# [ODE] Read out the contact forces?

Jon Watte (ODE) hplus-ode at mindcontrol.org
Fri Jul 6 00:45:21 MST 2007

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
>>>
>>>
>>>
>>>
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