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I started a treatise with air density calculations from the extra heat picked up by the rad, and was going to compare that to an equal mass of air at ambient temp, then figure in the possibility of a little boost in airspeed and momentum change from the extra energy picked up but decided that it probably was less significant than:

"The hood itself has a raised lip at the front of the openings to create turbulence, which creates low pressure and actually draws the air out of the engine compartment or the chimney, and that helps the air flow out from behind the radiator more efficiently and adds about 30 pounds of downforce, too." 

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Disclaimer: I have no formal aerodynamics training and most of this info I am going to share I have just picked up on the side for fun. Feel free to critique anything I am about to say.   I

one more before i hit the road.  not quite.  The air being directed over the top of the car does add downforce. It's not the temp to blame, it's the speed. So to make downforce, the general

Put a hole in the top of the fender above your control arm/spindle/whatever. Attach a rod to the top of your control arm that is long enough to protrude above the fender at full droop. Mark the rod at

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Disclaimer: I have no formal aerodynamics training and most of this info I am going to share I have just picked up on the side for fun. Feel free to critique anything I am about to say.

 

I believe @@NineLivesJohnny's explanation was correct about the increase in downforce/front end stability from venting the hood is more of a side effect from taking air that may be disrupting the underbody flow and putting it somewhere less disruptive.  If you are going to vent your hood, you want to vent it as close to the convex portion of the hood, which will most likely be at the front. This will be the area of lowest pressure and it will only get higher as you approach the windshield, which creates a concave shape with the hood.  This is why vented cowls are usually a very bad idea for heat extraction and a great idea for a cold air intake.  Another side effect of venting your hood for radiator airflow is that it may allow you to decrease the size of the radiator inlet duct, which can also improve front downforce by maintaining the high pressure zone above your front splitter (assuming you have one).  Ultimately, venting the hood is more of a cooling airflow improvement that may have lift reduction (downforce) benefits depending on how the engine compartment was originally vented on your vehicle.  You will notice that very high downforce cars like LMP1, LMP2, and DPi that don't have road car constraints will not have any cooling venting on the top of the car and will have all of it at the rear.  They also pull air into their side pods for the heat exchangers from underneath the car, which further promotes underbody airflow.

 

Minimizing the gap between the front of your car and the ground with a splitter is one of the easiest ways to create front downforce.  Forcing air into a very small area will cause it too accelerate in an attempt to maintain the same mass flow rate, which will lower the pressure under the front splitter.  The next step is to add diffuser tunnels that vent towards the inside of the front wheels and vent the rear of the front fenders to further increase airflow velocity under the front splitter.

 

This is where it gets interesting in my opinion.  You will notice a difference in design from what I described above in the most recent GT3, LMP1, LMP2 and DPi cars, and one-off hillclimb cars like the VW IDR.  Below is the progression of front splitter design for the Ferrari GT3 car (458, 488, and 488 Evo)

 

1927093308_ferraricomparison.thumb.PNG.25f44885035345df4470eed8eae151ec.PNG

 

Notice how the raised portion of the front splitter goes from being non-existent to being nearly the full width of the front-end.

The issue with running a flat front splitter as close to the ground as possible is that your front downforce becomes very pitch-sensitive. As soon as the splitter gets too close to the ground due to braking or bumps in the road, you will lose a large portion of your front downforce. This makes for a car that will be very unpredictible to drive, but still stable as the rear downforce will be maintained.  These are things you will not notice in CFD or in the wind tunnel, but on the track.  By having a raised front splitter and shaping like a nozzle, you still have high velocity air going under your front splitter, but it will be much less pitch sensitive. This is what they mean when you read the press releases of these cars and they say the car is "easier to drive at the limit" than the previous generation.  For reference, here are pictures of the underside of the 488 GT3 (middle picture):

 

image.png.eca0ba82aa82ffdc6c052ee9e53a87f9.png

 

One thing to note, this splitter design does not work very well unless you can maintain air velocity underneath the car. You cannot add a large raised front splitter on an old sports car and expect it to work.  You need a flat bottom like the one above as well as a large rear diffuser to keep air velocity high though the entire underside of the car.  As the air becomes turbulent from interacting with parts under the car like the exhaust, suspension, and, drivetrain, you will lose air velocity under the car, which will reduce air velocity at the front, which will greatly reduce front downforce and overall downforce.  There is also the problem of the boundary layer increasing in size towards the rear of the bottom of the car.  This can be combated by adding positive rake to the vehicle. The key to efficient and effective aero design is to get all of the parts of your aero package working together.

