rec.autos.simulators

Thanks for eveyone that helped out last week.  I've been thinking
about this over the weekend, and I'm still a bit confused.  Maybe it's
just the way I'm doing it.....

I've got an engine.  The engine produces a torque at a specific RPM.
The Idle RPM is 850.  The engine also has a flywheel that has inertia.
 When the engine is at idle (850rpm) and the throttle is at idle
throttle (5%), then the internal torque of the engine equals the
output torque of the engine (5% of the max torque at 850rpm) and hence
no output torque is produced.

If the throttle is pressed, then the output torque of the engine goes
up, and the rpm of the engine is calculated by working out the
acceleration of the flywheel.

Now, if the engine is put into gear, then the inertia of the
drivetrain and the drive wheels are added into the engine flywheel
inertia (using the relevant equations).  This seems to work pretty
well, but it is only modelling the car if there is no reaction from
the ground.

The problem I have is modelling the external forces on the wheels that
feed back to the engine.  The original version I had derived the
engine RPM from the angular velocity of the wheels which was derived
from the speed of the vehicle.  Now, I've got an engine that is it's
own object.  The engine can have an rpm when in neutral even if the
vehicle isn't moving.  So, the question is, how to work out how much
negative torque the wheels apply on the engine.

Cheers,

Nick.

Sounds good, inertia of engine itself is also included probably.

Glad you got that out of the way; the velocity of the vehicle has very
little to do with the speed of the wheels, in racing at least. ;-)

I think I explained quite a bit in a post before this one, hope you
didn't miss that. About Pacejka, reaction forces, sliptrac.crv etc.

In short, you just let the whole assembly rotate, using the total
inertia (geared etc). Then, as the wheels begin to rotate faster than
the ground is moving beneath them, a slip ratio appears.
The slip ratio decides the reaction force of the surface (see the
curves in RCVD), and you add that as negative force to the whole
assembly.

So now you have:
acc=total_torque/total_inertia

but you have to add the negative forces on the wheel to total_torque.
You just got to have those curves in, or approximate by 2 or 3 lines:

   /\
  /  \___
 /

Something like that. From 0 to 100% (maximum traction), and once it
starts slipping too much, it goes back to ~60% (~75% for braking).

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

When you get things right, then you should be able to slowly let out the
clutch with the engine at idle, and start the car rolling (unless you have
programmed in a very abrupt racing clutch that engages over a very small
distance).  This was one test we used during the development of Atari Hard
Drivin' (1st arcade game with a real physics model, c.1989).

-- Doug
         Douglas Milliken
         Milliken Research Associates, Inc.

  A masterpiece :-)  I drive it whenever I see it, as well as the sequel, Race
Drivin'.  

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

Any chance the source is opened up? ;-)
This way we could check out the clutch functions and such that are
often not in our sims. :)

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

|
| >>When you get things right, then you should be able to slowly let out the
| >>clutch with the engine at idle, and start the car rolling (unless you
have
| >>programmed in a very abrupt racing clutch that engages over a very small
| >>distance).  This was one test we used during the development of Atari
Hard
| >>Drivin' (1st arcade game with a real physics model, c.1989).
| >
| >  A masterpiece :-)  I drive it whenever I see it, as well as the sequel,
Race
| >Drivin'.
|
| Any chance the source is opened up? ;-)
| This way we could check out the clutch functions and such that are
| often not in our sims. :)
|
|

Ouuuuh that WOULD be nice <g>

Ash

  That would be great, but I doubt it :-)  What's your clutch problem?
Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://PerformanceSimulations.Com

Afaik, Atari Coin-Op still owns it (does anyone know who owns Atari these
days?).  A modified version of the Race Drivin' model was used (so I hear)
in San Francisco Rush, an arcade game from a few years ago.  I don't think
that there is any chance of the source being opened up in the near future.
That's not the Atari style...<grin>...the development building I used to
visit had separate electronic locks with credit-card-sized inductive-keys
for every separate lab and project.  I had to borrow my friend's "key" to
use the washroom.

-- Doug

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

The way I do it is similar to how I handle the locking wheels (and now
the differential as well :) ). I have a flag that tells you whether the
clutch is locked with the assembly or not. If the clutch is locked, then
it's all considered as a single assembly. However, once the torque on
the clutch exceeds how much the clutch can handle (which can vary with
how much the clutch is actually applied), I set the flag to unlocked,
and then treat the engine and the rest of the drivetrain as a separate
rotating assemblies, but coupled with a frictional torque that the
clutch produces (again, varies with clutch application) and points in
the opposite direction to the slippage of the clutch. The clutch is
again set to locked (with engine becoming a part of the assembly) when
the direction of the clutch slippage reverses sign.

 >

Actually, I never properly thought about this. Since, however, the diff
in my implementation is the highermost object in the rotating hierarchy,
it wouldn't work in my system. I think Chris is using a different
hierarchy (but the same system of equations) with engine on top, and
perhaps that way one can just couple a diff upon diff in a treelike
structure, and it could work. I don't know for sure, though, and still
have doubts. However, if you have a special mechanism (such as the one
above) for handling locked wheels, then it all becomes really, really
complex anyway as there are many combinations of locked yet interlinked
wheels possible and one should separately handle them all.

-Gregor