General BaseY-Rod Steward-Pickup-CG-Ride Height - the 1Millionths thread about it + salad cucumbers

[Dirk Steffen]

The more I read about the topic, the more I confuse myself :-( and I am searching for the uncomfortable hard facts to end the confusion.

BaseY (suspensions.ini)
- the definition to place the entire vehicle into relation to the cars center of gravity (at 0,0,0) on the world Y axis
CHECK, no questions

Rod Length (suspensions.ini)
- the length of the actual push rod - its relation to the vehicles ride height is dependent of its vector within the designed suspension system also unrelated to crazy hairdos a rough voice
CHECK, no questions

Rod length description is not right, it's vertical displacement at the wheel from the "0 point" of the suspension before deflection. Active position = rod_length - deflection.

Thanks @mclarenf1papa for the correction ;-)


Tire Radius (in tyres.ini)
straight forward and self explanatory
CHECK, no questions

Ride Height (in session/telemetry)
- exclusively displayed in session (or available through telemetry data), dependent on correct definitions of BaseY and Pickup Height and variable physics values influencing it
I think I understand the various influencing factors leading to the actual calculation of Ride height but I have a feeling I don't get the full picture.

Pickup Height (car.ini)
Now THIS is causing all my confusion.
I do not understand this part in the Kunos SDK:

"PICKUP_FRONT_HEIGHT=-0.281 ; Height of the front ride height pickup point in meters WRT cg"

What does "WRT cg" mean Washing Machine Rotational Torque in relation to Center of Gravity ?

Thanks @Stereo for the info ;-)

I think it defines the pickup point to measure ride height on the actual vehicle to the ground surface, much like Porsche defines those measurement pickup points on certain suspension parts in their motorsport vehicle handbooks.

Is this the actual dimension in the Y-axis from CoG to the actual pickup point on the car to measure my ride height? [EDIT] - yes it is.

I really would love to learn how to PROPERLY calculate these values so I can PROPERLY setup a car instead of bumbling my way through the files until the vehicle "looks right" :-( I hate "looks right".

 

[Stereo]

WRT is "with respect to"

 

[Dirk Steffen]

Fantastic, that answers all my questions then ;-)

 

[mclarenf1papa]

Rod length description is not right, it's vertical displacement at the wheel from the "0 point" of the suspension before deflection. Active position = rod_length - deflection.

 

[Dirk Steffen]

thanks guys for the clear answers ;-)
@mclarenf1papa - thats's awesome!

So I can actually use my wheel rate value from suspensions.ini combined with rod_length and wheel loads and calculate a direct correlation to the cars actual default ride height?

 

[mclarenf1papa]

thanks guys for the clear answers ;-)
@mclarenf1papa - thats's awesome!

So I can actually use my wheel rate value from suspensions.ini combined with rod_length and wheel loads and calculate a direct correlation to the cars actual default ride height?

Not wheel loads, no. Deflection dependent on sprung load only, not sprung+unsprung (wheel loads).

 

[Dirk Steffen]

;-) thanks, crystal clear it is now.

 

[aphidgod]

You can also use the Telemetry dev app to see the in-game deflection if you're lazy or just want to verify the maths.

 

[Kyuubeey]

I suggest using the app anyway, unless you have a sheet to calculate it automatically. For strut you should use it because strut has some weird automatic spring angle stuff or something in it.

 

[Dirk Steffen]

You can also use the Telemetry dev app to see the in-game deflection if you're lazy or just want to verify the maths.

Great stuff - I am slowly learning my ways around reading information from the DEV apps - I know, I am years behind on this ;-)

I suggest using the app anyway, unless you have a sheet to calculate it automatically. For strut you should use it because strut has some weird automatic spring angle stuff or something in it.

Good you mention this I believe I saw somewhere else the behavior of Struts to behave differently in some ways - got to take note of these quirks.

I just found out about the excellent Excel worksheet to get you started (I did so far everything like an old man with calculator, whiteboard, WolframAlpha app on an iPad and notes).
I have to rebuild that Excel more to my needs though and expand it + I need metric units - I cannot think in football fields, quarter pounders and such ;-) I need my millimeters ...

 

[prodarwin]

Is there a good diagram indicating these values? I am having trouble picturing them.

What I am specifically interested in at the moment is correcting the ride height on a car. I've made it 'look right' by screwing with rod length (adjusting it a crazy amount), but I hate doing that and not understanding what I am doing.

This is a car that had a spring rate of ~16k from the factory and was changed to ~60k with coilovers. When I make this change the car sits wayyyyy taller.

In reality I can adjust perch height, spring rate, and spring length. AC doesnt have corresponding values and I'm having difficulty understanding the correct method of doing these changes.

 

[Stereo]

The trouble with a diagram is AC has no concept of "spring length" like there would be irl, so they all get compacted together around the design height.

rod length is definitely the primary way you should adjust ride height though. That length is added to the current ride height to determine how far the spring is compressed (eg. if it's 2cm above 0, and rod length is 6cm, then the spring is 8cm compressed, for purposes of calculating rate*compression). In terms of secondary effects it's equivalent to changing the perch height, the soft bumpstop moves with it.

