rec.autos.simulators

Car Physics: Mass or no Mass

Colin Re

Car Physics: Mass or no Mass

by Colin Re » Wed, 03 Apr 2002 06:53:05

As my simulation develops I am continually haunted by one single
question;

Should I give my wheels mass?

At the moment I don't and the car body still has all of the subtle
movements that one would expect .... What exactly am I missing apart
from a huge headache trying to keep track of the trajectory of the
wheel CM through it's complex arc, especially as I head off into the
land of SLA with caster ...

Having watched some of the replays in GPL it would seem that are
keeping track of the wheels' mass, do they?

TIA,

Colin

-----------------------------
http://www.racesimcentral.net/

Haqsa

Car Physics: Mass or no Mass

by Haqsa » Wed, 03 Apr 2002 07:20:04

If you don't give the wheels mass they aren't going to respond to bumps
and dips correctly.


Colin Re

Car Physics: Mass or no Mass

by Colin Re » Wed, 03 Apr 2002 11:05:45


> If you don't give the wheels mass they aren't going to respond to bumps
> and dips correctly.

You would think so yes but it really doesn't seem to make that much difference.

Does GPL or N2002 model tire springs?

Colin

Dave Henri

Car Physics: Mass or no Mass

by Dave Henri » Wed, 03 Apr 2002 12:34:25


> If you don't give the wheels mass they aren't going to respond to bumps
> and dips correctly.



> > As my simulation develops I am continually haunted by one single
> > question;

> > Should I give my wheels mass?

   Well yes...if the car in question is Italian.  If it's British, then
probably not...and if it's Hungarian...why are you building it?
can cars be Catholic?
dave henrie
Haqsa

Car Physics: Mass or no Mass

by Haqsa » Wed, 03 Apr 2002 13:09:28



difference.

Under normal circumstances perhaps not, but what about when cutting over
kerbs?  Seems like having mass would change how well the tire could
follow the contour of the kerb, which would then change how much grip
you would get from it.  IOW a massless tire would allow somebody to cut
over the kerbs with no real penalty to grip.  Also remember wheel mass
is part of your drivetrain inertia.

Don't know about those two, but I do know F1 2001 models tire spring
rate, damper rate, and mass.

Ruud van Ga

Car Physics: Mass or no Mass

by Ruud van Ga » Wed, 03 Apr 2002 23:50:36

Hi Colin,

Hm, I have a setting in Racer which is close to this; debug.ini's
use_susp_as_load. If on, it skips tire rate and uses the suspension
instead (but still uses wheel mass to get the wheel's acceleration).

I don't have a trajectory btw, just a pure vertical suspension system.
For the moment I'll just have a simplistic form of trajectories, like
camber change/mm suspension movement.

The problem you might face is that with tire rate you need bigger
frequencies (more physics updates/sec), which is why I originally did
the use_susp_as_load setting, until the dampers were integrated
implicitly instead of explicitly.
I know use tire rates again, btw. It just explodes a bit sooner.

I don't know; that might have just been gyroscopic precession you're
seeing.
What GPL does have is fixed rollcenters, so you'd say no kinematic
suspensions. So at least you can come quite far without trajectories.
;-)

Perhaps Olley's calculations (the new book from Doug & Bill Milliken)
can also help. Haven't bought it yet.

Ruud van Gaal
Free car sim: http://www.racer.nl/
Pencil art  : http://www.marketgraph.nl/gallery/

Colin Re

Car Physics: Mass or no Mass

by Colin Re » Thu, 04 Apr 2002 02:56:28

Ahhhh kerbs ;-) Can you imagine how cool it would be to be sliding
sideways into a kerb, the two leading wheels hit and then boom, you
are flipping in a beautiful barrel roll.

I have rotational mass because as you say they are part of the
drivetrain inertia ;-)

Hmmm, I really should go and buy me a copy, I saw it in my local
software shop recently ... hmmmm

Colin

Colin Re

Car Physics: Mass or no Mass

by Colin Re » Thu, 04 Apr 2002 03:06:58

Are you trying to imply that even using implicit integration, using
tire rates introduces instabilities into the sim?

When I've finished I also won't have a trajectory as such because once
the arm lengths and positions have been specified then the wheel will
automatically move through the correct path of movement.

Indeed, but time to move on, I can't wait to get the effects of
dynamic camber working but it should be a nightmare to setup, so many
more parameters.

Todd seems to be coming along really well, can't wait for a demo (is
there one coming?)

