I have just downloaded a BMW blender model (http://www.blendswap.com/3D-models/vehicles/bmw-3-series-coupe/) and tried to find out how this is made. I tried to edit the model and started with the body. That went fine. After that I tried to switch to ‘Edit mode’ on the tiers. But it looks like the tiers do not have an edit mode.
How is this possible? And how can I edit the tiers?