Physics Center of gravity, inertia of rotating car etc

[Jach]

Does that mean does not have a concept of a given car spinning with a mid-engine layout has less inertia than a car with engine and transmission at the outer ends spinning at the same frequency?

It basically means you don't know what these figures mean...

 

[007DBR9]

AC calculates the MOI from those dimensions, so they need to be representative (the dimensions of the car are not).

KS cars shouldn't be the gold standard for doing things... It's nowhere near close enough, and it gets worse the more compact the car is internally. Some of the KS racing cars have up to 90% error per axis in their sprung MOI. That is, their MOI is 90% higher than the correct value.

So what I'm getting at is that there are so many factors that go into figuring out what uniform density box would fit the car, that getting something 100% accurate is almost impossible, because of factors like cars obviously not being a box, different parts of it being heavier than others, ie engine transmission and fuel tank being far heavier than bodywork, as well as things like the roof especially in race cars being vastly ligher than the lower part of the car. at least width should be almost only effected by the inertia being calculated from a complete box.

Side question which I assume it doesn't, but if there's more fuel in the car, does the inertia change on it's own based on where the fuel tank is positioned in car.ini?

 

[Stereo]

Fuel affects inertia by being a mass placed at the location from car.ini. I don't know offhand if it has a specific dimensional inertia box as well, I'd guess no. Even a point mass that's rigidly fixed to the body affects its overall inertia so it's just implicitly added that way.

 

[mclarenf1papa]

So what I'm getting at is that there are so many factors that go into figuring out what uniform density box would fit the car, that getting something 100% accurate is almost impossible, because of factors like cars obviously not being a box, different parts of it being heavier than others, ie engine transmission and fuel tank being far heavier than bodywork, as well as things like the roof especially in race cars being vastly ligher than the lower part of the car. at least width should be almost only effected by the inertia being calculated from a complete box.

Side question which I assume it doesn't, but if there's more fuel in the car, does the inertia change on it's own based on where the fuel tank is positioned in car.ini?

Essentially, yes. Unless you have the measured data (which you can solve for the dimensions of the box with), it’s very hard to get close.

 

[Tennisbiki]

The equation for moment of inertia about Yaw direction, (is it Y or Z in AC?) is

I-yaw = 1/12 * Mass * ( a^2 + b^2)
Now I think a and b are the x and z value.
Your thoughts?

In general, how do you assigned the X, Y, Z values in the beginning? Start with car's overall dimension width, height, and length and divide by say 2?

 

[Agelistrator]

Greetings and sorry for the necro, have a question in regards to the inertia box, on certain mod cars the inertia box instead of starting from the "road" as it does in most kunos cars it starts from the chassis bottom/wheel hub of even higher, as i try to "fix" in the best way that i can mod cars that the author allows it, is there any way to correctly position the inertia box or is it based on the 3d model only ? (i don't yet know how to use a 3d editing program)

 

[Stereo]

There's no "correct" position for the inertia box, so there's nothing to fix... if you have accurate inertia numbers it will never be the same size as the visual model, so it will never fit its outside dimensions. I don't know what tool you're using to draw an inertia box either, probably it's misleading you as to its importance.

 

[Agelistrator]

There's no "correct" position for the inertia box, so there's nothing to fix... if you have accurate inertia numbers it will never be the same size as the visual model, so it will never fit its outside dimensions. I don't know what tool you're using to draw an inertia box either, probably it's misleading you as to its importance.

Thanks for the answer, aye i completely understand the "not fitting" part!
Using content manager on some mod cars, instead of the inertia box starting from the ground as it does for most if not all kunos cars, it may start at bottom of the car/top of the wheels etc, i don't have a problem with the length/width part, i try to adjust it depending on engine position/weight distribution etc, i just want the inertia box starting point to "move" to the bottom of the wheels so i can make "correct" assumptions, here are two examples, notice how on the kunos 911 the box starts at the bottom of the wheels, while on the mod audi at the bottom of the chassis

 

[Stereo]

Probably have to get after the author of Content Manager to find out how that works, there's nothing in AC data to locate the inertia box because it's not that kind of box.

