Physics Change a car Mod‘s aero drag

[GaryUK]

I am making an “upgrade” to a car. It’s a low-power car, so it’s sensitive to aero drag and weight. One of the changes is removing the glass windscreen (road use) and replacing it with a small wind deflector. This saves weight but more importantly lowers the car’s frontal area and aero drag.
I don’t understand how to reduce aero drag in an AC mod. File aero.ini has “wings” although that seems a generic term. The aero.ini file has “wings” named Body, Front and Rear. The span and chord of “Body” comport with the width and height of the frontal area of the real-world car. However, I don’t understand the difference between “Body” and “Front”, nor how to make adjustments to these three “wings” in the case of removing a windscreen. Is it reducing the surface area (via smaller Cord):

1. On Body?
2. On Front?
3. On both?

Related question: are the aero lookup tables - .LUT files - specific to a car, or are they generic? Can the change I’m hoping to make be accomplished without editing the .LUT files? What software is required to edit .LUT files?

Thanks for any explanations you can provide.

 

[Stereo]

In general the intended usage is body for overall body drag, and front/rear for separated front/rear lift. (centered on front splitter, rear wing) That allows tuning of front-rear aero balance in a manner similar to real cars, as a common way of measuring lift is front/rear axle loads.

.lut files are text, you can open them in Notepad++ or anything else that does ini files. Each line in the .lut maps an input value to an output value; the file itself doesn't say what either of these values means, but for aero, input is either angle or ground height ("gh"), and output is a multiplier coefficient. So for each wing to determine its overall drag, you take the area times the ini coefficient times any relevant lut coefficients.

lut should be specific to a car but are more likely to be generic.

 

[GaryUK]

thank you Stereo.
Is aero.ini only calculating the aero drag restraining force (as a function of velocity), or is it also calculating a downforce/lift (as a function of velocity) for purposes of tyre grip? Your first paragraph seems to indicate the latter, while your second paragraph suggests the calculations determine an aero-drag for each "wing", and doesn't mention a downforce. there'd need to be two calculations, one for the vector in Z (downforce) and one for the negative force vector in X/Y

What are the units used in these calculations? Are the units standard within Assetto Corsa? Is there a developer's guide for the aero, engine, suspension, brakes, etc.? I'm very new to this but eager to learn.

 

[Deputy Dawg]

I'm looking for the same information.

 

[Deputy Dawg]

So the line you're looking for in aero.ini is:

CD_GAIN=0.9 ; Coefficient of drag multiplier (for easy fine tuning)

Just moving it 10% up or down will make a considerable difference.

 

[Deputy Dawg]

Thanks for all the tips BTW, I wouldn't have found this information by myself. I'd already given up. Lol.

 

[Stereo]

CD is coefficient of drag, CL is coefficient of lift, same process applies to both. As a matter of convention I believe positive lift is actually a down force. Area is in square metres (or rather, it's a length and width) and everything else is dimensionless so far as I know; to get to an actual force you use the normal equation for drag/lift, which is as per wikipedia, 1/2*rho*v^2*C_D*A. The stuff in aero.ini is either A (length+width) or C_D (everything else)

 

[Marcus1423]

Does this apply to a car with no wing or splitter? I am moddifying the body of a car and am wondering if I have to run sim flow to get the new CD to put in the .ini

 

[Tennisbiki]

My suggestion is to look up this Assetto Corsa Modding Manual. The pdf is big and bulky and not user friendly but actually contains a lot of useful information in how the codes works in AC. https://www.overtake.gg/threads/ac-modding-manual-3-0.252064/. Once you downloaded the huge pdf file, search for "aero.ini".
I feel that I need to read it several times and play with the codes and test them and watch the wing app while driving to get a feel of it. It takes awhile.

These are the things I learned:

1. AC models the aero forces using the wings and fins. There can be one or none, or as many wings/fin as you want. Each wings can have drag, forces in Z direction front and back, and lift, Y direction or up and down. Each fins also can have drag and lift, X direction left and right. Drag, CD, can be affected by AOA, angle of attack, and GH, ground height. Yes, it gets complicated.
2. Essentially each force is affected by [multiplier] x [area (span x cord)] x [ value in lut file (CD or LD at different angle or height]
3. Real world data for specific cars is not easy to find. CD and cross-section area and top speed can be found. But drag or lift curve over different AoA or ground clearance are rarely available. But as Stereo mentioned "lut should be specific to a car but are more likely to be generic." the lut files are more or less generic because an object in air stream behave more or less in a similar fashion. Thus AOA or GH lut may be kind of generic.
4. Use CM's custom showroom to visualize the "wing". There are huge range of ways to model the aero among Kuno and mod cars. Most Kuno's street cars has 3 wings. Stereo's very nice 912 has 1 wing. Abarth 1000 mod pack uses 2 wings. And the modern formula cars has many wings including spliters and diffusers.
5. I gets to understand why the 3-wing is a good way to model most street cars. Use Body to model just the drag, and use front and rear to balance the lift. To simplify things for slow street cars, I ignore the GH and AOA by making lut values constant over the range.
6. Use wing app during driving mod to observe how the forces change at different speed, and balance of lift/downforce.
7. Check top speed of the car, to match the real world data, to verify whether your power and aero modeling.

How do you model their aero?
Good luck in the rabbit's hole.