movement and slow the air on the bottom of the roof down, which would hurt the airfoil effect.

A windshield might help. You would have to have it completely flush with the roof/a pillars.. And you'll have to think about weight and raising the center of gravity of the car as well. I agree with Jim that you'd also need a rear window to feed the air down to the spoiler.

Are you allowed to add fins behind the rear wheels?

 

[Scottyk]

I guess I could add some pieces of sheetmetal to those rub bars that go around the 1/4 panel behind the rear wheels.. Is that your referring to? There is a rule that the widest part of the car has to be the tires. I didnt think there would be enough airflow behind the rear tires for fins to help, but maybe.. Anyone know of a air-tunnel for rent around iowa??

 

[key]

This is what I'm talking about:

http://www.fourtitude.com/news/uploads/Motorsport_News/new_dtm.jpg

http://i653.photobucket.com/albums/uu257/fuzz26/dtma4.jpg

I don't know if it would help or not, but pretty much every car in that series has little fins like that.

 

[key]

Do those C-pillar sheets angle out towards the front of the car and get narrower towards the trunklid/spoiler or is that just an optical illusion? If they do, they might be scooping up air to feed to the spoiler and putting a back window in would block that.

Can you run dynamic downforce components?

http://images.google.com/images?hl=en&q=brabham bt46b fan car&um=1&ie=UTF-8&sa=N&tab=wi

 

[James Snover]

Along those lines, TX_Diablous had an idea: take the car to a dragstrip, bring a buddy with a video camera. Tie a stick about four feet long in front of the car and put a smoke bomb on it, then make a run down the track. A 1/4 mile ought to tell the story. Look for the smoke to flow smoothly over the car, you don't want any abrupt dips in the flow. Presto: cheap wind tunnel.

Jim Snover

I guess I could add some pieces of sheetmetal to those rub bars that go around the 1/4 panel behind the rear wheels.. Is that your referring to? There is a rule that the widest part of the car has to be the tires. I didnt think there would be enough airflow behind the rear tires for fins to help, but maybe.. Anyone know of a air-tunnel for rent around iowa??

 

[oilwell1415]

Are these your rules?

Index of /manage/uploaded[pdf-0]northsportmod.pdf

If so, there is some working room there. The roof can't be curved upward on the edges and can't be dished. However, you can use a flat roof with the egdes curved downward. You can also have the front of the roof 5" lower than the rear, which would give you a lot of angle of attack. It also doesn't say the roof has to be solidly mounted, so you could make it adjustable as long as you stay within the overall height rules. Add a panel under it to help the air get around the leading edge and transition through the driver's compartment and maybe add a "stiffener" between the tops of the sail panels that is angled upward. Don't go to low with it or it will rob air from the spoiler, which may not be a bad thing. Some vortex generators on the undeside of the roof would help also.

I also notice that nitrous isn't illegal, but that may be covered in the general IMCA rules or track rules, and not in the class specific rules.

 

[stangin99]

Along those lines, TX_Diablous had an idea: take the car to a dragstrip, bring a buddy with a video camera. Tie a stick about four feet long in front of the car and put a smoke bomb on it, then make a run down the track. A 1/4 mile ought to tell the story. Look for the smoke to flow smoothly over the car, you don't want any abrupt dips in the flow. Presto: cheap wind tunnel.

Jim Snover

You can probably rent one of those huge fans that are used to cool down large events. Then just put the smokebomb on a stick and put it in front of the fan to cause the same effect.

 

[oilwell1415]

I guess I could add some pieces of sheetmetal to those rub bars that go around the 1/4 panel behind the rear wheels.. Is that your referring to? There is a rule that the widest part of the car has to be the tires. I didnt think there would be enough airflow behind the rear tires for fins to help, but maybe..

The rules I found say no fins allowed and that would likely be considered a fin.

Anyone know of a air-tunnel for rent around iowa??

You can count the number of wind tunnels that will do meaningful work on a full sized car in this country on one hand. They are thousands of dollars per hour to rent.

 

[James Snover]

You are a genius, stangin99. No foolin'.

OP, what he said.

Jim Snover

You can probably rent one of those huge fans that are used to cool down large events. Then just put the smokebomb on a stick and put it in front of the fan to cause the same effect.

 

[oilwell1415]

Or just put it on a trailer and haul it out here. The wind's been blowing 30-40 mph everyday. I'm not sure a fan would give good results because they air doesn't come off of one straight. It actually swirls around. That's why wind tunnels have the fan at the exit of the tunnel instead of the entrance. You could put it on a trailer and haul it out to the arrival end of the largest runway you can find and wait for a jumbo jet to come along.

Do you have any scales to measure the weight on the wheels? If so they would make it easy to see changes in downforce.

 

[lobra97]

Vteeeeeekkk!

 

[Scottyk]

Thanks for the ideas, I was hoping for some creative answers like that!!

Heres a link to the rules.
International Motor Contest Association :: Welcome to IMCA.com!

