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Rear wing theory


gr1vlet
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The new rules allow wings and splitters at a cost of 10pts each. This looks like a great way to increase a vehicles potential at a low point cost with the added benefit of encouraging the shade tree engineering that makes ChumpCar so awesome.

 

I'd like to gain a more thorough understanding of how and why a wing works and what the pros and cons are beyond the basic tradeoffs in downforce vs drag. Is there a reason wings are typically placed as far back as possible on a car? My cursory understanding is:

 

1) You can place it lower than the roofline since the air is coming off the roof and downward, allowing it to work in semi-clean air without increasing frontal area.

 

2) the further back it is the more mechanical advantage it has on the rear wheels allowing a lower drag/downforce angle of attack to have the same effect as if it were placed directly over the rear axle.

 

I'm sure this is an oversimplification and the only way to come to any real conclusion would be to put the car in a wind tunnel and/or do lots of testing.  I know we have a lot of great minds on this forum with a ton of experience and I'd be interested to hear any opinion on this as it is certainly a black art.

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Bigger is always better. And further back too. Cantilever that sprint car wing back off the trunk for max rear downforce. 

 

For a rear wing alone, yes.  If it's not balanced with the front, or the rest of the car's behavior, you can have some really strange results.  Two things I see a lot with rear wings:

 

1:  Rear wing is not balanced by the front

      Driver complains of an understeering car.  A large rear wing behind the rear axle will attempt to unload the front tires.  It's a lever, with the rear wheels as a fulcrum.  If you can't get enough downforce on the front axle to balance the big wing, you'll just be adding drag.  The car won't be any quicker because the front tires have a reduced normal load, and saturate well before the rears.  The lower grip axle determines your cornering capability, and that big wing reduces front grip and adds drag if it's not balanced.

 

2:  Rear wing creates no downforce

      This is a fun one.  People like to use changes in the car's balance as an indicator of adding grip.  If you add a rear wing, and the car understeers, if follows that you must have added grip on the rear, right?  What if you just put a big parachute on a stick above the rear axle?  It creates no downforce, but a lot of drag.  Because the center of pressure is above the cg of the car, the parachute will create a moment tilting the front of the car up and the rear down - in effect transferring weight from the front to the rear.  The car understeers now because the normal force on the front was decreased, but total normal force was not changed, so there is no increase in total grip.  The big parachute feels like it added downforce, when all it did was add drag.  I see this a lot with big planks used as wings, and oversized spoilers.  Downforce and drag on a rear wing feel the same to the driver - you have to take data and watch end of straight and mid corner speeds to tell the difference.  

 

The rear wing by itself can be used to balance out vehicle dymanic effects - take a car that is mechanically very over-steery, and make it driveable at high speeds.  This is especially applicable to FWD cars.  On a FWD, you want 100% rear lateral load transfer distribution at low speeds so that you can put power to the ground on corner exit.  100% rear LLTD is a heavily oversteering car, and can be very hard to drive at higher speeds.  Put the big wing on the back, and you have a car that oversteers as desired at low speeds, and can put power down leaving low speed corners.  At higher speeds, when the corner exit grip is less needed (most chump cars can't burn front tires at 90 mph), the wing loads the rear axle and makes the car much more driveable. 

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For a rear wing alone, yes.  If it's not balanced with the front, or the rest of the car's behavior, you can have some really strange results.  Two things I see a lot with rear wings:

 

1:  Rear wing is not balanced by the front

      Driver complains of an understeering car.  A large rear wing behind the rear axle will attempt to unload the front tires.  It's a lever, with the rear wheels as a fulcrum.  If you can't get enough downforce on the front axle to balance the big wing, you'll just be adding drag.  The car won't be any quicker because the front tires have a reduced normal load, and saturate well before the rears.  The lower grip axle determines your cornering capability, and that big wing reduces front grip and adds drag if it's not balanced.

 

2:  Rear wing creates no downforce

      This is a fun one.  People like to use changes in the car's balance as an indicator of adding grip.  If you add a rear wing, and the car understeers, if follows that you must have added grip on the rear, right?  What if you just put a big parachute on a stick above the rear axle?  It creates no downforce, but a lot of drag.  Because the center of pressure is above the cg of the car, the parachute will create a moment tilting the front of the car up and the rear down - in effect transferring weight from the front to the rear.  The car understeers now because the normal force on the front was decreased, but total normal force was not changed, so there is no increase in total grip.  The big parachute feels like it added downforce, when all it did was add drag.  I see this a lot with big planks used as wings, and oversized spoilers.  Downforce and drag on a rear wing feel the same to the driver - you have to take data and watch end of straight and mid corner speeds to tell the difference.  

