rec.autos.simulators

Back to some physics programming, I'm trying to get Richard Chaney's
idea (see http://www.racesimcentral.net/) of using a quaternion for
rotation, and angular velocities (adjusted using precession) and a
location/velocity for the rigid bodies to work.

It contains some easy & fast to use formulas for forces placed on a
point on the body (for example, aerodynamic drag). This leads to a
force on the center of mass, and a torque.

Now I wonder, before I haul over my source; can I just add resulting
forces and torques to get 1 end-result, and apply that to the body?
I mean like:

init:
  totalForce.SetToZero()
  totalTorque.SetToZero()  (both are 3D vectors)
calc_forces:

  wing_front_force=(x,y,z)
  totalForce+=wing_front_force (in body coords)
  // Calc torque because of wing_front_force using T=F*r
  // where 'r' is a vector from the CM to the point
  // of force application
  totalTorque+=wing_torque

  wing_back_force=...
  totalForce+=wing_back_force
  totalTorque+=wing_back_torque

integrate:
  // Apply force and torque to body
  cm_acceleration=totalForce/mass
  cm_angular_acceleration=... (not really totalTorque/inertia, but
using some more careful formulas because of precession)

I read at one point, IIRC, that you can see a total force system of a
rigid body as having a resultant force and torque, so you can combine
all forces and torques to come up with just 1 force and 1 torque.
Would this be the way?

BTW The Giancarlo Genta book (Motor Vehicle Dynamics), is it any good
on explaining the Pacejka Magic Formula? The formula seems great, but
I can't find too much raw info on it (to use in a simulation), and the
book, priced at US$98, is quite pricy if all I can find on the formula
is in an appendix, perhaps (seeing I also have RCVD and Gillespie's
Fundamentals of Vehicle Dynamics).

Thanks for any ideas,

Ruud van Gaal, GPL Rank +53.25
Pencil art    : http://www.racesimcentral.net/
Car simulation: http://www.racesimcentral.net/

-Gregor

How would this be represented then? (I currently just apply the engine
torque to the wheel, not applying that torque to the sprung mass
(body) itself in any way)

Thanks again,

Ruud van Gaal, GPL Rank +53.25
Pencil art    : http://www.marketgraph.nl/gallery/
Car simulation: http://www.marketgraph.nl/gallery/racer/

  I think you just add the engine torque along the appropriate axis.  I saw a
motorcycle drag race simulator similar to the program at my site, that wanted
the engine's location and direction of rotation.  Funny how the engine torque
could either increase or decrease the tendency of a bike to wheelie!  

  I'll be back to hear Gregor, in case I'm missing (another) something :0)

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

Todd extended that idea also to the engine torque. You can consider the
engine as another rotating mass that exerts torque on the body along the
appropriate axis, and that's why the wheelie tendency is very much
dependent on the engine orientation.

Also, without going into too much detail, I can tell you that you can
consider the body as a sort of 'sink & source' for torques; that is to
say, you don't need to worry too much about how the torque actually gets
to the wheels, if you input it along the appropriate axis into the body,
and then 'take it out' on the wheels (as per the above Newton's law
example), it's the exact way of doing it as far as the effects on the
body, wheels and engine go, even when you take the losses into account.
You will, however, still need to somehow be able to couple the engine
and wheels rotational velocities and inertias, as they are rigidly
coupled (with an additional degree of freedom in the differential). But
as far as torques go, that's the simplest and proper way to do it. I
would go into doing it all only after what you currently set out to do
works, as debugging will be much more messy if everything is taken into
account from the start.

Which is pretty cool but still pretty crazy ;)

- Matt

Ruud van Gaal, GPL Rank +53.25
Pencil art    : http://www.marketgraph.nl/gallery/
Car simulation: http://www.marketgraph.nl/gallery/racer/

As one would expect there appears to have been quite
a difference in thought about the which wheels to
land on as Denny Hulme gave the exact opposite
advice (the interested party was a young David
Hobbs) reasoning that regaining steering input
was more important than worrying about the rear
coming around.

The book itself is quite fascinating as the author
spent an entire month with 'Big John' around the
time of the 1967 German GP.  Surtees mentions
that the lighter more powerful Honda 'Electron'
motor gave 394 HP at best and that all versions
in '67 only revved to 10,200 RPM, making maximum
power at 9,500 RPM and simply not pulling at all
below 8,750 RPM.  Too bad the GPL engine
model seems to use a second order curve
to model the power band.

Also of interest was that he geared for 190 MPH
at Spa and 172 MPH at the 'Ring.  They also give
the fuel loads for the German GP as:

BRM: 40 Gallons
Brabham: 32 Gallons
Cooper: 37 Gallons
Lotus: 35 Gallons
Honda: 48 Gallons

The Japanese it appears were quite the opposite
of Colin Chapman when it came to estimating fuel
consumption. ;-)

 - George

Ofcourse, using the brakes seems a waste of precious GPLRank time. :)

Ruud van Gaal, GPL Rank +53.25
Pencil art    : http://www.racesimcentral.net/
Car simulation: http://www.racesimcentral.net/