GTS Wing's Aerodynamics

[admranger]

The one thing I've heard multiple times is that aero makes the car easier to drive. That results in confidence which results in better lap times.

Not discounting downforce, but take that for what it's worth. Smash the like button.

[Bartledoo]

Easier to drive because there is more grip, and because most of the time aero is put on with a bias to the rear.

If more grip doesn't result in better lap times (within a reasonable range of added drag and balance) then I dunno.

I'll smash yer like though.

[rhyary]

I really like the look of the swan neck rear wing.
I don't know if this is necessary, faster or will give me more confidence, but I like it.

I talked few times to APR and the main issue would be the mounting and generating enough downforce on the front on a street car. They wanted such a big splitter on the front that it makes it not practical for me. However, I also saw huge wings and front splitter that was not bigger than mine. I also not willing to lose my trunk space.

Originally, a wing was this year project, but the more I dug into the topic, the less clear the benefit seemed and the more complicated the trade offs became. So I gave up and did more SPL and brake ducts, solid diff and experimenting with trans mounts

This can be one solution. But I have not seen anyone doing it.

http://www.racingparts-bmw.de/start/..._gt4_e92_3.htm

My hunch tells me that next winter I will again drilling into the issue.

[Bartledoo]

That wing doesn't look much bigger than yours (also they have no info about it) and the splitter is too small with no tunnels. If you don't want a bigger splitter I'd just stick with what you have and buy some real tires instead.

[Navman56]

There’s some great discussion in this thread! I’m usually unqualified to comment on technical threads, but as an aerodynamicist this seems right up my alley, hopefully I’m not too late to the party 😊 Here are my 2¢:

A single element wing can only handle about 10-15° degrees of angle before it begins to stall out. In graphical form, the relationship looks like this:



However, angle of attack is not just a function of the bodywork.

Consider the following example, where the wing itself is at 10° of angle, but the incoming flow is coming down at 15°. What is the effective angle of attack of the wing?



It’s easier if I ‘rotate’ the image counterclockwise to straighten out the incoming flow angle.



So, if the flow is already at a 10° angle before it even meets the wing, if you still crank the wing to 15° of angle, the wing thinks its at 25° of angle, well beyond the stall point.

Some have picked up on this already, but by virtue of having to flow over the roof of a car and then down the rear glass, the flow impinging on the rear wing already has a substantial incident angle of attack.

With this in mind, the Schirmer wing is set to an overly aggressive angle of attack. There has been some discussion about this being done on purpose to have the wing act more like a spoiler. This is incorrect for a handful of reasons, but for sake of not getting too far into the weeds, I will not go into it. A properly designed wing >> spoiler.

How big of a deal too much angle is depends on the cross-sectional design of the wing, or the ‘airfoil’ shape. Some designs are more robust through stall than others (as shown by the various possible curves below). I don’t know what the cross-sectional shape of the GTS wing is, so I can’t make any judgements on how it will behave if you overdo it on angle.



The Sport Auto magazine screenshot has some good information hidden in German. I’ve translated the contents below:



A few notes:

• When a wind tunnel does not have a rolling road, the front downforce numbers should be taken with a grain of salt. Additionally, front downforce is extremely sensitive to front ride height. The rear downforce numbers are trustworthy enough, but unfortunately no specific angle of attack is quoted.
• I see a lot of comments about downforce only being produced after ‘X’ speed, the favorite number in this thread being ~100mph. This is incorrect. Downforce (and drag) follow a Velocity^2 relationship. So, at 200mph, bodywork produces 4 times the downforce and drag as at 100mph, and at 100mph, the bodywork produces 4x the DF and drag as at 50mph, and so on and so forth. So, the effect of aerodynamics diminishes exponentially with speed, but this doesn’t mean it’s negligible at lower speeds.

A race car spends a larger percentage of the lap in slower corners than in fast corners. A 5% improvement in a slower corner will yield a larger benefit than a 5% benefit in a fast corner. For any of you that enjoy looking at data, give this a read. Willem is a smart guy with some impressive credentials https://www.linkedin.com/pulse/formu...s-willem-toet/

• With the V^2 relationship in mind, I plotted the Sport Auto data below and showed how it scales with vehicle speed. Because the normal E92 produces lift, and the GTS produces a reasonable amount of downforce, the GTS has a considerable amount of extra load on the tires vs the vanilla E92.



