if you are a basher you want to lock the rear diff, the bashing ability increases at least 20%, drifting wise I would lock both the front and rear, its a much more predictable drift, usually a diff in the front is a bad idea, you loose turn in but gain run out, as for the guy that said add thicker oil thats fine except for 80% of rc cars don''t use oil diffs....sqaure racing makes a very nice spool for the tt 01, order a couple and your drifting will become much easier on pvc