Follow the timber trails from Arosa to Ochsenalp a steady uphill section with narrow single tracks and wide open fast curves on loose gravel. Then take the mountain signpost to Tschiertschen - this is a very rough trail across the high Swiss alps, so make sure you take supplies with you in case of a flat tire of a breakage. If disaster does strike you can take a post bus from Tschiertschen - but it could be up to 1.5 hour walk depending on the location. https://youtu.be/BwG_VQk9lPM
The remainder of the trail is very fast curves and hairpin bends down to Chur - the oldest town in Switzerland.
Then take the trail back up to Arosa, with your bike. Buy a half fare to cover the bike or ask at the station. Best of Luck Have fun!