rec.autos.simulators

1) I would like to know if the graph slip angle/lateral forces is the same
for every surfaces/tires by a coefficient on lateral force. Or the same by a
coefficient on slip angles . Or completly different.

IMO this how i see the different graphs

        TARMAC                                              

        Lat force                                                      
        ^                                                                                      
        |                                                                                      
        |     *                                                                
        |    * *                                                                                
        |   *     *                                                                            
        |   *      **                                                                          
        |  *         ***                                                                        
        |  *              *********                                                    
        | *                        **********                          
        | *                                                                                    
        |*                                                                                      
        |*                                                                                      
        |-------------------------------->   angle deg      
              6       15                                                                        

                    DIRT                                  

        Lat force
        ^                                                                                    
        |                                                
        |                                                
        |                                                
        |                                                
        |         **                                        
        |       **  **                                      
        |    *  ***                                  
        |   *      ****                              
        |  *             ******                        
        | *                              *********                
        |*                                                
        |-------------------------------->   angle deg    
                 10       20                          

2) If it is do you know where could i find those graphs for
dirt/snow/tarmac/gravel/etc... surfaces ?

Thanks in advance

--
Seb
Game Developer
GPLRank -41.68
http://www.racesimcentral.net/

From what I understand (from G.Genta's book) the peak force is reduced by a
ratio in these graphs.  This would lead to the sort of graphs that you have
presented below.  In terms of the Pacekja formula i'd say this translates to
multiplying the "D" coefficient by the ratio, eg. 1 = tarmac, 0.7 = grass,
0.5 = ice.  Sorry I dont know the specific ratios, although I will have a
look in the book tonight to see if there are any guidelines.

You will of course want to do this for the Longitudnal Force AND Lateral
Force Calculations

Please someone correct me if this is wrong, cause this is what I will be
using!!

Hope this helps a bit
Ash

| Hi,
|
| 1) I would like to know if the graph slip angle/lateral forces is the same
| for every surfaces/tires by a coefficient on lateral force. Or the same by
a
| coefficient on slip angles . Or completly different.
|
| IMO this how i see the different graphs
|
| TARMAC
|
|
| Lat force
| ^
| |
| |     *
| |    * *
| |   *   *
| |   *    **
| |  *      ***
| |  *     *********
| | *            **********
| | *
| |*
| |*
| |--------------------------------> angle deg
|       6 15
|
|
|
|
|         DIRT
|
| Lat force
| ^
| |
| |
| |
| |
| |   **
| | **  **
| |    * ***
| |   *    ****
| |  *       ******
| | * *********
| |*
| |--------------------------------> angle deg
|          10     20
|
| 2) If it is do you know where could i find those graphs for
| dirt/snow/tarmac/gravel/etc... surfaces ?
|
| Thanks in advance
|
| --
| Seb
| Game Developer
| GPLRank -41.68
| http://magicfr.multimania.com

Just had a quick look in the book it has the following (converted into
ratios) :-

Concrete - Dry 0.94 Wet 0.72

Tarmac -  Dry 1.0  Wet 0.88  Snow 0.42  Ice 0.16

This is for radial tyres.

Hope this helps
Ash

Hum, i was not clear in my question :o(

I know that the peak force is reduced when grip is reduced.

I would like to know if peak force is moved on slip angle axle with
different grips , for exemple:

on tarmac, the peak force appeared at 6 of slip angle, i would like to know
if the peak force on ice appeared also at 6 or another angle ? 10 ? 2 ?

I also would like to know if after the peak force the force decreased the
same amount on tarmac and ice , for exemple.

On tarmac on high slip angle ( > 15 ) coef is .75, does it the same on
grass, or less ?

anyway thanks for the peak force coefficients !

regards,

--
Seb
Game Developer
GPLRank -41.68
http://magicfr.multimania.com

I think that the peak will appear in exactly the same slip angle, however
the peak will be lower. This is based on the fact that u(yp) is the peak and
D = u(yp) x Fz.  So basically you are multiplying every point on the graph
by the ratio...which wouldnt move the peak, just raise or lower it.

If you see any probs with my idea, please tell me - i want to get it right
too :)

Hope this helps
Ash

| Hum, i was not clear in my question :o(
|
| I know that the peak force is reduced when grip is reduced.
|
| I would like to know if peak force is moved on slip angle axle with
| different grips , for exemple:
|
| on tarmac, the peak force appeared at 6 of slip angle, i would like to
know
| if the peak force on ice appeared also at 6 or another angle ? 10 ? 2 ?
|
| I also would like to know if after the peak force the force decreased the
| same amount on tarmac and ice , for exemple.
|
| On tarmac on high slip angle ( > 15 ) coef is .75, does it the same on
| grass, or less ?
|
|
| anyway thanks for the peak force coefficients !
|
| regards,
|
| --
| Seb
| Game Developer
| GPLRank -41.68
| http://magicfr.multimania.com

In principle there is no reason why the peak should be reached at the
same angle on all surfaces, and indeed I would think it in fact isn't.

Let's assume that the behaviour of the tire in the linear regime (very
small slip angles) depends entirely on the structure of the tire and not
on the surface properties itself. It's a plausible assumption since a
very small portion of the contact patch is actually slipping in this
regime, and the maximum ammount of grip doesn't come into the picture
yet. In this case, the cornering stiffness of the tyre, which is the
slope of the force vs. the slip angle curve (dF/dS) close to S=0, should
not depend on the maximum grip available and should be constant for all
surfaces.

