rec.autos.simulators

Please somebody help me or point me on some ras archive i didnt find.

So i have probleme modeling differential.

Lets start with an open differential :)

lets assume those datas

TorqueIn  .. torque given to the diff
TorqueOut[2] .. torque out of the diff to the both axles
ResistiveTorque[2].. torque that come back from axles

When reading books i can read that an open diff split  torque_in in 2 to
the TorqueOut[i].

So TorqueOut[i] = TorqueIn / 2

Its also say ( Race Car Vehicules Dynamics page 734 ) "if one wheel
loses traction the total drive force available will be just twice the
force available from the tire with least grip". i.e. ResistiveTorque is
weak ?

So i modify to

TorqueMaxPossible = Min( ResistiveTorque[0], ResistiveTorque[1] )
Torque = Min( TorqueIn, TorqueMaxPossible )
TorqueOut[i] = Torque / 2

The probleme is when the driving wheels are not sliding at all ( i.e.
slip ratio ~0 ) the ResistiveTorque is almot zero , so TorqueOut[i]
become slower, so slipratio become slower, etc... the car doesnt accell
anymore :(

So what should i do ?

1) Always use Torque = TorqueIn ?
2) Use Torque = Min ( TorqueIn, TorqueMaxPossible ) only if
Abs(SlipRatio ) > 1 ( if we assume SlipRatio is normalize with Optimum
Slipratio ).
3) Maybe i should use a ResistiveBiasInstead ? TorqueBias =
ResistiveTorque[Grippy] / ResistiveTorque[Slippy] ? How should i use it
then ?

Limited Slip Diff

Ok, one i'll get Open Diff working , i think limited Diff are easy.

lets assume

OmegaOut[2] are angle velocity of axles
OmegaIn are anglular velocity Engine side
OmegaS , angulare velocity of internal diff

OmegaIn = ( OmegaOut[0] + OmegaIn[1] ) / 2
OmegaS = OmegaOut[1] - OmegaOut[2]

Td = Differential torque.

Torque = ?? , depend of how its works with open diff lets assume we have
2 grippy wheels
Torque = TorqueIn

Td = OmegaS * DiffFriction
TorqueOut[0] = ( Torque - Td ) / 2
TorqueOut[1] = ( Torque + Td ) / 2

Is that right ?

Now, should i clamp Td so Abs( Td ) is not > to Torque ??

thanks in advance for any help.

Seb.

On Mon, 15 Mar 2004 12:03:02 +0100, Sebastien Tixier

Hard to imagine what you did in VRally3 considering your question! ;-)

Read that carefully; 'driving' force means the effective force being
used to accelerate the car. This does NOT mean forces/torque just
disappears! Just that the excess force is used to spin the wheels.

So this is not the case at all. TorqueOut[i]=torque/2. That's it.

Consider the example of one tire on the road, one on perfect ice.
torqueIn=torqueEngine*gearing
torque[i]=torqueIn/2

Suppose torqueIn=100Nm (engine), torqueResistive[0]=50Nm (road) and
torqueResistive[1]=0Nm (ice). What would happen?
The sum of resistive torques add up to torquein, so on the engine
side, no accelerating will take place; the engine will keep rotating
at the same speed.
The diff will split torque 50/50: 50Nm on both sides. The [0] side
will stay still. However, the [1] side gets 50Nm from the engine
(torque[1]=50Nm), plus torque transfer happens from [0] to [1].
Totalling 50Nm of torque transfer means 100Nm is used on the [1] side
to spin the wheel on ice.

The transfer is a bit odd, and this example evens up too nicely. A
different example:
torqueResist[0]=100, torqueRestist[1]=30, torqueEngine=200.
The torque that transfers back to the engine is the resistive torque
pointing in the same direction; in this case 30 from [0] and 30 from
[1]. The remaining torque transfer (this is the diff acting) of
100-30=70Nm goes to spin up [1].
So [0] gets 200/2=100Nm engine torque, road reaction 100Nm, summing to
0Nm = no motion.
And [1] gets 200/2=100Nm engine torque, road reaction 30Nm, torque
transfer from [0] = 70Nm. 100+70-30=140Nm spinning the wheel.

Not sure if that last sum is totally correct. :) Would be good to read
up on Casillo's post. Search for 'Ruud van Gaal physics torque through
the driveline' for a post of mine quoting Stefano (otherwise I have it
in my archives here as well).

LSD is then just adding a torque resisting the motion of the axles, I
think. ;-)

Hope that helps,

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

hey hey, well, in fact, i only modeled LDS in VRally3, and the diff were
never open. The minimum slip was 50% ;)
In VR3 i split torque equaly all the time plus the resistorque from
diff. I know its good on very low grip :(

arf, lack of English for me :)))

Hey, great explaination :) and i'm quite happy, because,  as i didnt saw
this answer i try to figure out myslef looking to the mechanical scheme
of an open diff :) and i almost find the good answer except for the
torque going back to the engine :)
Thanks a lot for this good explanation :)

Well i didnt find any archive no where, so it would be so great if u

Again thanks u.