This article has appeared on a prior thread, but it's written by a master (Budd Davisson) and is worth a close look!
He makes the point about the CG relationship to the main gear (CG forward of the mains in a tri-gear, CG aft of the mains in a taildragger) and that is all-important to understanding ground handling.
You can demonstrate this to yourself next time you're at the grocery. Find a cart that rolls straight - no dragging wheels etc. Give it a shove down the aisle and let go - it should roll straight ahead.
Now turn it around and shove it backwards down the aisle. In less than a second it will do the prettiest ground loop! The difference is the position of the CG relative to the "main gear" (i.e. fixed wheels).