Aero Imbalance and Possible Solutions

[FlyingLow78]

My car has the OEM front lip (PN 51192350711) and the OEM carbon trunk spoiler (PN 51712240832). I added RW Carbon canards to the front and now the car has an aero imbalance toward the nose, especially at high speed, which makes the rear feel unsettled.

Rather than augment the existing rear spoiler with a high-mount wing, I'm going to explore ways to subtly stall the canards first. I can't easily remove them since they're riveted on. I have a track day in a little over a week and it will be a good opportunity to test the changes. I want the downforce, though, so clearly I need to improve rear aero.

The problem with the rear wing as-is, is that it sits in an area that gets turbulent, detached airflow, so its performance hasn't been maximized. The effect is visible when driving through rain, because the rear window just holds droplets and they don't clear easily...except for directly behind the sharkfin antenna and a bit closer to the sides.

To improve airflow to the rear wing, I'm going to explore the use of vortex generators attached to the rear glass (because I refuse to stick them to the roof). In principle, the VGs disrupt the free-stream air passing over the roof and energize it so that it attaches to the rear glass all the way to the rear wing. An increase in downforce comes with an increase in drag, but they're removable. F1 cars have been using them for years to attach airflow over the bodywork all the way to the rear.

Here's an example of the effect:

My plan is to do some testing similar to the video first - install tufts of yarn on the car and film the effect to see what works and what doesn't. It will also help identify which areas need the most improvement.

Does anyone have any ideas what shapes might help stall the canards?

[icegrill]

Watch ford vs Ferrari again go from there.

[NYG]

So whatever you're trying to achieve with diverting fluid path or altering behavior will only end up being marginally successful. It's the nature of the beast and I deal with wind tunnels for skyscrapers regularly. There's nothing really you can do to eliminate vortex/fluttering from the canards and stalling the fluid will lower the pressures and reduce downforce which makes their presence unnecessary.

If your rear window droplets aren't moving then you don't really have any significant turbulent air flow. It means the pressure gradient from roof to trunk is basically zero, or possibly negative balancing uplift with gravity. Unless there's some sort of adhesion happening with your window and the water.

I would start with the simple steps first, lower your front springs or raise your rear springs so that the streamlines are hitting the wing. If that doesn't help, add a gurney flap to give it a booster.

The problem with that video is that there are a ton of variables at play: wind speed, wind vectors, temperature, turbulent flow from the car in front.

If you want to run through real specifics here private message me, send me a side profile of your car. It'll be easier to approximate the behavior.

[nicknaz]

How about oem Cs front and rear parts?

[maddmatth]

I'm sure the OEM lip spoiler is more for looks than downforce. To balance the front aero you've added, I think you need a proper wing spoiler. The GTS one looks nice.
BMW also has an M Performance wing spoiler, it's expensive and possibly not adjustable, probably also made more for looks than performance.

Although maybe you're better off removing the canards, if balancing out the aero to match them is going to be such a mission.

[Suds]

Generally race cars have big wings near the roof line of the car. Just saying the air flow is pretty straight back from there.

Funny thing is I’m thinking back to airfoil testing in engineering classes. You can’t bring the air down along the rear glass of the car. Your just breaking it up so that it flows straight back.

[F80Speed77]

What suspension/tire setup are you on? Maybe try to dial out the instability mechanically?

The front felt a little floaty with the oem front springs, switching to something stiffer kind of firmed up the front end feel for me.

APR GTC300 Wings will definitely had some confidence at speed tho haha.

[bimmerboyE92]

I do like the path you are going with the VGs to add just a bit more downforce. If you are equipped with coilovers, also look at raising the rear height 1/4" and see if that's enough to make a difference.

[CanAutM3]

Originally Posted by nicknaz

How about oem Cs front and rear parts?

It is indeed a proven solution.

The front lip works in conjunction with the hood to provide downforce that is balanced with the rear gurney flap, so it can become an expensive solution.

[nicknaz]

Originally Posted by CanAutM3

It is indeed a proven solution.

The front lip works in conjunction with the hood to provide downforce that is balanced with the rear gurney flap, so it can become an expensive solution.

Adding up the test and tune time and cost of after market parts and install labor, it might actually make using oem parts seem like a good deal

[FlyingLow78]

Originally Posted by NYG

So whatever you're trying to achieve with diverting fluid path or altering behavior will only end up being marginally successful. It's the nature of the beast and I deal with wind tunnels for skyscrapers regularly. There's nothing really you can do to eliminate vortex/fluttering from the canards and stalling the fluid will lower the pressures and reduce downforce which makes their presence unnecessary.

I would start with the simple steps first, lower your front springs or raise your rear springs so that the streamlines are hitting the wing. If that doesn't help, add a gurney flap to give it a booster.

