Aerodynamic forces

silente

Hi all,

this is my first post in this forum. i have found very interesting and useful info till now, but nothing really connected to my doubt. So hope you can help me.

I have modeled a F3 car on the base of data i had from my previous experience as race engineer.

I have found very good insight reading throug the rf modding tutorial from Bristow when it was on the RSC forum, but still have some confusion about how rf calculate aero forces.

I have modeled correctly downforce for different wings/rideheight settings and i am now quite happy with that, althoug it has been somehow a trial and error procedure, since i didn't find the right info to do it "scientifically".
Now i have the same problem for drag. I could make it right trying different settings untill the speed is finally correct, but i would like to understand where i am doing wrong instead and to be able to do it properly next time.

Doing the calculations as show in Bristow posts and checking them with Carfactory, i should have a too low drag force in all the configurations; but instead my speed is too low, although the engine power is correct. So i guess i am missing something in the aero modeling, since with the forces i find from my calculation, i should have even a too high top speed instead.

My suspect is that there is something wrong with the ride height also, but honestly i don't think it could justify the big difference i am finding. Or maye anykind of drivetrain efficiency factor i am ignoring? i didn't find any number referring to it in my files.

can anybody point me on same concrete reference on how to deal this topic or help me to shed some light?

I think i have summed up all the drag influencig factors, because my results are anywaz very close to that showd by carfactory; the problem is that also carfactory results seem to be wrong...

Thanks!
 
Maybe the rolling resistance value is too high... what kind of topspeed difference are we talking about ?
 
I wouldn't trust Carfactory with anything. I just took the time to move my suspension into a proper program for testing bump steer and the results differ immensely from that of K!, something on the order of 4x. On top of that, Roll center calculations have also been found to be incorrect.

Trust your instincts.
 
Yep, I also noticed the same with roll centers, the results weren't exact at all.
 
@silente

Total drag comes from:
- aero drag
- drivetrain (mechanical drag)
- rolling resistance from tyres

CarFactory calculates aerodynamics as if you do a static aerodynamic test (without rolling floor). If you have data from wind tunnel with rolling floor, you have to use rFactor itself to check, if forces are OK.

Check, if mechanical drag is OK. If you can't measure that on real F3 or just have no data, you can use those as your baseline values:
- ClutchFriction=3.0
- for non-driven wheels: FrictionTorque=1.0
- for driven wheels: FrictionTorque=8.0

The same goes for inertia:
- ClutchInertia=0.025
- non-driven wheels: SpinInertia=1.2
- driven wheels: SpinInertia=1.5

If you know all the weights for drivetrain parts, tyres and rims, it's possible to calculate inertia in an easy way. It won't be *perfect* but still should be pretty close (i think, it won't be off by more than just a several %).

Remember to set proper weights and inertias into PM (suspension) file. SpinInertia in HDV overrides inertia in direction of rotation from PM.

As for rolling resistance - if you have data packages for F3 tyres, you probably have info about rolling resistance (kg/ton). From there, you can calculate values you then put into rFactor.
 
Hi guys,

thanks all for the info.

first point is, the general behaviour of the car is very good. It fix very well to the real car data and also the driving behaviour seems to be correct. I have used some "non common" setting for the model coming from the knowledge i had (and from real data) and they worked quite well.

Regarding the drag, my biggest point is that calculating all the drag force contributions as shown in tutorial and guides and as done by Carfactory the results are wrong. So:
1) the formulas i am using are wrong;
2) there is some other drag contribution i am ignoring;
3) there is something influencing the final result that i am ignoring (like ride heights, as i said: they are already in the calculation, but maybe the ride heights i read ingame are not the one the code use for calculations?).

The point to me it´s not only the top speed, which can be also related to the power at the wheel (so engine power and drivetrain efficiency) and could be tuned quite easily, but also the fact that the formulas i have seem to produce inconsistent results.

I am quite sure the rolling resistance is at least in the ballpark, so it should not be the point.

Do you guys have some concrete reference for aeroforces calculation starting from the numbers we insert into the HDV file?

What about drivetrain efficiency (not inertia)? Do you know if there is something that, given a certain engine power, produce at the wheel a certain drop (like: Power at wheel = 0.9xx*engine power)? Normally gearbox efficiency is never 100%.

As i said, i believe overall my model behaves in a very realistic way, i am pretty happy about that. But i would be sure to be able to use rfactor "language" properly.

Out of it, you mentioned rolling and no rolling wheels wind tunnel aero data. Do you have any data on how big the differences should be one to the other?

