rec.autos.simulators

I was wondering if you could give me a hand with a couple of problems.
How is lateral slip angle calculated?
I have been trying to use this formula that I came across:

slip_angle = sideslip_angle + omega * b / wheelbase + wheel_angle

(where b is the longitudinal distance from the CG to the wheel position)

Is this correct? Because it doesn't seem to be giving me the correct
answers.

My second question is what range of values does the pacejka magic formula
take for slip angle (and camber angle)? is it in radians or degrees?

Any help would be greatly appreciated.

George.

  That's not quite right because it doesn't take into account the true velocity
at the tire.  You need the rotational velocity of the car included in the calcs
so rotational damping occurs.  If the car was sitting in one spot and spinning
around its center of mass, could you find the direction each tire is moving,
along with how quickly they're moving?  (Velocity vectors)

  Use 3-D vectors for this (if you're doing it in 3-D).  Given the rotational
velocity of the car, find the true velocity vector for the tire's contact
patch.  Add the car's translational velocity vector to this.  Then, using the
normal to the surface the tire is on, project the vector onto the plane (so you
subtract the part of the velocity that is going directly towards or away from
the plane, this component BTW, can get you the shock absorber/damper velocity.

  I define my tire orientation with one vector, and a rotation angle around it
(sticking out one side of the wheel).  Project this vector against the plane in
the same manner (this eliminates the effect of camber angle for slip angle
calculation.)  Once you've got these two projected vectors, which are giving
tire contact patch velocity relative to the surface it's rolling on, as well as
the orientation of the wheel with camber not included, you DOT these two and
subtract 90 degrees to get the slip angle.  

  Slip angle is really the angle between the direction the tire is facing and
moving, so the above stuff gets the solution in 3-D.

  I don't have an answer for your Pacejka question.  

  Hope that helps,

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

Thanks alot for the reply, it helped alot. One day I might buy myself "Race
Car Vehicle Dynamics" for help with stuff like this - but at the moment I'm
a poor student :(.
Can anybody give me a hand with this Pacejka stuff?

Thanks again,

George

It's degrees. And oh my, it takes the normal force as kiloNewtons. And
slipratio as a percentage. What WAS he thinking! ;-)
Anything to give you a hard time getting it right.

Ruud van Gaal, GPL Rank +53.25
Pencil art    : http://www.marketgraph.nl/gallery/
Free car sim  : http://www.marketgraph.nl/gallery/racer/

<hiding from Doug ;)>

-Gregor

Ruud van Gaal, GPL Rank +53.25
Pencil art    : http://www.marketgraph.nl/gallery/
Free car sim  : http://www.marketgraph.nl/gallery/racer/

  It's an awesome book, although it doesn't cover any of the vector stuff
above.  That's all 3-D graphics stuff, but works great for physics too.  Can't
help ya with Pacejka yet though :-)  The book covers the Raadt
non-dimensionalized technique.  I've played with that a little, and it looks
good.  It can be used in combination with Pacejka if you want.  There are
others here a lot more knowledgeable on that, so I'll pass on trying to help on
that one... :-)

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

I got a basic combined-slip, load-dependent model with a
non-dimensional Pacejka curve working okay in my final year project -
and I can provide numbers for that based on some rough Dunlop tyre
maps, although longitudinal stuff was all guessed.

LMK if info's wanted - perhaps the easiest way would be to email my
FYP, or maybe just the relevant chapter.

Jonny

I believe the formula is mentioned, although somewhat quick & rough.
Still, IIRC there's a picture of a hand somewhere and that's the hand
of Pacejka himself! Not that that will get you a great tiremodel. ;-)

Ruud van Gaal, GPL Rank +53.25
Pencil art    : http://www.marketgraph.nl/gallery/
Free car sim  : http://www.marketgraph.nl/gallery/racer/

Ruud van Gaal, GPL Rank +53.25
Pencil art    : http://www.marketgraph.nl/gallery/
Free car sim  : http://www.marketgraph.nl/gallery/racer/

Thanks for the help,

George

I'm a little hesitant about supplying too much information since
technically the University owns the copyright on my project, but I'm
assured that the report itself is public domain so this should be
okay...

Jonny

  Yes, it's the shorter version with (I think) four constants instead of
200,000 or so ;-)  Pacejka is holding the pencil on page.....  well, I'd know
it if I saw it.  Doug pointed that out for us all one day.
Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://PerformanceSimulations.Com

Just read about Pacejka '97 yesterday, hm. I use Pacejka 96, with some
30 or so constants. Still, every constant has its direct influence on
the curve, which I intend to explain (as far as possible) on my site.
It's not really such rocket science after all it seems, with often
good physical backgrounds.
Anyway, the Pacejka '97 methods seem to define combined slip
operations. Need to check that out later. Another 10 constants or so.
;-)

Ruud van Gaal, GPL Rank +53.25
Pencil art    : http://www.marketgraph.nl/gallery/
Free car sim  : http://www.marketgraph.nl/gallery/racer/

| >>>  It's an awesome book, although it doesn't cover any of the vector
stuff
| >>>above.  That's all 3-D graphics stuff, but works great for physics too.
| >>Can't
| >>>help ya with Pacejka yet though :-)
| >>
| >>I believe the formula is mentioned, although somewhat quick & rough.
| >>Still, IIRC there's a picture of a hand somewhere and that's the hand
| >>of Pacejka himself! Not that that will get you a great tiremodel. ;-)
| >>
| >>
| >
| >  Yes, it's the shorter version with (I think) four constants instead of
| >200,000 or so ;-)  Pacejka is holding the pencil on page.....  well, I'd
know
| >it if I saw it.  Doug pointed that out for us all one day.
|
| Just read about Pacejka '97 yesterday, hm. I use Pacejka 96, with some
| 30 or so constants. Still, every constant has its direct influence on
| the curve, which I intend to explain (as far as possible) on my site.
| It's not really such rocket science after all it seems, with often
| good physical backgrounds.
| Anyway, the Pacejka '97 methods seem to define combined slip
| operations. Need to check that out later. Another 10 constants or so.
| ;-)
|
|
| Ruud van Gaal, GPL Rank +53.25
| Pencil art    : http://www.marketgraph.nl/gallery/
| Free car sim  : http://www.marketgraph.nl/gallery/racer/

Ruud,

Is Pacekja '97 available anywhere on the web? (In English <g>)

Thanks
Ash