 

With all of that said, lets go to side skirts. This is an area that I think a lot of people mess up.  Fully-sealed side skirts are only effective if you are able to maintain underbody air velocity from the front of the car to the diffuser. Otherwise, you are cutting off valuable airflow that the diffuser may need.  Norbert Singer briefly describes this phenomena in this video: 

 

 

 

Below is a comparison between the Porsche 919 and 919 Evo:

 

porsche919.jpg.6bf7d57c1a210dcc693f377fd13f6202.jpg

 

IMG_0342.thumb.jpg.0c945b92940e25dee93f7028ec2c3373.jpg

 

porsche919evo.thumb.jpg.24b4784ffa82cb6768254076ff59f560.jpg

 

You will notice the side of the Porsche 919 is very high off the ground. Due to the rules restricting the diffuser size as well as what can be done with the side skirts, they found that rounding the edge of the side skirts and directing the inlet of the rear diffuser towards the side (shown in the second picture, this is actually from the Cadillac/Dallara DPi car) was their best way forward. You will also notice the car has a lot of positive rake to effectively make the entire underbody a diffuser.  The 919 Evo is an unrestricted version of the 919.  No longer hindered by the rules, they were able to greatly increase the size of the rear diffuser.  They also made the rear wing larger and moved it rearward.  This created a very large low-pressure area at the rear of the car, which helped maintain air velocity under the car.  This allowed side skirts to be beneficial, so they were added. By feeding the rear diffuser from the front instead of the side, they were able to shift the aero balance forward. They then shifted it back to neutral using the giant rear wing and increased total downforce significantly.  This design can also be seen on the VW IDR below, which is also unrestricted by rules:

 

IDR.thumb.jpg.2247a64331222256f3670ffc3c2652c9.jpg

 

If you add side skirts to your old sports car, you may see a small drag reduction, but probably no downforce.

 

One last note is that you want to try to match your downforce balance to your weight balance.  If your car is 55/45 Fr/Rr, you want 55% of your downforce at the front.  There can be exceptions to this, but it is a good rule of thumb.  You should also be aware of how the downforce balance (center of pressure) changes as speed increases. You don't want the balance to shift forward, as it will reduce rear stability at high speed.

 

There is so much more I could get into, but I should probably call it a night. I hope this post is informative and creates some more discussion.

 

 

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9 hours ago, Scribe said:

Disclaimer: I have no formal aerodynamics training and most of this info I am going to share I have just picked up on the side for fun. Feel free to critique anything I am about to say.

You are obviously an amateur at this. 

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On 2/22/2019 at 5:58 AM, E. Tyler Pedersen said:

How do you make the boxy front end of an e30 more aerodynamic?

make the nose pointy, can you ditch the windshield?

 

 

On 2/22/2019 at 8:15 AM, SonsOfIrony said:

 

Excellent info, Thank you.  Most of the headlights are covered.  Just one side is open for the air filer, and it's boxed in to create high pressure there.  Is that causing more harm than good?

 

Moving the Antenna is easy.

 

Not sure if nets are allowed on the passenger side, Lexan is not.  Is there a way to make a deflector that will kick air around the opening without causing too much turbulence/drag?

 

 

FB_IMG_1550840890848.jpg

 

try to cover the air filter hole, use duct tape if need be.  go out and do a few laps, then pull it off and do a few more. see if the car goes faster. we are looking to see if the drag penalty is worth the added hp. 

 

 

On 2/26/2019 at 8:59 AM, zack_280 said:

You are obviously an amateur at this. 

