Thanks for the help.
I found that on the wiki :
How can an immovable body be created?
In other words, how can you create a body that doesn’t move, but that interacts with other bodies? The answer is to create a geom only, without the corresponding rigid body object. The geom is associated with a rigid body ID of zero. Then in the contact callback when you detect a collision between two geoms with a nonzero body ID and a zero body ID, you can simply pass those two IDs to the dJointAttach function as normal. This will create a contact between the rigid body and the static environment.
Don’t try to get the same effect by setting a very high mass/inertia on the “motionless” body and then resetting it’s position/orientation on each time step. This can cause unexpected simulation errors.
But i don’t know how to do that. Cause when i don’t set a body to the geom then it doesn’t interact with the environment (no collision)