well, i've recentley started working with gmax and battlefield , getting myself familiar with the meshes in the game, but this one stumps me.

i was recentley modifying some meshes to test out in BF , to check i was doing it right, but i found my meshes collisions were not working after i had extruded it, the model collapsed through the ground after i spun over in a jeep, if it weren't for the wheels which i had not fiddled with i would have went straight through.

can anyone point out what im doing wrong? thanks. :)