I tried to restrain myself and still ending up typing a massive essay, so I’ll end it there for now. Happy to answer any questions anyone may have.

[berns]

Originally Posted by Navman56

There’s some great discussion in this thread! I’m usually unqualified to comment on technical threads, but as an aerodynamicist this seems right up my alley, hopefully I’m not too late to the party 😊 Here are my 2¢:

A single element wing can only handle about 10-15° degrees of angle before it begins to stall out. In graphical form, the relationship looks like this:



However, angle of attack is not just a function of the bodywork.

Consider the following example, where the wing itself is at 10° of angle, but the incoming flow is coming down at 15°. What is the effective angle of attack of the wing?



It’s easier if I ‘rotate’ the image counterclockwise to straighten out the incoming flow angle.



So, if the flow is already at a 10° angle before it even meets the wing, if you still crank the wing to 15° of angle, the wing thinks its at 25° of angle, well beyond the stall point.

Some have picked up on this already, but by virtue of having to flow over the roof of a car and then down the rear glass, the flow impinging on the rear wing already has a substantial incident angle of attack.

With this in mind, the Schirmer wing is set to an overly aggressive angle of attack. There has been some discussion about this being done on purpose to have the wing act more like a spoiler. This is incorrect for a handful of reasons, but for sake of not getting too far into the weeds, I will not go into it. A properly designed wing >> spoiler.

How big of a deal too much angle is depends on the cross-sectional design of the wing, or the ‘airfoil’ shape. Some designs are more robust through stall than others (as shown by the various possible curves below). I don’t know what the cross-sectional shape of the GTS wing is, so I can’t make any judgements on how it will behave if you overdo it on angle.



The Sport Auto magazine screenshot has some good information hidden in German. I’ve translated the contents below:



A few notes:

• When a wind tunnel does not have a rolling road, the front downforce numbers should be taken with a grain of salt. Additionally, front downforce is extremely sensitive to front ride height. The rear downforce numbers are trustworthy enough, but unfortunately no specific angle of attack is quoted.
• I see a lot of comments about downforce only being produced after ‘X’ speed, the favorite number in this thread being ~100mph. This is incorrect. Downforce (and drag) follow a Velocity^2 relationship. So, at 200mph, bodywork produces 4 times the downforce and drag as at 100mph, and at 100mph, the bodywork produces 4x the DF and drag as at 50mph, and so on and so forth. So, the effect of aerodynamics diminishes exponentially with speed, but this doesn’t mean it’s negligible at lower speeds.

A race car spends a larger percentage of the lap in slower corners than in fast corners. A 5% improvement in a slower corner will yield a larger benefit than a 5% benefit in a fast corner. For any of you that enjoy looking at data, give this a read. Willem is a smart guy with some impressive credentials https://www.linkedin.com/pulse/formu...s-willem-toet/

• With the V^2 relationship in mind, I plotted the Sport Auto data below and showed how it scales with vehicle speed. Because the normal E92 produces lift, and the GTS produces a reasonable amount of downforce, the GTS has a considerable amount of extra load on the tires vs the vanilla E92.



I tried to restrain myself and still ending up typing a massive essay, so I’ll end it there for now. Happy to answer any questions anyone may have.

Thanks for the awesome info.

I suppose the one thing you left out is... well then, is the GTS aero a substantial upgrade and, does it seem its effects will be noticeable and beneficial to laptimes, or in the grand scheme of things, is it just a slight upgrade to a stock car that has lift rather than downforce?

[rhyary]

^ great contribution.
Can you help us set the AOA?
if the air flow comes down, should AOA be parallel to the road and still have "wing" for the down slope air and act as a wing rather than spoiler?

[Epoustouflant]

Nominated for post of the year.

Hate to ask, how about the GT4 wing? 140x19cm

[//steve\\]

Originally Posted by berns

Thanks for the awesome info.

