# [ODE] Read out the contact forces?

Ke kxubb at yahoo.com
Mon Jul 9 13:58:26 MST 2007

```Thanks.
But to us, it would be easy to calculate the relative
position of contact point in body-fixed coordinate
system. Since the wheels and car are moving, it would
not be easy to find out the absolute position of
contact point.

Ke
--- "Jon Watte (ODE)" <hplus-ode at mindcontrol.org>
wrote:

> You want to get the point of contact with the
> ground, in absolute
> (world) coordinates. The relative position will vary
> in velocity
> depending on whether it's up or down in world space.
>
>
>
> Ke wrote:
> > Thanks.
> > For the velocity of the contact point, we used
> >
>
> >>From car kinematics, all the contact point
> velocity
> > should be zeroes or very close to zeroes if the
> wheel
> > is pure rolling on the ground. But the read-out
> from
> > dBodyGetRelPointVel gave us non-zero numbers.
> > And we set the contact[i].surface.slip1/2 to a
> very
> > small number (1E-12), which should gives us almost
> > non-slip. but the read-out non-zero numbers says
> we
> > still got slip velocity. what happened?
> > Thanks again.
> >
> > Ke
> >
> > --- "Jon Watte (ODE)" <hplus-ode at mindcontrol.org>
> > wrote:
> >
> >> This is a FAQ, and it is answered in the manual.
> The
> >> forces are zero,
> >> unless you are inside the execution of your step
> >> function.
> >>
> >> However, if you can get the velocity of the point
> of
> >> the body that
> >> touches the other geometry, and you know your mu,
> >> you can calculate some
> >> useful bounds on the friction force.
> >>
> >> Cheers,
> >>
> >>           / h+
> >>
> >>
> >>
> >> Ke wrote:
> >>> Hi,
> >>> When we simulate a 3-wheel car on ground,we need
> >> to
> >>> read out the contact/friction forces between
> >> wheels
> >>> and the ground. But when I used
> >>> dBodyGetForce(Wheel_body_ID), what I got is
> >> (0,0,0)
> >>> all the time. Am I wrong? or any other way to do
> >> that?
> >>> Thanks.
> >>>
> >>>
> >>> Ke
> >>>
> >>>
> >>>
> >>>
> >
>
____________________________________________________________________________________
> >>> Need a vacation? Get great deals
> >>> to amazing places on Yahoo! Travel.
> >>> http://travel.yahoo.com/
> >>> _______________________________________________
> >>> ODE mailing list
> >>> ODE at ode.org
> >>> http://ode.org/mailman/listinfo/ode
> >>>
> >>>
> >>>
> >
> >
> >
> >
>
____________________________________________________________________________________
> Research Panel today!
>
>
> >
> >
> >
> >
>

____________________________________________________________________________________
Looking for earth-friendly autos?
Browse Top Cars by "Green Rating" at Yahoo! Autos' Green Center.
http://autos.yahoo.com/green_center/
```