In reply to ow arm:
Look on a map for bridleways. There's a bridleway marked going to the summit of Moel Hebog but the line on the map isn't where you go on the ground - it goes over a 300ft cliff for starters!
Legally there aren't many summits that you can bike to, most of the bridleways take passes - makes sense as that'd be where the old byways would go rather than over the tops. Moel Siabod definitely doesn't have a bridleway up it.
There's a loop from Capel Curig over to Llyn Cowlyd and back via Crafnant. Also from near Capel is Sarn Helen which heads up a rough track from Pont Cyfyng and goes over to Dolwyddelan, you then come back via the trails above Betws y Coed.
Post edited at 09:58