Thanks a lot for your help.
 
just saw that LesiU probably already told something about drivetrain efficiency. I guess this voices:

- ClutchFriction=3.0
- for non-driven wheels: FrictionTorque=1.0
- for driven wheels: FrictionTorque=8.0
Click to expand...

refers somehow to the mechanichal loses you have in your drivetrain system and on your hubs.

Do you know anything ele connected somehow to mechanical losses?

Still hope you can also help me to figure out how to take out drag from rF parameters.

Thanks!
 
As for gearbox efficiency, you can add a line under [engine] in .hdv like TotalTorqueMult, dont remember exact syntax, and set that to 0.9 or whatever. Or you can just multiply the torque curve in rf engine physics editor, most modders prefer that.
 
thanks Osella!

any idea of how the other parameters from LesiU work?
 
LesiU said:
You probably want to read whole thread:
https://community.racesimcentral.net/showthread.php/7108-Inertia
Click to expand...


Thank you LesiU, but i think we talk about different topics.

Inertia is working when accelerating something, losses are always present.

Do you know where mechanical losses or overall efficiency are playing a role? Actually what i am looking for is something that:
1) Produce some friction i am not considering at the moment and create the "drag" that exist in the car but it's not shown in my aero calculation
2) Aero drag contributions i am neglecting or wrong formulas.

Anyway, this forum is really cool. I was looking for a place like that where to discuss about modding and for people to share this interest.

Thanks for your feedback!
 
From that thread:

LesiU said:
(...)

EDIT:
EngineInertia is about total inertia of all engine parts, like guys already said, but I'd also add to that inertia from gearbox when in neutral... if you have such precise data. Because if you want to just try to match videos from the internet, then there is a chance most of them are recorded in neutral without engaged clutch (so, inertia from gearbox in N is already there).

As for drivetrain inertia distribution (in the current rF2 version), I see it like this:
- ClutchInertia: clutch itself + [main shaft from engine to gearbox, if applicable] + gearbox (when in gear - I'd take something like an average from inertia values from 3 to last gear and use it),
- SpinInertia (for driven wheels): [main shaft from gearbox to diff, if applicable] + (differential inertia/2) + halfaxle + wheel + disc bell
- SpinInertia (for non-driven wheels): wheel + disc bell

- ClutchFriction: clutch itself (very small though) + gearbox friction (when in gear - I'd take something like an average from friction values from 3 to last gear and use it),
- FrictionTorque (for driven wheels): (<gearbox friction when in N> / 2) + (<diff friction> / 2) + ball bearing friction (loaded)
- FrictionTorque (for non-driven wheels): just ball bearing friction (loaded)
Click to expand...
 
LesiU said:
From that thread:
Click to expand...

I just have "ClutchFriction" in my files, not the other parameters ("FrictionTorque"). Do we need to consider them into clutch friction?

Could you please help me to understand how this parameter work? Is this expressed in Nm? Is it a constant value that is always applied (as subtraction) to the torque output from the engine or it works a different way?

On the other hand, do you guys normally use the "TotalTorqueMult" Osella was referring to?

Thanks!
 
Odp: Aerodynamic forces

FrictionTorque is for each wheel.
No, i don't use the enginetorquemult (along with enginepowermult) as no engine is exactly the same. Some have slightly more power than specified, some have less, so I just stay with advertised (SAE Net, where possible).

Yes, frictiontorque and clutch fricion are in Nm and they are constant.
 
thanks for the insight!

I will try something on these parameters and see how the things change.

I´ll let you know!
 
silente said:
thanks for the insight!

I will try something on these parameters and see how the things change.

I´ll let you know!
Click to expand...

Just tried to decrease clutch friction and i saw a quite sensitive speed increase (around 3-4 km/h at 254 km/h).

Don't know at which gearbox efficiency the value i have now corresponds.

Anyway, i still would like to understand how to properly model aero loads (drag and downforce) without too much trial and error. Also to properly fix them vs wing configurations.

Do you guys have any insight to give me?
 
Indeed it would make things easier. AFAIK the plugin only will output what the devs allow it to, I doubt who made the plugin held back aero logging, and so I also doubt that it is exposed/available to be output.

Interesting thought that stuck in my head: If the wings produce moments, at some point in rf's code those are linked back to the entire body, thus producing the appropriate result (and can be measured by increase of load at the wheels). Considering a computer program is logic and math, we should be able to pull out the math/logic and do this manually. Seems like that is a more efficient method than build, test, repeat until proper.

I think we would need to get info from Devs to see how exactly the wing moments are being applied.
 
Since the way rF is modeling aero is not still totally clear to me, to develop the proper downforce i have mainly used data acquistion of sessions on a straight line. With simple math channels using Tyre loads as a base, you can exctract front and rear downforce (at the wheels) and verifiy these results vs your data.