 

[Agelistrator]

Probably have to get after the author of Content Manager to find out how that works, there's nothing in AC data to locate the inertia box because it's not that kind of box.

Ah thanks, i thought that it could be a visual bug as there is no way to move the starting point, so i assumed that it was based on the 3d model, although i remember in the past, i downloaded a very low quality porsche 944 mod, it appeared in content manager that the inertia box started above the wheels but the value seemed "correct" at about 1.20m for the height, that car would tip over super easily!

Edit : found a particularly bad (or good) example, wonder what causes this and if its just visual in content manager!

 

[mclarenf1papa]

Ah thanks, i thought that it could be a visual bug as there is no way to move the starting point, so i assumed that it was based on the 3d model, although i remember in the past, i downloaded a very low quality porsche 944 mod, it appeared in content manager that the inertia box started above the wheels but the value seemed "correct" at about 1.20m for the height, that car would tip over super easily!

Edit : found a particularly bad (or good) example, wonder what causes this and if its just visual in content manager!

Inertia box is centered on the center of gravity of the car. All there is to it, but visual dimensions don't tell you much. No idea what CM does to display it.

 

[Agelistrator]

Inertia box is centered on the center of gravity of the car. All there is to it, but visual dimensions don't tell you much. No idea what CM does to display it.

I researched it a bit, Ιt seems that the problem is tied to hasty 3D model conversions/low quality mods with inaccurate dimensions of the 3D model.
For example the Koenigsegg in the car ini had a positive graphic offset in height, most if not all kunos/quality mod cars are negative there (suspension basey @ -0,55 in that case)
I made changes to the graphics offset in the car ini and ended up with the "correct" inertia box below the start of the wheels, wonder if keeping the inertia box starting above the wheels
and adjusting it accordingly can "accurately" model inertia or if the whole 3D model has to be adjusted/changed

 

[mclarenf1papa]

I researched it a bit, Ιt seems that the problem is tied to hasty 3D model conversions/low quality mods with inaccurate dimensions of the 3D model.
For example the Koenigsegg in the car ini had a positive graphic offset in height, most if not all kunos/quality mod cars are negative there (suspension basey @ -0,55 in that case)
I made changes to the graphics offset in the car ini and ended up with the "correct" inertia box below the start of the wheels, wonder if keeping the inertia box starting above the wheels
and adjusting it accordingly can "accurately" model inertia or if the whole 3D model has to be adjusted/changed

Graphic offset doesn't mean anything, it's just where the model is placed in the 3d relative to the origin when it's exported, 0 relation to the quality of the car. CM then is just putting the inertia box in the wrong spot (relative to model instead of physics). Regardless, as I said before, I wouldn't use the visual box for anything. It's nearly meaningless.

 

[Agelistrator]

Graphic offset doesn't mean anything, it's just where the model is placed in the 3d relative to the origin when it's exported, 0 relation to the quality of the car. CM then is just putting the inertia box in the wrong spot (relative to model instead of physics). Regardless, as I said before, I wouldn't use the visual box for anything. It's nearly meaningless.

Thanks very much for the answer! Once again you cleared everything up

 

[Tennisbiki]

For those who care, I just discovered that CM showroom actually shows this box visually. Thus to change the inertia of a car, or the dimension of this box. Open a text editor and car.ini. Increasing X will increase the width, Z increases length, and Y increase the height.

From a quick survey of many Kuno's car, the width is typically a little less than the width of the whole car, even less for open-wheeler. (Because the inertia in AC counts only for the sprung weight not the wheels). The height mostly closely matches the height of the car. And the length is a little less than the overall length.

Keep in mind that the equation of MOI for a given axis is something like C x ( A^2 + B^2). So a changing engine from tail end of a car to middle of car, one would reduce Z, but the calculation is Z square, so be mindful of that. An inline engine changed to boxer, one reduces Y value.