I suppose the one thing you left out is... well then, is the GTS aero a substantial upgrade and, does it seem its effects will be noticeable and beneficial to laptimes, or in the grand scheme of things, is it just a slight upgrade to a stock car that has lift rather than downforce?

My gut says on a car like yours it's going to be minimal benefit. I don't see it being something that drops a ton of time.

[berns]

Originally Posted by //steve\\

My gut says on a car like yours it's going to be minimal benefit. I don't see it being something that drops a ton of time.

My gut says the same... but I'll probably always be curious. Just not curious enough to spend the time, drill the holes and spend $2500+ to find out.

[rhyary]

Originally Posted by berns

My gut says the same... but I'll probably always be curious. Just not curious enough to spend the time, drill the holes and spend $2500+ to find out.

I want to be you.
When I get curious it is always expensive :-(

[Navman56]

Originally Posted by berns

Thanks for the awesome info.

I suppose the one thing you left out is... well then, is the GTS aero a substantial upgrade and, does it seem its effects will be noticeable and beneficial to laptimes, or in the grand scheme of things, is it just a slight upgrade to a stock car that has lift rather than downforce?

In my opinion, regarding lowering laptime, investing in aero is the best bang-for-the-buck, second only to more seat time.

For an amateur driver, these two things are about equally important:
• More car capability
• Larger margin of error between setting a fast lap and crashing out

With better braking, more speed carried through corners, earlier acceleration out of a corner, and a more planted car in general, downforce gives you both things (the effect of additional confidence in the car is hard to quantify, but very important).

However, there’s a spectrum for performance/$, and investing in a GTS aero package is on the lower end of the value spectrum imo.

I’m not quite sure why BMW and AMG insist on such lukewarm ‘track oriented’ models. A comparison to the gold standard of track-oriented cars, the Viper ACR, demonstrates this point well. There’s nothing particular genius about its design. FCA just fully committed to it.




The ACRs wing is much wider, and it splits into a two-element design at the outboard ends. A two-element design produces a preposterous amount more downforce than even the best single element design.

Just for fun, here's something I pulled from one of my CFD cases. It's showing the static pressure field around a single element wing & and a very conservative multi element wing. More blue = lower static pressure, more orange = higher static pressure.




At the front, among other gadgets, the Viper has vents in the hood to relieve underhood pressure, which is a massive source of lift in almost every road car. The M3 has no such provisions, and the splitter is…lame. As a result, the Viper ACR makes 10x (!) the downforce of the GTS.



To demonstrate the difference this makes, we can do some quick napkin math. Imagine a car on a skidpad, doing a constant radius turn in steady state conditions. The tires produce inward lateral force, but that’s counteracted by the outward centrifugal force trying to push the car out of the corner. The more downforce acting on the car, the more lateral force the tires can produce, increasing the speed the car can hold through the constant radius corner.

Keeping all parameters of the car the same and just changing its just downforce levels, we can compare a regular E92, GTS, and an E92 with Viper levels of aero. Quite a spread. I should reiterate that this is quick napkin math, and it shouldn’t be looked at as gospel. If anything, this is still vastly underpredicting the benefit of downforce.



Now, the ACR had to make a few concessions due to its massive aero focus. The front springs on the ACR are three times the stiffness of the base car, and the rear springs are over double the stiffness. They have to be, to keep the car from rapidly bottoming out due to all the aero load. Probably makes for a harsher ride, but again, if you’re going to make a track day car, I can appreciate a full-on commitment to it.

I think the aftermarket will better serve owners looking for the best performance/$ much better than the GTS package. It’s really not that hard to make downforce when you really try. A NASCAR CUP car, which isn’t necessarily known for being the pinnacle of aerodynamics technology, makes 50% more downforce than even the Viper ACR, and that’s with rules that try to handcuff the engineers at every step.



I know I haven't answered a few other questions, but I'll try to get to them in time.

[rhyary]

^ Please keep going.
This is what this tread needed all a long

[leftfootbr8king]

Is the nascar's large amount of downforce owed to its flat bottom and low ride height? It's certainly not from a BFW or insane front splitter.