5. BODY: (See diagram) Must be same width, front to rear, and parallel to OEM frame. Nose panel must be flat with no fins or scoops allowed. Cooling holes allowed. Nose panel may be no wider, or lower than two inches outside of front frame horns. Nose panel must remain within confines of front bumper. Engine compartment must remain open (no side panels). Hood must be enclosed at rear. No panel in front of right door to engine compartment. No inner panels. No car covers. Must have front windshield and rear window support posts. Driver and passenger side windows must have at least 12 inch opening (height and width), measured at center of window, between lowest point at top of window, whether roof or roll cage, and highest point at bottom of window, whether interior or body. May use full windshield. Roof must be fiberglass or aluminum, full size and rounded down in all directions (see diagram). No dished roofs allowed. Driver roof hatch allowed. Window side panels must resemble all aspects of drawing. May use lexan in window side panels. Rear spoiler (optional) may be maximum 5 inches in material height and maximum 66 inches wide. Spoiler may have rear stiffener, must be 1 inch or more down from top. Maximum 3 spoiler braces allowed, must be mounted in line. Spoiler braces must resemble all aspects of drawing. Spoiler must be mounted within confines of spoiler braces. No fins, lips or wings allowed. No reflective doors or quarter panels. Tires must be widest part of car.

No fins, lips, or wings.. So those fins behind the rear wheel would be out, the tech officials dont understand the rules very well and REALLY dont like it when they see something different on the race track. I havent even seen anyone run a windshield yet and I think they are missing out, like I said the rules are written for dirt track cars and they cant run a windshield "too much mud and rocks" I like the idea of a big fan but I would need some type of a duct with straight vanes in it to get rid of the swirl motion from the fan, I also have 4 corner scales that are accurate to 1 or 2 pounds.. We race 1/4 mile and have plenty of power so drag isnt too much of a factor compared to down force in the corner.

Im thinking a smoke bomb with a bunch of stringers taped on the car in key areas, then putting equal size tires on the rear and finding a country road and driving down it with a buddy video taping it.. That should be interesting

 

[Ferris Bueller]

Aerospace engineer here. Looks like your car creates downforce mostly at the nose and the rear wing. The nose is out front and in clean air, so no worries there. (bigger wing in front = more downforce) The rear wing is fed with a smooth laminar air from the flat area through the "passenger area". The rear wing seems to be designed with this in mind. Since the wing is a fixed size and at a fixed angle of attack, the way to increase downforce for the wing is to increase the air velocity passing over it. A windshield will prevent the air from taking a straight shot to the wing by making it go up and over the roof and separating the airflow from the car creating a turbulent area over the wing. Slower air = less downforce. You may be able to prevent this by adding a rear window at a shallow angle (think fastback). However, adding a front windscreen will significantly increase drag. Drag is affected by the cross sectional area the airstream will see, which can be visualized by looking at your car from directly in front. Without the wind screen, this area is fairly low, but add a big section that the air has to go around (windscreen) is going to make some additional drag.

If you want to increase the air velocity over the rear wing, you could add a teardrop shaped cowl behind the driver's head. This shape has a lower coefficient of drag and will keep the airspeed high when it passes over the wing.

If you want a windshield to protect the driver from the wind, you might be able to put in a short, narrow screen that will direct the air up and over the top of the driver's helmet. This, along with the teardrop cowl, should keep the wind away from the driver and keep the airspeed over the rear wing high.

Sorry if this explanation is a bit of a mess, just finished a 10.5 hour day crunching on aerodynamic problems and my brain is a little spongy.

 

[Scottyk]

Aerospace engineer here. Looks like your car creates downforce mostly at the nose and the rear wing. The nose is out front and in clean air, so no worries there. (bigger wing in front = more downforce) The rear wing is fed with a smooth laminar air from the flat area through the "passenger area". The rear wing seems to be designed with this in mind. Since the wing is a fixed size and at a fixed angle of attack, the way to increase downforce for the wing is to increase the air velocity passing over it. A windshield will prevent the air from taking a straight shot to the wing by making it go up and over the roof and separating the airflow from the car creating a turbulent area over the wing. Slower air = less downforce. You may be able to prevent this by adding a rear window at a shallow angle (think fastback). However, adding a front windscreen will significantly increase drag. Drag is affected by the cross sectional area the airstream will see, which can be visualized by looking at your car from directly in front. Without the wind screen, this area is fairly low, but add a big section that the air has to go around (windscreen) is going to make some additional drag.

If you want to increase the air velocity over the rear wing, you could add a teardrop shaped cowl behind the driver's head. This shape has a lower coefficient of drag and will keep the airspeed high when it passes over the wing.

If you want a windshield to protect the driver from the wind, you might be able to put in a short, narrow screen that will direct the air up and over the top of the driver's helmet. This, along with the teardrop cowl, should keep the wind away from the driver and keep the airspeed over the rear wing high.

Sorry if this explanation is a bit of a mess, just finished a 10.5 hour day crunching on aerodynamic problems and my brain is a little spongy.

Makes alot of sense.. So in your opinion, increasing air speed to the spoiler would create more downforce than adding a windsheild? The only reason I am pondering a windsheild is for downforce, not wind noise or anything else. I am limited on spoiler size but it seems a windshield would be a large area wing creating downforce.

How about this idea I just had, no windsheild, but a partial rear window with about 6" open between deck and bottom of rear window to guide air to the spoiler, and it would actually create a funnel effect aiming straight toward the spoiler. Hopefully that made sense, if not I can draw a simple model at work tomorrow and post up on here.

 

[mineralgrey01gt]

i watch these on dirt quite often at a local track. Awesome little cars thats for sure.