Craft 4 wheller and terrain from terragen

[Octavian]

Hello,

I wish to thank you all that use this forum for the solutions provided when you response. I came with a question, I have a problem using 4wheller rig and a terrain imported from another application called terragen. The terrain was exported as obj and is ok in 3dsmax (topology is made of quad poly). After I convert the terrain to poly an interesting problem come out. When the rig of the car is set in place and after press record button the car rig is going thru the terrain and fall. I want to mention that all the polygons of the terrain have all normals unified, and there are no poly's that re turned. After some tryouts that have fail I think that the terrain was the problem so i optimize the object from 900k poly to a lower rate of 40k poly but the result was the same, the car fall again. Than I decide to reconstruct the terrain by modeling a new terrain based on the original one that was exported so I start to push edges of a small plane and than weld vertices. After I create a new terrain that has all the vertices snapped to the old vertices of the original terrain the car rig was ok he stay on the plane when I press record. So my question is simple how can I use the original terrain because to recreate the terrain from scratch based on this technique is hard I need to recreate 900k poly's!!!!. What am I doing wrong and how can I use a terrain generated by another application.
I hope that someone came with a solution at this problem.
Thank you in advance for the support.