I've been advised that in car circles "spring preload" means the amount it's still compressed at full droop, otherwise I would say that the rod length is the amount of preload at design height.

 

[prodarwin]

I am trying to model this myself to comprehend.
This is the wheel & suspension at their nominal ("zero point") settings straight from the .ini files.

In game the way the wheel location is calculated (and ride height follows with it) is that wheel

Move that wheel downward by rod_length, (0.05m in this case)

then back upward by corner weight/wheel rate (376kg /6118 Kg/m in this case [60N/mm spring rate]) and it would come back up by .06m

(Yes, I know thats a mini shown with a DWB suspension, something I would like to correct once I have a better grasp of how this math works within AC)

Edit, ignore the pre-edited post if you saw that, this is exactly how it works, I just was off by a decimal point on my rod-length entry

Thank you for your patience

 

[mclarenf1papa]

then back upward by corner weight/wheel rate (376kg /6118 Kg/m in this case [60N/mm spring rate]) and it would come back up by .06m

Sprung corner weight, not total, for anyone following this at home.

 

[prodarwin]

Sprung corner weight, not total, for anyone following this at home.

Thanks, I forgot about that.

 

[Agelistrator]

Greetings and sorry for the necro but the thread is very helpfull in setting a suspension ini and i believe it's good to expand it, Would love an answer as to how to calculate :
PICKUP_FRONT_HEIGHT
PICKUP_REAR_HEIGHT
in the car ini file, is it the rim radius with tyre taken from tyres ini minus/in relation to center of gravity and ROD_LENGTH for suspension ini?
Cant figure it out, any help is greatly appreciated

 

[mclarenf1papa]

Greetings and sorry for the necro but the thread is very helpfull in setting a suspension ini and i believe it's good to expand it, Would love an answer as to how to calculate :
PICKUP_FRONT_HEIGHT
PICKUP_REAR_HEIGHT
in the car ini file, is it the rim radius with tyre taken from tyres ini minus/in relation to center of gravity and ROD_LENGTH for suspension ini?
Cant figure it out, any help is greatly appreciated

It's just the vertical distance from the car's sprung CoG to the ride height measurement point. Actual ride heights (from the ground) will work out if the basey/rod_length numbers were set correctly.

 

[Agelistrator]

It's just the vertical distance from the car's sprung CoG to the ride height measurement point. Actual ride heights (from the ground) will work out if the basey/rod_length numbers were set correctly.

Thank you very much for the answer! so if im not mistaken to accurately calculate it you go wheel radius/2 (to find F/R wheel center) + baseY (axis vertical center of gravity in relation to the center of the wheel) + assumed distance of the center of the wheel to the ride height measurement point in the vertical?
That's pretty clear but i can't find a way to calculate accurately at what distance from the f/r center of gravity the car floor should be, even in a flat floor car i believe it's up to assumptions unless you know exactly the rod length and the (per axis) center of gravity of the real car, it's still possible to do it semi accurately by eyeing it but i believe i'm missing something

Ps seems like i'm out of my depth, i'll keep reading and trying and i hope i'll understand in some time

 

[mclarenf1papa]

Thank you very much for the answer! so if im not mistaken to accurately calculate it you go wheel diameter/2 (to find F/R wheel center) + baseY (axis vertical center of gravity in relation to the center of the wheel) + assumed distance of the center of the wheel to the ride height measurement point in the vertical?
That's pretty clear but i can't find a way to calculate accurately at what distance from the f/r center of gravity the car floor should be, even in a flat floor car i believe it's up to assumptions unless you know exactly the rod length and the (per axis) center of gravity of the real car, it's still possible to do it semi accurately by eyeing it but i believe i'm missing something

Ps seems like i'm out of my depth, i'll keep reading and trying and i hope i'll understand in some time

You may be overthinking it. Math seems fine (besides minor issue/typo I fixed in bold) but may be unnecessarily jumping through hoops.

If the dot is the CoG and the car here is shown at 0 rake (as far as the physics are concerned), it's just the distances shown (and they can be completely arbitrary, they do not affect the physics at all besides for aero, which use the ride height values). You don't necessarily need to know anything else about the car.

As far as not knowing what numbers they should be (which really just means you're either not sure how high the CoG is or not sure where your suspension points mount on the car relative to the chassis itself), that's the difficulty of simulation in a nutshell. You either have data or you don't have data, and if you don't, you guess.

 

[Agelistrator]

Thank you again for your answer! everything is much clearer now just one more question for the car ini PICKUP_FRONT_HEIGHT & PICKUP_REAR_HEIGHT settings.
Is it better to put the numbers in assuming no rake or is it better to give values assuming a neutral ("stock") car rake of some mm (5-20mm)?
I believe if i understand it correctly that it's better to give values assuming no rake just based on wheel size and CoG (for cars with different wheel sizes front/rear) so that aero/drag has a neutral point to be calculated from (and playing with rod length/setup you adjust the rake).