 

Aero is the most humbling race car science out there. you think the air does one thing and then it makes you eat crow. that's why CFD scans we offer are so important. 

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On 2/25/2019 at 11:25 PM, Scribe said:

...

Minimizing the gap between the front of your car and the ground with a splitter is one of the easiest ways to create front downforce.  Forcing air into a very small area will cause it too accelerate in an attempt to maintain the same mass flow rate, which will lower the pressure under the front splitter.  The next step is to add diffuser tunnels that vent towards the inside of the front wheels and vent the rear of the front fenders to further increase airflow velocity under the front splitter..

2

 

yes and no. so when we are building cars splitter height is critical. yes making it close to the ground is good, making it touch the ground is bad. if it touches the air stalls and you go from very fast moving air to no air. you lose all the downforce.  so having height is important. 

 

Another thing to consider is height in relation to suspension movement and consistency. say you have a splitter that sits 1" off the ground, and your suspension allows the nose to move 1/2". So the height range is 1/2" at full compression and 1" at full droop.  when your suspension cycles your front downforce levels are changing 50% at a time. let's say your splitter makes 200lbs of downforce, it's changing from 100-200-100-200.  this can make your front downforce levels change constantly. makes it very hard to drive. Now let's say we keep everything relative but raise the car to 3" off the ground. so splitter height changes from 2.5" -3" off the ground.   now your front downforce only changes 16% or you only lose 30lbs of downforce with a 200lbd-df splitter.  makes it much easier to drive. 

 

 

 

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On 2/22/2019 at 6:15 AM, SonsOfIrony said:

 

Excellent info, Thank you.  Most of the headlights are covered.  Just one side is open for the air filer, and it's boxed in to create high pressure there.  Is that causing more harm than good?

 

Moving the Antenna is easy.

 

Not sure if nets are allowed on the passenger side, Lexan is not.  Is there a way to make a deflector that will kick air around the opening without causing too much turbulence/drag?

 

 

FB_IMG_1550840890848.jpg

The main benefit of the external air inlet for the engine is not pressure but temperature.

 

The engine will see a much higher increase in air density from the cold air than it will from any ram air or pressure effect. 

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So slightly different direction here if I may- We've talked about 'fast backs' and sedans, how about something a tad more hopeless: the classic 2 box hatchback (Civic hatch, Fit, Mazda 2, Mini, etc). 

 

I'm assuming the usual tricks still apply (reduce frontal area, lay down the windshield, chop the roof if you can, smaller front openings, etc) but what about wings, drag and the rear end shape? I've seen numerous efforts like the below - big but rather flat wing and as much diffuser as you can stuff under it.

 

Image result for mini cooper aero

 

For us, we're more interested in drag than anything else, but I have a hard time wrapping my head around adding a wing to reduce drag (instead of leaving it a slick-top). Or am I barking up the wrong tree?

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10 minutes ago, mender said:

 

The engine will see a much higher increase in air density from the cold air than it will from any ram air or pressure effect. 

 

So before the radiator would be a "bad" place since it blocks the radiator and the radiator might radiate some heat.

 

What about Having a dedicate intake duct, that duct could be much smaller than your intake right? Say the duct is only 1" but the large cone filter might be 5 inch.

 

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2 minutes ago, turbogrill said:

 

So before the radiator would be a "bad" place since it blocks the radiator and the radiator might radiate some heat.

 

What about Having a dedicate intake duct, that duct could be much smaller than your intake right? Say the duct is only 1" but the large cone filter might be 5 inch.

 

If you have radiator inlet ducting you could run an intake air duct off that and get the lowest temp and highest pressure air for the least drag.

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1 hour ago, @NineLivesJohnny said:

 

yes and no. so when we are building cars splitter height is critical. yes making it close to the ground is good, making it touch the ground is bad. if it touches the air stalls and you go from very fast moving air to no air. you lose all the downforce.  so having height is important. 

