Just got back. Though most of my time was watching the TT and being a tourist. We stopped at Port Erin on the South West Coast. Bradda Head is a 'local' spot and was pretty good. Great if you like riding cliff tops with the sea crashing down below!! Also consider Berrule Forest, though I didnt get to that spot. Try getting in touch with the local bike shops in Douglas sure locals will give you more info?