Probably, if i could understand better how all the parameters are linked together (including ride height mesurement for aero purposes) i could have done it right at the first attempt, but i had to go with trial and error instead. I am now quite happy about the downforce.

I have more problems with drag, as i said, and it is difficult to extract a clear number for it out of a session data file: i will probably do some coast down test, but the results will always include also friction and rolling resistance.
 
Ride height from telemetry is taken from where you have put respective measure points at undertray.
In HDV, you define points from defining undertrays (flat bottom of a car) and UnderTray00-03 are measuring points for ride height (for telemetry).
Ride height you set up in garage has nothing to do with that, as it is set at wheel centers at each car corner.

About aero - i probably already provided that link but here's again:
http://koti.mbnet.fi/tspartan/gp1975/airoopas/index.php?id=functions.php


silente said:
Probably, if i could understand better how all the parameters are linked together (including ride height mesurement for aero purposes) i could have done it right at the first attempt, but i had to go with trial and error instead. I am now quite happy about the downforce.
Click to expand...
So, you are asking for source code, how each thing is related to another... ask ISI for that ;-)
Seriously, it all depends on where you specify places for things like front/rear wing, diffuser as forces acting on those parts (until they are located directly at axles) will create torque around front and rear axle. And all that stuff have an effect on one another.
As a racing engineer, you probably have done such calculations already. You can do such in Excel, for example.

silente said:
I have more problems with drag, as i said, and it is difficult to extract a clear number for it out of a session data file: i will probably do some coast down test, but the results will always include also friction and rolling resistance.
Click to expand...
You can set 0 mechanical friction and rolling resistance and you will end up with only aero drag in rFactor.
 
LesiU said:
Ride height from telemetry is taken from where you have put respective measure points at undertray.
In HDV, you define points from defining undertrays (flat bottom of a car) and UnderTray00-03 are measuring points for ride height (for telemetry).
Ride height you set up in garage has nothing to do with that, as it is set at wheel centers at each car corner.

About aero - i probably already provided that link but here's again:
http://koti.mbnet.fi/tspartan/gp1975/airoopas/index.php?id=functions.php



So, you are asking for source code, how each thing is related to another... ask ISI for that ;-)
Seriously, it all depends on where you specify places for things like front/rear wing, diffuser as forces acting on those parts (until they are located directly at axles) will create torque around front and rear axle. And all that stuff have an effect on one another.
As a racing engineer, you probably have done such calculations already. You can do such in Excel, for example.


You can set 0 mechanical friction and rolling resistance and you will end up with only aero drag in rFactor.
Click to expand...


LesiU thanks again.

As you think, i did all this calculation in excel. But, as i said, using the formulas i have found, results are not consistent to how the car behaves on track. Above all for drag.

I am not saying the simulation is wrong, but that probably i miss some pieces in my equations. I was simply trying to find them. But didn't succed, so i think i will have to go by trial and error. But that's not the right way to me.
 
Have you tried to remove (set to 0) all the mechanical drag and rolling resistance and work just on aero drag, starting with just the body and then one by one, adding the rest of the aero parts to the equation?
 
LesiU said:
Have you tried to remove (set to 0) all the mechanical drag and rolling resistance and work just on aero drag, starting with just the body and then one by one, adding the rest of the aero parts to the equation?
Click to expand...


Not yet, i will probably use this or a similar approach to isolate aero drag in a simulated track test.

But since it is about modeling (and a model is baically a set of equations that simulate the behaviour of one or more systems) i would have been happier to know the "right" equations to use to model aero drag to be able to do it again and properly next time, instead of being again in a condition where i will need to do again track test to be sure to do it correctly.
I though the info provded by Bristol were correct, but i still have a mismatch, so probably i am missing somethig.

Thanks anyway again for all your advices, it´s very interesting and useful for me to discuss with you guys!
 
Odp: Aerodynamic forces

I can't help you, because I don't know what issues you are having. For me, drag works ok as I've done...well, more than several cars already. Of course, for some parameters (like inertia and friction for gearbox) you have to do some roundings as rf1 and current version of rf2 do some calculations in a more general way but still, you should still be able to get your in-sim results pretty close to real data.
 
LesiU said:
I can't help you, because I don't know what issues you are having. For me, drag works ok as I've done...well, more than several cars already. Of course, for some parameters (like inertia and friction for gearbox) you have to do some roundings as rf1 and current version of rf2 do some calculations in a more general way but still, you should still be able to get your in-sim results pretty close to real data.
Click to expand...

Just to double check, could you tell me the parameters in HDV file that have an influence on the overall drag?

As i said, i based my calculations on Bristow posts (both the parameters to tune and the equation to use to model them).

