I have a GridDataController class that handles all of my data requests from my jqGrids. I think my application would be easier to maintain if I busted that class apart, put the relevent actions in their respective controllers, keep the grid specific functions in a base controller class and then inherit from that class when I need to retrieve grid data in my controllers.
It doesn't seem right for the Base Controller class to reside in the Controllers folder as I do not want any of it's actions to be called.
Is there a convention for this?
No conventions. It would be easier for everybody though if you put it under Controllers, since its a controller, want it or not :). However, you can put it anywhere and refer to it inside your controllers by using and by inheriting from it (if that's what you want to do).
If you don't want its actions to be called, declare it appropriately as abstract class ...
So, no right answer to your question, but before putting it anywhere, thing twice and change your mind and put it back into Controllers :)