I'm making a race demo and I use the terrain as the track.But when I edit the level in WED,if the car is uptowards the terrain,it doesn't drop down on the terrain,so I couldn't make it move.
And I set the gravity of it,when I change the position of the terrain,the car can move a small distance and sometimes stop suddenly.Why?

Its collision box or polys are catching on edges of the terrain segments.
Raise the collision box or model untill it no longer 'catches'