Creating Flexible body in hm Optistruct

I am creating flexible body in optistruct of a truck chassis with trailer which is very big model with so many number of nodes and elements.

CBN method is used UB_FREQ given 20

but i am getting 131 modes and fbi file size is about 15GB which is too much to import in SIMPACK

and then i gave NMODES 40 but still its giving 131 modes which is very high

So please help me with this

Thank You