 

Another thing to consider is height in relation to suspension movement and consistency. say you have a splitter that sits 1" off the ground, and your suspension allows the nose to move 1/2". So the height range is 1/2" at full compression and 1" at full droop.  when your suspension cycles your front downforce levels are changing 50% at a time. let's say your splitter makes 200lbs of downforce, it's changing from 100-200-100-200.  this can make your front downforce levels change constantly. makes it very hard to drive. Now let's say we keep everything relative but raise the car to 3" off the ground. so splitter height changes from 2.5" -3" off the ground.   now your front downforce only changes 16% or you only lose 30lbs of downforce with a 200lbd-df splitter.  makes it much easier to drive. 

Do you have any evidence to back up this claim that changes in splitter height cause a proportional change in downforce (in your example, going from 1” to 1/2” DOUBLES the downforce)? Or did you just pull numbers out of the air because they sounded good?

 

If accurate, how close to the ground does the splitter need to get before this ‘proportionality rule’ no longer holds?

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Piling on with the splitter design question, in addition to splitter height considerations, at about what point do the returns from adding splitter length start to taper off or possibly harm performance?

 

Our rules allow for a 12 inch splitter - that seems awful big for cars taking corners at a max of maybe 100mph.  

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On 2/25/2019 at 7:25 PM, mender said:

I started a treatise with air density calculations from the extra heat picked up by the rad, and was going to compare that to an equal mass of air at ambient temp, then figure in the possibility of a little boost in airspeed and momentum change from the extra energy picked up but decided that it probably was less significant than:

"The hood itself has a raised lip at the front of the openings to create turbulence, which creates low pressure and actually draws the air out of the engine compartment or the chimney, and that helps the air flow out from behind the radiator more efficiently and adds about 30 pounds of downforce, too." 

Shouldn't talk about this in public....

 

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16 hours ago, Wittenauer Racing said:

So slightly different direction here if I may- We've talked about 'fast backs' and sedans, how about something a tad more hopeless: the classic 2 box hatchback (Civic hatch, Fit, Mazda 2, Mini, etc). 

 

I'm assuming the usual tricks still apply (reduce frontal area, lay down the windshield, chop the roof if you can, smaller front openings, etc) but what about wings, drag and the rear end shape? I've seen numerous efforts like the below - big but rather flat wing and as much diffuser as you can stuff under it.

 

Image result for mini cooper aero

 

For us, we're more interested in drag than anything else, but I have a hard time wrapping my head around adding a wing to reduce drag (instead of leaving it a slick-top). Or am I barking up the wrong tree?

 

There is no free lunch, so you would be correct that adding a wing will add drag. if the overall low drag is what your after you might want to reconsider where your driving. unless you're on the salt flats no drag shouldn't be the goal. in Road racing, the Goal has been and always should be to decrease lap times. items that are causing drag and not creating downforce should be removed or modified to either make downforce or remove it altogether. 

 

12 hours ago, Originalsterm said:

How important is overall splitter shape? Does it need to stick out from the front of the body work uniform all the way across? How far under the car should it extend?

 

14 hours ago, gavro said:

Piling on with the splitter design question, in addition to splitter height considerations, at about what point do the returns from adding splitter length start to taper off or possibly harm performance?

 

Our rules allow for a 12 inch splitter - that seems awful big for cars taking corners at a max of maybe 100mph.  

 

Everyone is introduced to splitter sizing via a rule book. Almost all of the rules are there for Safety or reducing cost. very few rules are in place to limit performance. we see this alot in GTA the rule book says for the lower classes you can only have a 3" splitter protruding from the bumper. so when racers step up into the unlimited classes they bolt on a 48" splitter thinking the organization was limiting their performance. unfortunately, that's not the case.   From our work, we haven't seen a connection to the distance a splitter protrudes and the performance.  We saw that a splitter should stick out around 3-5" anything more than that it wasn't doing anything.  What is very important is how deep the splitter goes under the car. at a minimum it should run to the centerline of the rear axle.  the splitters area underneath the car helps speed air and give the low pressure air something to attach to. Run that splitter as far back as you can. 

