Move a rigid body forward localy

Hello,
I am working on a small project where I have a battleram.
I managed to control it as an arcade car so it can move around the map.
Now I want to move the timber inside it backward and forward but I do not manage to keep the timber to move only along the local z axis.
Either it works only in initial position and then when my vehicle move it does not keep the loca z axis as its move orientation, either it gets inertia when I move the vehicle.
I am started to get lost with all the local vs global Vector3.

My idea was to do something like this :
//rb : timber's rigid body
if (Input.GetKey(KeyCode.LeftAlt))
{
rb.MovePosition(rb.transform.localPosition - rb.transform.forward * speed * Time.fixedDeltaTime );

}
rb.velocity = Vector3.zero; // Remove inertia
But with that, my timber teleport far from the ram.

Any help would be appreciated.

Regards.

 

Why
transform.localPosition - transform.forward?

If you want to move something in the local z axis try with just transform.forward and then multiply it by the speed.

Also watch out with rigidbody.movePosition afaik it ignores collision, so you can get inside walls.

 

MovePosition does not do this.
https://docs.unity3d.com/ScriptReference/Rigidbody-position.html

"If you want to continuously move a rigidbody use MovePosition instead, which takes interpolation into account."