[//steve\\]

One giant problem with aero though is it does mask bad driving/habits. Much better off maxing the car out without aero and then adding it.

This was super common in the S2000 world.

[rhyary]

Originally Posted by //steve\\

One giant problem with aero though is it does mask bad driving/habits. Much better off maxing the car out without aero and then adding it.

This was super common in the S2000 world.

This is true with everything we do.

[berns]

Originally Posted by leftfootbr8king

Is the nascar's large amount of downforce owed to its flat bottom and low ride height? It's certainly not from a BFW or insane front splitter.

All that paired with the fact that there are virtually no holes or gaps for air to sneak into.

[rhyary]

I did a lot of thinking and reflecting on what wing I should go with and why. Aesthetics are important to me and this is a subjective matter.

What convinced me to make the leap to a more effective wing than the GTS is analysis of my braking coming into turn 1 of LRP, PMP and NYST. Ray Phillips built for me a math channels that show ABS on and off and the duration that the ABS is on.

This shows the rears are longer on ABS than thr fronts at 130-120mph. This is a speed that wings already produce a sizable downforce.

I dislike that look of the 3D wings. I like clean straight lines. So a 2D wing. But a 2D wing can't sit low on the trunk. It wants to be in clean air.

That calls for the Hard Mounts that connects to the rear bumper.

So this is what I am pursuing. It will also help balancing the car better. After taking a lot of weight from the car, it is a tail light when the gas tank goes below half. This is because more of the easy weight off on the E92 M3 come from behind the driver. With a supercharger and not much that can take of the front, the wing can restore some balance at 80mph and above with out going extreme splitter on the front.

Attached is coning to big bend at LRP on the brakes
.

[DRLane]

Originally Posted by rhyary

I did a lot of thinking and reflecting on what wing I should go with and why. Aesthetics are important to me and this is a subjective matter.

What convinced me to make the leap to a more effective wing than the GTS is analysis of my braking coming into turn 1 of LRP, PMP and NYST. Ray Phillips built for me a math channels that show ABS on and off and the duration that the ABS is on.

This shows the rears are longer on ABS than thr fronts at 130-120mph. This is a speed that wings already produce a sizable downforce.

I dislike that look of the 3D wings. I like clean straight lines. So a 2D wing. But a 2D wing can't sit low on the trunk. It wants to be in clean air.

That calls for the Hard Mounts that connects to the rear bumper.

So this is what I am pursuing. It will also help balancing the car better. After taking a lot of weight from the car, it is a tail light when the gas tank goes below half. This is because more of the easy weight off on the E92 M3 come from behind the driver. With a supercharger and not much that can take of the front, the wing can restore some balance at 80mph and above with out going extreme splitter on the front.

Attached is coning to big bend at LRP on the brakes
.

Quite the change in perspective!

[Bartledoo]

Lol I thought you wanted less understeer. So you're gonna throw a big wing on with no front splitter?? This is all very confusing, especially when you add in wider, stickier tires in the rear vs the front.

Also, if you are doing a GT-250 wing you can just mount it directly to the trunk, unless you have a carbon trunk without reinforcements. Mounting further back like that hard setup in the photo will give you even more understeer.

[leftfootbr8king]

Is the conclusion that gtc 300 wing is the more effective wing vs gt250 at gts riser heights?

[rhyary]

Originally Posted by Bartledoo

Lol I thought you wanted less understeer. So you're gonna throw a big wing on with no front splitter?? This is all very confusing, especially when you add in wider, stickier tires in the rear vs the front.

Also, if you are doing a GT-250 wing you can just mount it directly to the trunk, unless you have a carbon trunk without reinforcements. Mounting further back like that hard setup in the photo will give you even more understeer.

Most of my understeer discussions revolved around low speed mechanical grip.

Here we are addressing 80mph and above grip.

I don't have high speed understeer issues. My apologies if I was not clear.

For low speed mechanical grip understeer, I am learning to be patient with the throttle and trail brake as long as I can to keep the weight on the nose. At low speed it is easy to induce understeer and it is not easy to distinction between the car understeer and driver induced understeer. But I have help and I am working on this. Regardless, it is not related to aerodynamics and wings discussion.