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15 hours ago, enginerd said:

Do you have any evidence to back up this claim that changes in splitter height cause a proportional change in downforce (in your example, going from 1” to 1/2” DOUBLES the downforce)? Or did you just pull numbers out of the air because they sounded good?

 

If accurate, how close to the ground does the splitter need to get before this ‘proportionality rule’ no longer holds?

 

Pulled general numbers to give a general example. 

if you want exact numbers we can run your car through a virtual wind tunnel.

if interested send an email to info@9livesracing.com

pricing is 3500 without a 3d model and 1200 with a 3d model. 

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2 hours ago, @NineLivesJohnny said:

 

There is no free lunch, so you would be correct that adding a wing will add drag. if the overall low drag is what your after you might want to reconsider where your driving. unless you're on the salt flats no drag shouldn't be the goal. in Road racing, the Goal has been and always should be to decrease lap times. items that are causing drag and not creating downforce should be removed or modified to either make downforce or remove it altogether. 

 

 

 

Everyone is introduced to splitter sizing via a rule book. Almost all of the rules are there for Safety or reducing cost. very few rules are in place to limit performance. we see this alot in GTA the rule book says for the lower classes you can only have a 3" splitter protruding from the bumper. so when racers step up into the unlimited classes they bolt on a 48" splitter thinking the organization was limiting their performance. unfortunately, that's not the case.   From our work, we haven't seen a connection to the distance a splitter protrudes and the performance.  We saw that a splitter should stick out around 3-5" anything more than that it wasn't doing anything.  What is very important is how deep the splitter goes under the car. at a minimum it should run to the centerline of the rear axle.  the splitters area underneath the car helps speed air and give the low pressure air something to attach to. Run that splitter as far back as you can. 

Minimum is REAR axle?  Dang!

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3 hours ago, mindspin311 said:

Top mounted (swan neck) vs. standard bottom mount. All pro GT stuff has gone to swan neck. Should us amateur guys be doing the same?

 

There is a good article on that here: http://www.mulsannescorner.com/rearwingLMPCFD2009.html

 

Short answer: yes, but there is a weight and CG height penalty for it (small, but its a consideration)

The Ford GT and Corvette GTLM cars still do bottom-mounts, but they also contour their wing mounts to reduce the separation on the bottom surface of the wing.

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23 hours ago, Wittenauer Racing said:

So slightly different direction here if I may- We've talked about 'fast backs' and sedans, how about something a tad more hopeless: the classic 2 box hatchback (Civic hatch, Fit, Mazda 2, Mini, etc). 

 

I'm assuming the usual tricks still apply (reduce frontal area, lay down the windshield, chop the roof if you can, smaller front openings, etc) but what about wings, drag and the rear end shape? I've seen numerous efforts like the below - big but rather flat wing and as much diffuser as you can stuff under it.

 

Image result for mini cooper aero

 

For us, we're more interested in drag than anything else, but I have a hard time wrapping my head around adding a wing to reduce drag (instead of leaving it a slick-top). Or am I barking up the wrong tree?

 

ChampCar is less restrictive than most series as far as building an effective wing on a  hatchback. The rules are a foot above the roof and a foot behind the bodywork so that tells you where the trailing edge of your wing should be.

 

 

38 minutes ago, Scribe said:

 

There is a good article on that here: http://www.mulsannescorner.com/rearwingLMPCFD2009.html

 

Short answer: yes, but there is a weight and CG height penalty for it (small, but its a consideration)

The Ford GT and Corvette GTLM cars still do bottom-mounts, but they also contour their wing mounts to reduce the separation on the bottom surface of the wing.

 

 

The ChampCar rules say that the maximum wing height is 12 inches above the roof, but doesn’t say that includes the brackets to mount them. When I was thinking about building a hatchback I was considering a swan neck mounting bracket mounted to the roof rack area. That will leave the bottom surface of the wing in a god spot to be energized by flow from the roof, in this case a small OEM spoiler would probably actually be a big help combined with the wing. And you would be mounting the wing to a part of the car that is already reinforced and strong.

 

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