Having studied the Tour de France stage schedule a bit further I now think that the stage ending at Alpe d'Huez on July 18th may be easier to get to then the one ending at Mont Ventoux.
It still may be necessary to rent a car I guess. But I read up on getting there at Wikitravel and there is a ski lift from Oz-en-Oisans that is sort of the recommended place to park your car. Is it possible to get from Grenoble (or any other city accessible by train) to Oz-En-Oisans with public transportation?
You can go by bus from Grenoble to the Alpe d'Huez. On the 18th of July 2013 you can leave Grenoble at 6:55, 10:27 or 13:00. You will arrive at the Alpe d'Huez at 9:13, 12:43 or 14:43. On that day there is no bus leaving between 13:00 and 18:55, for obvious reasons ... Schedules can be consulted here. The site is in French only. Let me know if you need help with it.
If you are using public transport from Grenoble, there is no need to travel via Oz-En-Oisans. Time- and budgetwise it is a bad deal.