Yeah tempting isn't it .... ;-)

Colin

J. Todd Wass

Car Physics: Mass or no Mass

by J. Todd Wass » Thu, 04 Apr 2002 04:34:50

 Thanks, Colin :0)  No demo just yet.  I don't use DirectX for anything 'cause
this is written in PowerBasic, which doesn't have much in the way of DX
wrappers.  I just got Wav files to load in manually as a matter of fact, so I
should be able to do a little sound mixing.  GPL's Ferrari engine sounds GREAT
at 15,500 rpm!  lol  

 I'll need to use standard Windows messages to allow controllers to be used.
This works fine with my Thrustmaster SuperSport wheel with it's 15 pin
connector, but I don't know if USB controllers will work this way..  Do you?
I'll be using MM_JOY1MOVE, joySetCapture, etc...  That won't allow force
feedback, but will it allow most USB wheels to work anyway?

 What I really need is some sort of DirectInput wrapper Dll or something.  I
think I've got sounds pretty well***ed, but it's done in software which is
probably not as fast as DirectSound would be :-(  Should get a wrapper for that
too, or maybe switch to FMod like Ruud did.

I was under the impression you already had this running.  Where are you right
now with your suspension system?

Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://www.racesimcentral.net/

My little car sim screenshots:
http://www.racesimcentral.net/

J. Todd Wass

Car Physics: Mass or no Mass

by J. Todd Wass » Thu, 04 Apr 2002 04:41:15

You could still get this right with a massless tire, I think.  The big effect
there you're talking about is how the wheel might jump off the curb, causing a
total loss of grip momentarily, right?  This can still be done by noting the
wheel mass in order to limit the suspension force on rebound, and at the same
time ignoring it's effect in the regular loop of calculations (meaning
essentially you're still using massless tires).  You'd wind up with the
vertical force going to zero if the wheel should leave the ground, but you
wouldn't need all the complex and CPU hungry constraint stuff to do it, just a
quick IF block to take care of it.  Should get the best of both worlds this
way, I think.

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

My little car sim screenshots:
http://performancesimulations.com/scnshot4.htm

Colin Re

Car Physics: Mass or no Mass

by Colin Re » Thu, 04 Apr 2002 09:38:27

Yeah, the first time I got the sound working I deliberately
disconnected all drive to the wheels and just played with revving the
engine for about an hour, sad isn't it but you should have seen the
smile on my face ;-)

I just ripped loads of code from one the DX SDK samples and shimmied
it into my engine, there is still a bug in it where either dinput or
dsound is causing the application to not terminate properly !?!?!?

Ahhh alas no, currently I have massless wheels (but with rotational
inertia) coupled with a vertical ray casting "lets find where the
ground is and if it is less than the spring rest length then put the
wheel there" kind of thing ... I then find the vertical velocity of
the wheel by doing vertVel = (currlen - oldlen) / dTime. A total hack
but I wouldn't be suprised if most games so far use this method ...

I am currently toying with a new method which I call "valid
configuration" suspension which allows me to specify limits in the
suspension travel as well and should be a whole load less hacking than
now.

Colin

J. Todd Wass

Car Physics: Mass or no Mass

by J. Todd Wass » Thu, 04 Apr 2002 11:23:58

LOL!  I did the same thing with my other system, I know how it goes :0)  Life's
little triumphs and pleasures...

Beats me...  Maybe Ruud, Ashley, or Gregor would know.  (Matt Jessick would too
I'm sure, but he's been scarce lately.)

Anyway, it sounds like your suspension works pretty much the same way mine
does.  I still need to convert it to truly use the full 3-D geometry so
anti-dive/squat, etc., are included.  Right now it's just a front view virtual
arm system, but it works great and is almost as fast as using a vertical wheel
travel model.  You're right though, it makes car setup much more complicated.
I counted about 16 different ways at each wheel to adjust camber, for instance,
and they all have slightly different impact on the handling..  Sheesh..  lol

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

My little car sim screenshots:
http://performancesimulations.com/scnshot4.htm

Colin Re

Car Physics: Mass or no Mass

by Colin Re » Fri, 05 Apr 2002 02:21:51

:-)

Do you have cast setup yet?

Colin

J. Todd Wass

Car Physics: Mass or no Mass

by J. Todd Wass » Fri, 05 Apr 2002 04:59:08

Caster? Yep, all setup, but it doesn't change with wheel travel yet.  For that,
I need the full 3-D system.   Are you doing McPherson struts or A-arms in yours
first?

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

My little car sim screenshots:
http://performancesimulations.com/scnshot4.htm

Colin Re

Car Physics: Mass or no Mass

by Colin Re » Fri, 05 Apr 2002 08:51:01

Doh!!! Yes I did mean caster ;-) Stupid keyboard ... stupid hands ....

A-arms first, they are easier, I want to get the suspension module
setup first then I can just add suspension types as I think of them.

Colin


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