Vehicle Collision issue

I am having an interesting problem that I cannot find a solution to.

I have wheeled vehicles, tanks basically, that have proper collision meshes. If I spawn one, the player is stopped by the collision mesh. However, if a tank runs into a player, normally he can drive right thru them. Sometimes tanks can drive thru tanks as well. Their integration is set to 8, and they arent going much faster than around 15 meters per second.

Oh, and I have 3 wheeled vehicles, which you can see in the links proctored below. They all exhibit the same symptoms.

Also, tanks seem to be stopped by terrain and interiors.

Any idea what could be wrong?

http://www.kuattech.com/warr.jpg
http://www.kuattech.com/abr.jpg
http://www.kuattech.com/t80.jpg