Agreed that the canards make "dirty" downforce...probably strong vortices and high drag for just a little more front grip. I hesitate to attempt a mechanical solution for an aerodynamic problem that can be undone in a few minutes with a drill, and would rather see if I can balance the increase in front downforce with an economical increase in rear downforce. Besides, whatever spring adjustment I do will only change the canards' angle of attack a tiny bit, and won't make the rear wing any more exposed to the air passing over the roof.

Originally Posted by Suds

Generally race cars have big wings near the roof line of the car. Just saying the air flow is pretty straight back from there.

Funny thing is I’m thinking back to airfoil testing in engineering classes. You can’t bring the air down along the rear glass of the car. Your just breaking it up so that it flows straight back.

Agreed that the most unobstructed air is going to be up high. But you can get the air to follow a curved profile, when properly manipulated. F1 cars have been using vortex generators on their diffusers and above the radiator inlets for years, in addition to leveraging the Coanda effect for exhaust-blown diffusers. Aircraft also use them, including the F-15C to keep air attached along the underside, and several airliners to keep airflow attached ahead of the Fowler flaps.

Originally Posted by F80Speed77

What suspension/tire setup are you on? Maybe try to dial out the instability mechanically?

I have Nitron coilovers that are fantastically stable. I used them last fall for several track days and had a blast. The tires are Yoko Advan A052s on OEM 513M wheels. The unsettled feeling at the rear didn't appear until I started using canards up front - this is a problem of my own making. I think it actually improved wet handling, but the dry is a little unnerving.

Originally Posted by bimmerboyE92

I do like the path you are going with the VGs to add just a bit more downforce. If you are equipped with coilovers, also look at raising the rear height 1/4" and see if that's enough to make a difference.

As stated above, it won't change the AoA/rake of the car in any meaningful way, and raising the rear would actually increase the AoA of the canards, which could amplify their effect.

Originally Posted by nicknaz

Adding up the test and tune time and cost of after market parts and install labor, it might actually make using oem parts seem like a good deal

It does, but for $4 in plastic and a few hours of time, I was able to convince a coworker to mock up some vortex generators at work.



It's soaking wet outside, so I'm going to wait until tomorrow to test them.

[NYG]

Originally Posted by FlyingLow78

Agreed that the canards make "dirty" downforce...probably strong vortices and high drag for just a little more front grip. I hesitate to attempt a mechanical solution for an aerodynamic problem that can be undone in a few minutes with a drill, and would rather see if I can balance the increase in front downforce with an economical increase in rear downforce. Besides, whatever spring adjustment I do will only change the canards' angle of attack a tiny bit, and won't make the rear wing any more exposed to the air passing over the roof.

Why do you say that? Remember your car is fully submerged in an invisible fluid. The streamline will follow the slope of your rear window in a transitional flow. By lowering the front, you're increasing the angle of attack and allowing higher velocity streamlines to hit the wing. The angle of attack on your canards are not affected nearly as much because of their shape vs the rear wing.

[Geez3]

I've seen some GT4s only run the lowers depending track condition. Those canards would give a noticeable impact to the front with that much surface area and angle.

I think a proper rear wing is the only way to balance out. At minimum a substantial gurney flap to the rear spoiler or high kick rear spoiler.

Regarding the MPP front lip, I remember seeing BMW recommending installing the MPP spoiler in conjunction to keep aero balance. So based on I think they are designed to be more function than form.

[RSXDC5]

Your vortex generators + a high kick rear spoiler with a gurney flap, if imbalance is still there remove 2 of the 4 canards.

[FlyingLow78]

Originally Posted by NYG

Why do you say that? Remember your car is fully submerged in an invisible fluid. The streamline will follow the slope of your rear window in a transitional flow. By lowering the front, you're increasing the angle of attack and allowing higher velocity streamlines to hit the wing. The angle of attack on your canards are not affected nearly as much because of their shape vs the rear wing.

I’ve got a degree in aviation maintenance, so I’ve had a few physics and aerodynamics lessons over the years. I appreciate the knowledge you’re bringing to this thread.

Viewed as a right triangle, the F80 has a 111” wheelbase and I measured the rear wing at 36” behind the rear axle centerline. The rear wing also sits 41” above the ground. a=41, b=147, which means angle alpha is 15.584 degrees. If I adjust the rear spring perches enough to raise the rear wing an inch, alpha gains only about 0.361 degrees. Although there may be some aero benefit, it will do little to unshroud the rear wing and make it more effective, because the roofline is so much higher than the wing, and air has a tendency to detach around the rear glass unless it’s properly energized.

Originally Posted by Geez3

I've seen some GT4s only run the lowers depending track condition. Those canards would give a noticeable impact to the front with that much surface area and angle.

