I'm sure I read it somewhere and I know it's a rule but I was hoping someone can put point me in the right direction.

I can't find the ruling where is says bases need to stay flat on the terrain or playmat.

An extension on this with speederbikes, I thought I saw somewhere if the model contacts terrain from the compulsory move and cannot lay flat but the base isn't touching it still counts as running into the terrain thus trigger damage.

How would this affect if a speederbike ran into another speeder bike?

Ran into this today and wasn't sure how to proceed, just a friendly game so let it go but was curious because my opponent kinda made a comment that some speederbikers were slightly off because they were just touching the base of terrain that was maybe half a centimeter high on the one side so they were not flat.

Any help would be appreciated

Thanks