rec.autos.simulators

Car physics; steering caster angle

Gregor Vebl

Car physics; steering caster angle

by Gregor Vebl » Wed, 25 Jul 2001 19:32:02

Hi Ruud,

the pain of thinking about combined full 3D rotations is too hard for
myself to get into right now during these hot summer days :). That's why
everything I've written is a linear approximation, where rotations can
be done interchangeably. In all seriousness, when angles involved are
lower than 0.1 radians (that's about 6 degrees), if taking 1 instead of
cos(x) and x instead of sin(x), you are making an error of about 0.5
percent, which is insignificant. Of course, all the formulas can be
given in an exact way and are numercially relativelty easy to do, but
would bring very little in accuracy and a lot in headaches to get them
absolutely right ;).

-Gregor


> Ah, so that would be cos(casterAngle)*tireRadius. No approximation
> needed probably, since this can be precalculated. Unless ofcourse you
> build in steering compliance. Skip that, I'll just precompute it and
> keep it static. ;-)

> >Indeed, it's mostly about the feedback, and the camber behaviour in
> >steering inputs. Definitely a secondary thing to consider in a car sim.
> >If you want to see the effects in a sim, Nascar 4 does this extremely
> >well. It makes a huge difference in FF and control of the vehicle, but I
> >couldn't notice any performance differences due to additional camber, at
> >least not with my untidy driving style ;).

> Sounds good, I don't want too many primary driving behavior anymore!
> Still, it seems nice to build in, since I noticed that the force
> feedback is very important to feel your limits, especially now that my
> skid volume code is in bad need of repairs! ;-)

> BTW I think that:
> additionalCamber=caster*wheelRotation should be
> additionalCamber*sin(wheelRotation), not sin(caster). But I may be
> wrong here. :) And besides, perhaps it's so close to linear for normal
> steering that the sin() isn't worth the calculation. Getting some
> optimization vibes up my spines.

Ruud van Ga

Car physics; steering caster angle

by Ruud van Ga » Wed, 25 Jul 2001 21:09:28

On Tue, 24 Jul 2001 12:32:02 +0200, Gregor Veble


>the pain of thinking about combined full 3D rotations is too hard for
>myself to get into right now during these hot summer days :).

Indeed, I should go out and eat lunch in the sun right now! And you're
probably right, when nitpicking about sin(a) instead of a, I should
then immediately take the full 3D steering angles and then just give
up and eat a sandwich and enjoy the hot sun.

Ofcourse, I must sometimes remind myself I'm making a game here, which
is always off by more than the 0.5% probably. ;-)

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

J. Todd Wass

Car physics; steering caster angle

by J. Todd Wass » Thu, 26 Jul 2001 06:54:01

  Ok, just making sure I hadn't missed something :-)

- Show quoted text -

  Ok.  What I was planning on doing was letting the body rotate around the
center of mass instead of the roll center.  I have a hunch that the combination
of jacking, lateral, longitudinal, anti-forces, and torques will happen to move
the body right the way.  After all, a large lateral force will lift the chassis
more and the body won't end up rolling around the same roll center as before
anyway since it'll move differently.  I might drop you an email here before too
long if you wouldn't mind discussing our methods further.  

  I don't have the anti or jacking forces in yet, but I tried a really
outrageous suspension system anyway once to see how it would work.  The right
rear wheel had double/triple length arms and the spring rates were all set to
wierd values.  The car ended up lifting the front right tire when cornering in
one direction, then slamming down hard and lifting one of the rear tires when
turning the other.  In a straight line it was rolled perhaps 10 degrees and
pitched quite a bit.  Anyway, it looked really cool :-)

  Thanks,

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

Doug Arna

Car physics; steering caster angle

by Doug Arna » Thu, 26 Jul 2001 09:47:38

GPL does not model caster.....I'm not sure what you're feeling there . N4
was Dave's first caster driving model.

--
Doug Arnao
www.vehiclecraft.com



> On Mon, 23 Jul 2001 17:56:00 GMT, "Stephen Smith"

> >I'm just now getting around to mastering FF.  Unlike every other
FF-equipped
> >racing sim, GPL models caster return.  Thus, it's a remarkably effective
> >teaching tool.  If you're driving correctly (at least, if your steering
> >input is correct), you'll never feel the caster return.  If you're doing
it
> >wrong, you'll feel it: the c.r. practically rips the wheel out of your
> >hands.

> >[GPL's caster return also provides some otherwise-hidden input on how
your
> >shocks are set up.  If you don't have the suspension properly damped, the
> >caster return will oscillate, which you can definitely feel thru the
wheel.]

> Ah, I always thought the oscillations were caused by the delay set too
> high or low in the FF section. Now that I reread the previous
> paragraph, no I never felt anything remotely like it! ;-) Uh...

> >Remarkable.  We're still discovering new stuff in GPL 3 years on.

