[ODE] which kind of parameters can influence the slip velocity?

[=?GB2312?B?xe3Qy87E?=]

These days, I want to set the contact.slip with reason in my car simulation.
So I want to find out which kind of parameters will influence the slip
velocity.

First, I just add one wheel in the ODE. The wheel mass is 5kg and the
gravity is 9.81, the torque is add on the rolling axis. Then I set the
contact parameters like this:

 ontact[i].surface.mode = dContactFDir1 | dContactMu2 | dContactSlip2 |
dContactBounce | dContactSoftERP |  dContactSoftCFM | dContactApprox1;

        FDIR1 =  wheel rolling axis (not the direction that the wheel
rolling in)

        contact[i].surface.mu = 0.7;
        contact[i].surface.mu2 = 0.7;
        contact[i].surface.slip2 = 0.01;
        contact[i].surface.bounce = 0.001;
        contact[i].surface.soft_erp = 0.9;
        contact[i].surface.soft_cfm = 0.01;

And the wheel's radius is 0.1m, the wheel width is 0.15m.

I get these data from ODE:
torque = 0.5 slip2 = 0.01 Angular velocity - linear velocity = 0.01667
torque = 1.0 slip2 = 0.01 Angular velocity - linear velocity = 0.03333
torque = 1.0 slip2 = 0.02 Angular velocity - linear velocity = 0.06667

there is no effectives when I changed the wheel width, the wheel mass, the
mu  mu2, bounce soft_erp and soft_cfm. But when I changed the wheel radius
to 0.2,it will be:
torque = 0.5 slip2 = 0.01 Angular velocity - linear velocity = 0.00833

So it seems the torque , contact.slip2 and the wheel radius will influence
the slip velocity(Angular velocity - linear velocity). Is this true? And can
someone help me
to understand how these parameters influence the slip velocity?
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