rec.autos.simulators

My sim: differentials

Stefano Casil

My sim: differentials

by Stefano Casil » Thu, 22 Aug 2002 00:40:26





> >> Stefano, what do you do to add braking torques? How do you clip the
> >> maximum brake torque when one of the axles stops moving?

> >I just add the brake torque after the diff.. so they come back
> >together with the reaction torque. I had a quick read to the code and
> >I see a state prediction and a brake torque cap based on inertia seen
> >by the wheel... the code is a bit unclear.. i wrote it ages
> >ago...hmmmm...

> Perhaps a brake torque cap so that the integration step won't make it
> do velocity (spin) reversal?
> Hm. Perhaps very difficult to predict with multiple diffs.

I'd say impossible.. maybe for 4WD I'll need some kind of "locked
wheel" state... and start from there... there must be a solution..
modelica and ADAMS do it.. I can do it as well...

yes. .let's be positive ;-)

Stefano Casillo
www.drivingitalia.com/netkar

Jonny Hodgso

My sim: differentials

by Jonny Hodgso » Thu, 22 Aug 2002 02:34:49


In practice, bearings and gears aren't perfect (neither are they
designed to be in a diff) so there's no such thing as a truly
open diff.  In car terms, though, the amount of torque transfer
in a gear diff (you don't want to know about ball diffs!) is
pretty small if there's no LS device.

If the engine delivers 200 Nm of torque, the zero-traction wheel
accelerates at [100/I_wheel] rad/sec^2 until the torque reaches
zero (electric motor) or the rev-limiter kicks in (IC engine).

During this time, the grippy wheel *does* see the same amount of
torque (an image comes to mind of a 2WD off-road buggy, beached
on a track marker with one rear wheel on the ground - repeatedly
punching the throttle causes it to move forward slightly as the
free wheel accelerates, and will *sometimes* get it moving
again).  However, [full_engine_torque/2] will accelerate I_wheel
pretty damn quickly ;-) so the impulse (force x time) acting on
the car is not often sufficient to start it moving.

Yup, which it doesn't, usually.

Good, I haven't argued myself into a corner yet, so that suggests
my thinking may be correct - you're always welcome to test it,
though!

Jonny


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