I am not in front of my pc at the moment so i cannot check my excel, but i would say that things having an influence on drag should be:

front wing

rear wing

body

radiator

brake ducts

Am i missing something?

Is there any other source around where i can find the equation to use to calculate the drag contribution of each of these parts out of the numbers you are giving to them? Just to check if i used a wrong formula anywhere...

Thanks!
 
There are also left/right fenderflares but if you don't use them, then they are out of equasion.

Apart from Bristow's posts and the link I gave you in post #21, you can look into CarFactory application, but it calculates just the aero part (doesn't take drivetrain friction and rolling resistance into account).
 
LesiU said:
There are also left/right fenderflares but if you don't use them, then they are out of equasion.

Apart from Bristow's posts and the link I gave you in post #21, you can look into CarFactory application, but it calculates just the aero part (doesn't take drivetrain friction and rolling resistance into account).
Click to expand...


A short feedback.

I checked again my calculation and did some coastdown test and it seems i was wrong...and my results were right.

Now the overall drag calculation make sense, as the standig still acceleration.

I don´t know if the split between mechanical and aero drag is correct, but the overall value is and at least now i am fairly confident it can be properly calculated in advance.

So thanks for your help.

I will come back with other questions soon! ;)
 
Glad you have sorted those things out and now it works like expected :)
 
Hi guys,

i did some more calculations/test about this topic.

First of all, i tried to build up a useful tool to work out aero loads in a predictive way and to do so i have had to review all the formulas used to calculate each contribution to Aero Loads (Downforce in this case).

Doing so, i have found that the formulas provided in the old Bristow Post about the diffuser didn't work in my case and, on the other hand, the results from carfactory were much closer to what i could see on track with Motec.

The main factor contributing to this difference has proved to be the Diffuser. Actually, the formula i have found in the old post from Bristow to calculate the Diffuser Downforce contribution is:

DiffuserCoL = (DiffuserBase) * (1-DiffuserRake) * (1-DiffuserStall)

DiffuserBase = DiffuserBase1 + DiffuserBase2*RRH + DiffuserBase3*RRH^2 + DiffuserFrontRideHeight*FRH



DiffuserRake = DiffuserRake2*((RRH - FRH) - DiffuserRake1) + DiffuserRake3*((RRH - FRH) - DiffuserRake1)^2


what i have found is that the formulas to calculate DiffuserBase and DiffuserRake worked properly, but the overall formula for the Diffuser COL was underestimating significantly the dowforce coming from the Diffuser.

Actually, the bahaviour i have seen on track is better described by something like (ignoring for now the stall effects):

DiffuserCoL = DiffuserBase + DiffuserRake


and that is also what carfactory shows. Actually the results from carfactory matches very well what i have measured on track.

Did anybody have any similar experience in calculating downforce, in particular from the diffuser?

Of course the overall downforce is made out of many contributions, so it could be that now my diffuser calculations are wrong and also all the other components contribution are not calculated correctly and i am so lucky that anyway the overall results are matching with track results, but it is very strange that this happens with several different rakes and wing configurations. And another interesting thing is that now my results match very well both with carfactory and track test results.

Another small question is if anybody knows how rF calculates the effect of wing center overhang in longitudinal direction compared to wheel contact patch.

Thanks!
 
Domi said:
You have the full formula here : http://koti.mbnet.fi/tspartan/gp1975/airoopas/index.php?id=functions.php

It seems to be copy/pasted from the engine code.
Click to expand...

I knew that formula from that site, and used that. But it seems to produce for me different results than what i have measured on track, while the other one i have posted seems to better match both with track results and with carfactory.

Anyone had similar issues with the base formula (results from calculation not matching what measured on track)?

also Carfactroy speaks about an additional element for the DiffuserRake result...
 
Last edited:
Does anyone know where the drag and lift from tires and wheels should be included?

the link above says:
Open wheels generate a lot of lift. Unfortunately there is no aerodynamic function for this. Adding the lift to some other object would be very incorrect (suspended vs unsuspended mass). Best compromise is to use the SpeedEffects function in .tbc file.

SpeedEffects1/(v + SpeedEffects1)*SpeedEffects2*v​

But the tTool tutorial/forum posts says only initial radius and temp are taken form the TBC file so I assume this doesn't apply anymore (for the player at least, maybe it still does for the AI).

Does the physics motor do some sort of parametric calculation based on the tire geometry, position, orientation, air speed, rotation rate, etc?

If not, then its seems less wrong to at least include the drag effect in the HDV here:
BodyDragBase=​

As for the lift, although its incorrect, I think its better to include it in the HDV here:
BodyFore=​
and get unrealistic ride height, but more realistic tire load.

any suggestions or help would be very much appreciated.
 
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