I think a proper rear wing is the only way to balance out. At minimum a substantial gurney flap to the rear spoiler or high kick rear spoiler.

Regarding the MPP front lip, I remember seeing BMW recommending installing the MPP spoiler in conjunction to keep aero balance. So based on I think they are designed to be more function than form.

You’re absolutely correct about the wings needing to be matched, and I recall reading the same. I’ve got the OEM lip and high-kick rear spoiler, and the car felt fine before. The canards are definitely responsible for the balance shift, and now I’m playing catch-up.

Originally Posted by RSXDC5

Your vortex generators + a high kick rear spoiler with a gurney flap, if imbalance is still there remove 2 of the 4 canards.

Can anyone point me to a gurney flap available online? It’s the simplest thing ever, but I wasn’t able to find something universals that could be trimmed to fit. I’d have to figure out a way to bond it as well (possibly betalink).

[FlyingLow78]

Test complete, video below. Because of the fish-eye effect, it may help to know that the Nurburgring sticker on the back is centered on the glass, directly behind the roof antenna. The vortex generators are visible at the top of the frame (four ABS in black, and the orange prototype made of PLA).

[CanAutM3]

Originally Posted by NYG

So whatever you're trying to achieve with diverting fluid path or altering behavior will only end up being marginally successful. It's the nature of the beast and I deal with wind tunnels for skyscrapers regularly. There's nothing really you can do to eliminate vortex/fluttering from the canards and stalling the fluid will lower the pressures and reduce downforce which makes their presence unnecessary.

If your rear window droplets aren't moving then you don't really have any significant turbulent air flow. It means the pressure gradient from roof to trunk is basically zero, or possibly negative balancing uplift with gravity. Unless there's some sort of adhesion happening with your window and the water.

I would start with the simple steps first, lower your front springs or raise your rear springs so that the streamlines are hitting the wing. If that doesn't help, add a gurney flap to give it a booster.

The problem with that video is that there are a ton of variables at play: wind speed, wind vectors, temperature, turbulent flow from the car in front.

If you want to run through real specifics here private message me, send me a side profile of your car. It'll be easier to approximate the behavior.

Originally Posted by NYG

Why do you say that? Remember your car is fully submerged in an invisible fluid. The streamline will follow the slope of your rear window in a transitional flow. By lowering the front, you're increasing the angle of attack and allowing higher velocity streamlines to hit the wing. The angle of attack on your canards are not affected nearly as much because of their shape vs the rear wing.

Changing the suspension "rake" will screw up the handling way more by offsetting the front/rear roll center proportions than it will improve the aero...

[CanAutM3]

Just a thought, but wouldn't keeping the airflow "attached" to the rear glass actually increase lift ?

[NYG]

Originally Posted by CanAutM3

Changing the suspension "rake" will screw up the handling way more by offsetting the front/rear roll center proportions than it will improve the aero...

It'll change it for sure but sometimes for the better. Changing the aero profile of a car, if done properly is such a pain in the ass process without a wind tunnel.

Originally Posted by CanAutM3

Just a thought, but wouldn't keeping the airflow "attached" to the rear glass actually increase lift ?

Yep! If there's no abnormal adhesion happening on his windows there's a good chance he's either got small vacuum or uplift.

[CanAutM3]

Originally Posted by NYG

It'll change it for sure but sometimes for the better. Changing the aero profile of a car, if done properly is such a pain in the ass process without a wind tunnel.



Yep! If there's no abnormal adhesion happening on his windows there's a good chance he's either got small vacuum or uplift.

Agreed, aerodynamics is a very empirical process and without a wind tunnel it becomes practically impossible. That's why I prefer to stick with OEM solutions when it comes to aero and avoid the aftermarket. Very few aftermarket tuners have the financial means to do proper research for aero. However, playing with the roll centers has a very direct and tangible effect on handling and grip. Anything done playing with rake for aero purposes is more than likely to result in lesser overall performance. It's best to start with an ideal suspension setup.

[NYG]

Originally Posted by CanAutM3

Agreed, aerodynamics is a very empirical process and without a wind tunnel it becomes practically impossible. That's why I prefer to stick with OEM solutions when it comes to aero and avoid the aftermarket. Very few aftermarket tuners have the financial means to do proper research for aero. However, playing with the roll centers has a very direct and tangible effect on handling and grip. Anything done playing with rake for aero purposes is than likely to result in lesser overall performance. It's best to start with an ideal suspension setup.

100% with you.

Standardized solutions for a field of engineering that outside of Bernoulli/Stokes/Euler are completely empirical end up in a really wide range of results.

[RSXDC5]

Google “JURAN Racing Gurney Flap - Universal”, this is bendable and can take to the contours of of your current rear lip or a high-kick alternative.