 

So if I'm understanding this correctly, in order to have an effective rear aero package you:

 

1) need an element that makes efficient use of the air going past it (DF vs. Drag)

 

2) Must ensure you are not creating more downward force than necessary to balance the front end of the car, or you will lift the front creating understeer.

 

Where things get muddy to me are:

 

Splitters typically reduce drag and lift at the front (I think). Would this be considered downforce since at a given speed there would be more "downward force" on the front axle vs no enhancement at that speed or is it just a reduction of lift? Is this the action you are trying to balance or should we be seeking a greater force on the front axle than when it is sitting at rest in the parking lot before we try to balance it with a rear force?

 

Thanks again, you're a huge asset to this forum - especially to us deskjockey bmw drivers...

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So if I'm understanding this correctly, in order to have an effective rear aero package you:

 

1) need an element that makes efficient use of the air going past it (DF vs. Drag)

 

2) Must ensure you are not creating more downward force than necessary to balance the front end of the car, or you will lift the front creating understeer.

 

Where things get muddy to me are:

 

Splitters typically reduce drag and lift at the front (I think). Would this be considered downforce since at a given speed there would be more "downward force" on the front axle vs no enhancement at that speed or is it just a reduction of lift? Is this the action you are trying to balance or should we be seeking a greater force on the front axle than when it is sitting at rest in the parking lot before we try to balance it with a rear force?

 

1.  Yes

 

2.  Yes, more than necessary to balance the front isn't necessarily bad, just inefficient.  With less than pro drivers, its usually worthwile to add a little extra rear wing to make the car easier to drive, even if it lowers the car's limit slightly.  Often you can make up the difference by making it easy for all the drivers to drive right up to the car's (slightly lower) limit, and make fewer mistakes.

 

Adding downforce and reducing lift are the same thing, it's just a question of magnitude.  Having a lot more normal force on the front wheels at speed than at rest is ideal, but having less lift than you started with is still making the car go faster.  The ideal for driveability is something close to aerodynamically neutral - the downforce on the front and rear, and the drag on the rear wing, are balanced such that the car handles neutrally regardless of speed.  If the car gets more over-steery as you go faster, you need more rear downforce.  If it understeers at high speed, you need to find more front downforce or reduce rear drag (usually by trimming the wing flatter).

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My .02 from what I think might be right (will be doing more track testing later this year to see) coupled from what I've learned from talking to a couple of people in bonafide racing series where big $$ is spent on this stuff is this: 

 

Wing = more df than a spoiler but a wing also = more drag. On a "short track" road course, go nuts with a big wing, on a medium to big track, you might want to rethink and track test the theory of "let's put on as big of a wing as possible"... with 175-300 hp being a "normal operating range" for a competitive chumpcar, I think BIG wings are complete overkill at our HP levels when looking at the complete set-up package as a whole.

 

But, this would not be the first time I've been wrong about a road course set-up. I'll be sure to not share my test results, but I guess whatever is on the car for Sept. Sebring will be a giveaway of which produced the net fastest lap times (for our particular set up & car)  :)

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Where things get muddy to me are:

 

Splitters typically reduce drag and lift at the front (I think). Would this be considered downforce since at a given speed there would be more "downward force" on the front axle vs no enhancement at that speed or is it just a reduction of lift? Is this the action you are trying to balance or should we be seeking a greater force on the front axle than when it is sitting at rest in the parking lot before we try to balance it with a rear force?

 

 

To give an even simpler answer to the part in bold, you can think of lift as the counterpart to downforce (it is "negative" downforce).  Similar to engine braking as the counterpart to engine acceleration.  Ok, now that I read what I wrote, I'm not sure my answer is any easier to understand than Will's....... (it made a lot of sense in my head!)

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Airplanes work on High aspect ratios and foils. Cars are low er and run more AOA for the most part. 

 

I know that, but you can't dispute that there are a lot of wing ribs we can purchase off the shelf of all different varieties. 

 

I am hoping to avoid skinning a wing and hoping to see what is available of the shelf. If it's a lot of Chinese tuner Honda crap. Well I guess I am making a wing.

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There's no size restriction on the wings... I think I'll make mine about 15' wide (detachable so that I can fit it on the trailer) and mounted high enough that other wings and raised shark fins don't run into it :)

I am cleaning the soda off my key board after this one.  Dying laughing at the visual  if your car dragging all the easyups down pit road

Edited by JDChristianson
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