TriMeshes should only be used for static objects in reality, they're too slow compared to primitives. If you want to create non static custom geometry, you need to build composite objects using multiple primitive geoms like cubes, spheres and capsules with geom offsets. Look at the Geom Offset demos to see how it works. Once you've built your custom physics geometry, update your custom mesh (the visible geometry) with the position/rotation of the body or one of the geoms.