Therefore, if the same (rescaled) shape of the force vs. slip curve is
used for all surfaces, the maximum grip and the optimal slip angle
(where peak force is reached) are directly proportional in the above
model, assuming the vertical load is kept constant. The model could be
completely wrong, of course :).

-Gregor

Okey, i understand that. Moreover i'll say that the angle where the peak is
reached depend of the couple tyre/surface.

Waow ... my english is not enough well simulated to understand completly
what you said ... could you try with word more simple ? :o)))

thanks in advance.

Maybe you could try in French ;o)

--
Seb
Game Developer
GPLRank -41.68
http://magicfr.multimania.com

I drove real cars on different kind of tire on different kind of surfaces,
and the feeling i had was in contradiction with this.

On earth, for exemple, everything is smoother, you have to turn more the
wheel to find the peak of force , i mean longer and smoother motion than on
tarmac. IMHO.

Well, currently this is the model i used, and the feeling is not good on
dirt/ice tracks, the sliding is too sharp but with smaller grip.

IMO a graph on earth/ice will be like a tarmac graph but larger i.e. the
peak force will be reach at higher slip angle, and also the force will be
slower at high slip angle.

for exemple, if this array is MAX = F( slip angle ) and lat = MAX * grip

                tarmac            earth                 ice
slip angle
        0          0                    0               0
        6          1                    .5      .5
        10       .75                    1               1
        15       .65                    .5      .5
        20       .6             .45     .4
        25       .55                    .4      .35

grip                1               .5                  .15

to be sure, we need graph of real experience, do you know somebody working
at GoodYear or Michellin ??? ;o)))

regards,
--
Seb
Game Developer
GPLRank -41.68
http://magicfr.multimania.com

(sorry for misspelling your name the first time; still, I can't produce
the accents ;) )

In simpler terms; slip angle is produced because the tyres flex under
sideways forces. The stronger the sideways force, the more the tyre
twists and the higher the slip angle. If the maximum force is lower due
to the lower coefficient of grip, then the corresponding peak slip angle
will also be lower.

In the theory I've given, the peak slip angle is directly proportional
to the maximum grip (if the general shape of the curve remains the same).

-Gregor

  I agree with Gregor's posts.  According to a lateral "brush" model I wrote a
couple months ago, his descriptions are valid.  The slope of the graph in the
"linear" part (not yet sliding much, or at low slip angles below the peak,
before the graph curves towards the peak grip), should remain approximately
constant at a given load/vertical force.  The peak force slip angle (according
to my model anyway) varies along according to the peak grip coefficient
available.

  For instance, my brush model shows results something like this:

       Tarmac
                  *   ***********
          *                      
      *
    *
   *
  *
 *
*

       Mud

         *   ********
     *
   *
  *
 *
*

  The peak force slip angle moves along the initial sloped line.  

Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://PerformanceSimulations.Com

I had thought that the slip angle effect was a combination of the sidewall
flex and also the transition of each piece of *** from sliding to
gripping to sliding as it passes under the contact patch.  So the sidewall
flex component would lead to an early peak on slippy surfaces, while the
slip/grip effect would probably cause the peak to be later...
From personal experience, it feels like snow and dirt have very wide slip
angles, just because you can really hang the back end out and get it to come
back with just getting off the gas.  I've used higher peak slip angles when
modeling dirt/offroad, and it feels okay... the bigger problem being the
lack of a more interesting surface with deformable bits and ruts and all
that.
Wet asphalt seems weirder--sometimes it feels more like the low-friction,
narrow-angle thing, but occasionally you can get some large slip angles that
still feel like you're on the near side of the peak--I suppose wet asphalt
has a very high variability because many surfaces that have similar grip
while dry (e.g. *** and concrete) have completely different grip levels
while wet (as I saw a wrecked Merkur at Brainerd), plus the effect of
hydroplaning.

I also have often wondered with snow and dirt if there is a combination of
more traditional tire-deformation slip angle stuff at low angles, and more
of a viscous drag-type of effects at wider slip angles.  Another factor is
that on loose surfaces there is motion within the driving surface, so that
will also widen the peak--here's a thought experiment that may or may not
relate to any real-world phenomena--picture a car driving on a thin but
large-area sheet of concrete laid over a dirt surface... a car could be
driving in a large circle on this object at the peak-friction slip angle,
_relative to the concrete_, however, the concrete is also sliding laterally
on the dirt due to the force of the car, and if this was say a linear
viscous drag it would cause the car to be driving at a wide slip angle
relative to the dirt...

A very interesting post. What you say about loose surfaces is absolutely
true, the rear can be hanged out more than on asphalt, we all see that
in rallying. I posted a while ago that in such circumstances the force
doesn't depend on the slip angle anymore, but that there must also exist
a component that is proportional directly to the slipping velocity,
which is exactly the viscous component you suggested. I'm glad that you
find it in accord with real world experience as well (it's what it's all
about, isn;t it?).

To summarize, I still believe that in a low grip situation the component
of the force that comes from the slip angle peaks at a lower slip angle
that on a high grip surface, but if that surface is also loose (dirt,
snow), an additional component exists that is proportional to the
slipping velocity itself and as such allows cars to drift at large
angles.

-Gregor

<snip>

I'm glad numerical experimentation confirmed my thoughts. Phew :).

-Gregor

Bob

On Thu, 17 May 2001 09:43:47 +0200, Gregor Veble

actually, that's what is implied with the component of force that is
proportional to the slip velocity. Such a component arises since some
material is accelerated in proportion to the slipping velocity, and this
is exactly the buildup you are talking about.

-Gregor