> It's amazing at the complexity that complex systems give; you can
> never really understand the results your own product somehow, but it's
> there. :)

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

J. Todd Wass

Car physics; steering caster angle

by J. Todd Wass » Thu, 26 Jul 2001 11:35:39

  Was he using a predefined steer-camber "curve" based on caster or running the
calcs around the actual kingpin axis, causing steer camber with both kingpin
and camber angle?

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

Gregor Vebl

Car physics; steering caster angle

by Gregor Vebl » Thu, 26 Jul 2001 16:25:46

Hi Todd,


>   Ok.  What I was planning on doing was letting the body rotate around the
> center of mass instead of the roll center.  I have a hunch that the combination
> of jacking, lateral, longitudinal, anti-forces, and torques will happen to move
> the body right the way.  After all, a large lateral force will lift the chassis
> more and the body won't end up rolling around the same roll center as before
> anyway since it'll move differently.  I might drop you an email here before too
> long if you wouldn't mind discussing our methods further.

That's how you really should be doing things. The roll center
description is really a steady state one, and the car should settle to
this position if you apply the forces and torque correctly to the center
of mass. So you hunch is perfectly correct. ;)

What the roll axis does tell you is the direction of travel of the wheel
relative to the body, which is perpendicular to this axis (vectorial
product of the roll axis direction vector times any vector from the roll
axis to the point on the wheel (is this the contact patch or the wheel
center, I can't recall right now?)). The forces in the direction of
wheel travel are transfered to springs, while those perpendicular to it
are transfered directly to the body (via suspension links, but this
doesn't interest you until you need to calculate suspension stress).

Drop me an e-mail if you seriously believe I can answer some further
questions :).

Sounds a lot like the Renault 4 I got to drive once ;).

-Gregor

Stephen Smit

Car physics; steering caster angle

by Stephen Smit » Thu, 26 Jul 2001 21:32:39

That's not what he told me.  You were there.  ;o)

--Steve


> GPL does not model caster.....I'm not sure what you're feeling there . N4
> was Dave's first caster driving model.

> --
> Doug Arnao
> www.vehiclecraft.com



> > On Mon, 23 Jul 2001 17:56:00 GMT, "Stephen Smith"

> > >I'm just now getting around to mastering FF.  Unlike every other
> FF-equipped
> > >racing sim, GPL models caster return.  Thus, it's a remarkably
effective
> > >teaching tool.  If you're driving correctly (at least, if your steering
> > >input is correct), you'll never feel the caster return.  If you're
doing
> it
> > >wrong, you'll feel it: the c.r. practically rips the wheel out of your
> > >hands.

> > >[GPL's caster return also provides some otherwise-hidden input on how
> your
> > >shocks are set up.  If you don't have the suspension properly damped,
the
> > >caster return will oscillate, which you can definitely feel thru the
> wheel.]

> > Ah, I always thought the oscillations were caused by the delay set too
> > high or low in the FF section. Now that I reread the previous
> > paragraph, no I never felt anything remotely like it! ;-) Uh...

> > >Remarkable.  We're still discovering new stuff in GPL 3 years on.

> > It's amazing at the complexity that complex systems give; you can
> > never really understand the results your own product somehow, but it's
> > there. :)

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

Doug Arna

Car physics; steering caster angle

by Doug Arna » Thu, 26 Jul 2001 21:42:45

Not sure Todd, but I believe he did modelled king pin inclination.

--
Doug Arnao
www.vehiclecraft.com



J. Todd Wass

Car physics; steering caster angle

by J. Todd Wass » Fri, 27 Jul 2001 06:17:40

  Great!  Thanks.  Things will progress perfectly then :-)  Look out world!
Mu-ha-ha-ha-ha!! :-P

  >What the roll axis does tell you is the direction of travel of the wheel

  I think with my current system, this will all work itself out just fine.  The
only drawback is vertical tire flex can only be indirectly modelled by making a
sequential spring system.  Not the most accurate way, but this should be a big
improvement over what I've currently got and it works really well with
high-downforce setups.  I'm thinking having accurate camber change, progressive
spring travel, caster, bump steer, etc., would probably be better than having a
linear system with a tire spring model that ignores all that or makes a rough
guess at it.  This first, then maybe a major rewrite later with everything.

  I might email you in time on a couple things.  Thanks for confirming my
hunches.

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

J. Todd Wass

Car physics; steering caster angle

by J. Todd Wass » Fri, 27 Jul 2001 06:21:08

  Thanks, Doug.  Say, you wouldn't happen to have some geometric sketches
and/or suspension data for one of your racing Porches laying around, would you?
 :-)  I need a real suspension system to try out, so far I've got to make my
own.  If it's an easy mouse click away for you, I'd appreciate it.  If not, no
problem of course.
Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://PerformanceSimulations.Com


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