Re: [Flightgear-devel] Abnormal surface friction factor

[Dany <da...@nu...>]

On 15/06/2023 à 20:06, Wayne Bragg wrote :

> Couple of things here, static VS rolling. Typically you would always 
> think static should be higher than rolling, it’s not. In a very short 
> test I saw ranges from .7 to 1 for static values. I need to test more 
> terrain types. I’m not sure exactly what relationship comparison these 
> exact properties have in the sim though.
>
>
> Typically
>
> Static, how much force does it take to make the tire slip from a 
> standing position.
>
> Rolling, how much force does it take to make the tire roll.

Force = weight x [static friction coefficient].
Force = weight x [rolling friction coefficient].

(Up to here, that's general physics, not particularly FG)

Where, *in FG*:
(if I understand well, extrapolated from rolling for the correction 
factor on static)

[static friction coefficient] = (FDM) static_friction x 
static_friction-factor
[rolling friction coefficient] = (FDM) rolling_friction x 
rolling_friction-factor

static_friction is the value given in the FDM,
rolling_friction is the value given in the FDM.

static_friction-factor and rolling_friction-factor are seen in FG 
Properties. They are *_correction_ factors*, possibly changing the 
effective [static friction coefficient] or [rolling friction 
coefficient], depending on the terrain.

*From geodinfo to FG*: (pay attention to variable names)

Friction:
I think that the (geadinfo) friction_factor is directly the correction 
factor.
friction_factor = static_friction-factor.

Rolling:
The (geadinfo) rolling_friction is used very differently. This is the 
source of many confusions.
In geodinfo, the rolling_friction is given the value 0.02 on asphalt. 
This is the [rolling friction coefficient] for usual tyres.
For the same tyres on other materials, the (geodinfo) rolling_friction 
values can be considered as relative to the one on asphalt.

(geodinfo) rolling_friction ---> (FG) rolling_friction-factor
*Very important*: here, the [rolling friction coefficient] for usual 
tyres on asphalt, that is (geodinfo) rolling_friction = 0.02, is taken 
as a reference.
This value will be multiplied by 50 for every terrain material for 
giving the (FG, correction) rolling_friction-factor. On asphalt, it will 
give 0.02 x 50 = 1.
*Which means that, on asphalt, there will be no correction*.
If a particular wheel is given rolling_friction = 0.03 in the FDM, its 
final [rolling friction coefficient] on asphalt will be 0.03 (NOT 0.02 
!!). *One can control this value via the FDM rolling_friction*.

On another terrain material with (geodinfo) rolling_friction = 0.1, the 
(FG, correction) rolling_friction-factor will be 0.1 x 50 = 5. The final 
[rolling friction coefficient] for the particular wheel above will be 
0.03 x 5 = 0.15. That is, FDM rolling_friction x 5.

Even more generally,
final [rolling friction coefficient] on some terrain material = FDM 
rolling_friction x rolling_friction-factor = FDM rolling_friction x 
(geodinfo) rolling_friction x 50.

*Attention* : this assumes that the rolling_friction_coeff given in the 
FDM for a particular wheel is the value *on asphalt*.


> When I see a static friction factor of 1 and a rolling friction factor 
> of 50, I automatically think something is wrong.

The  rolling friction _factor_ is not 50. it is 50 x (geodinfo) 
rolling_friction. Moreover, it is not a rolling friction coefficient, 
but a correction factor from one terrain to another.
Also, the static friction factor is not calculated by 50 x (geodinfo) 
friction_factor.


@wlbragg: Hard for me to explain something that I don't fully 
understand... Please, be critical.
Also hard because it needs a lot of concentration (risk of error).
Again, thank you Erik if you can fix me in case of errors.

Note that I've done digging for rolling coefficients, much less for 
friction coefficients (static or dynamic).

Dany