collision before touching surface

[Fabrice]

Can't distill much from a description alone. Please post (or send) the scene file.

You're right

But I found out :
the shape was a group of 4 differents flakes 
and I guess the group's pivot was used instead of each group's member local pivot
wich was generating a visual offset.

I now use the 4 flakes separatly as 4 shapes REF and it works fine!