To tidy up a thread I started, I tried to access the Copper Canyon from the western side and the city of Los Mochis. I rode up to Choix and a little further (to Nacimiento) before I was forced back.
The basic problem was that my maps were not up to the task and better ones couldnot be obtained locally. With hindsight I have discovered that there are two good possibilities for riding into the canyon from the west.
Firstly, the northern route that wouldbring you out ultimately in Creel: it is possible to ride up following a similar route to the railway line. On my map there was a gap in the road between Bahuichivo and Divisidero. I saw a very nice track as I passed by a day later and regretted not trying this route. One word of caution though, I checked with the police about the safety on this route and they confirmed that people had been attacked and robbed here. I was advised this would be ok in a groupbut not recommended for a lone rider.
The second and southern route: This would bring you out in Batopilas. There is a track from Nacimiento (the point I turned around) that leads 100kms or so to Batopilas on the canyon floor. It is tricky to find the way from the west but on speaking with the German guy who lives just up the track from Batopilas in La Bufa, the track is in reasonable condition for an enduro bike to complete in about 3 hours.
Another point worth making is that most of the maps of the canyon show a dead endat Batopilas. This is certainly not the case. Batopilas is essentially a crossroads with access to the north (Divisidero), west (Choix and Los Mochis) and a whole load more dirt to the south. All routes are suitable for an enduro bike.