Cycling in Montseny mountain is one of the best rides you can do if you are staying in Barcelona city. But it is also a very good area if you are staying at Barcelona northern coast, or even Costa Brava.
For the locals, cycling from Barcelona to Montseny is a classic ride, even if it is about 140 km long.

The loop Coll Formic-Santa Fe del Montseny is a classic, and possibly one of the best rides you will do in your life. The roads are very well surfaced and especially during the week (working days), you won’t find any traffic. 90 km to enjoy cycling. The highlight is the 25 km descent from Santa Fe del Montseny to Sant Celoni, which is absolutely stunning. I always recommend doing the tour clock-wise, especially to ride this descent.
But there are other possibilities. One of the most interesting is climbing the whole way to Turo de l’Home (Sant Celoni-Mosqueroles-La Costa del Montseny-Fontmartina). Then, instead of taking the bad road to Turo de l’Home you reach Coll de Santa Helena (where the big antennas are), and then you follow a precious tiny narrow road that leads to Santa Fe del Montseny.
The highest point of Montseny mountains is Turó de l’Home, at 1700 mts height, although in the last 5 km the road is in very bad shape.
Montseny mountains get together with the Guilleries range of mountains which is linked to Girona province. All the roads in the area are simply beautiful and solitaire. I would like to mention Collsesplanes, Sant Hilari Sacalm and Coll de Ravell as some of the rides you should not miss if you are in the area.