A middle mountain route that is nevertheless quite Alpine. To be done preferably in dry weather. This route is to be hiked in the indicated direction so as to traverse the two true difficulties (the couloir of Petit Som and the southern ridge of Grand Som from La Suiffière) while going uphill. The remainder is in forests or mountain pastures.
From Ruche à Gîter (S), follow the Tour de Chartreuse PR Trail south passing through Le Tracol (1) until Col de la Ruchère (2). In the pasture, leave the trail you're on taking the steep couloir of Petit Som (3) in the NNE direction to reach the summit (4).
Descend (east) towards Col de Léchaud (5) (limestone pavement, watch your ankles) then cross a rocky ridge to descend to Col de Bovinant (6). At Habert de Bovinant (11), continue south in the Mauvernay valley (obvious) to Col de Mauvernay (7).
A small trail in the scree leads to the ridge of La Suiffière and Pas de La Suiffière (8).
You are on the southern ridge of Grand Som. A small trail climbs (north) to the summit (9).
After a picnic break and well-deserved panorama, follow the yellow-marked trail south. At the point where it seems to want to turn to cross over (west) the rocky outcropping, continue a bit longer until you come across a moderately steep valley (10) where you will quickly find the trail that easily crosses an opening in the outcropping.
Reach Col de Bovinant and Habert of Bovinant (11) for the second time and again follow the Tour de Chartreuse (PR trail) in the SW direction. At the point where you meet an outcropping, leave the trail you're on and head in the NNW direction following the route of Pas du Loup (12). Then reach Col de la Ruchère (2). Set out in the SW direction to reach Prairie de La Folie and Habert Billon. Set out again north to cross Col de l’Aliénard (13). Keep descending north to reach the Tour de Chartreuse which brings you back to the starting point (E).