Karolina is historical road connecting Karlovac and Bakar. It was built from 1726 till 1728. Today road is surfaced except small part, few kilometres, which is still forest road. Road is very suitable for cycling due to low traffic and it is passing through nice mountain surroundings.
On a section from Bosiljevo to Vrbovsko, due to unsurfaced part of Karolina, ride goes via another historical road called Lujzijana. Lujzijana has a bit more traffic then Karolina, but it is still suitable for cycling. Nowadays, most of traffic from Karlovac to Rijeke goes via highway.
Occasional, you will came on few steep sections (+14%) on a ride but they are fully rewarding when you tackle them. Whole ride can be managed in one day. I took an early morning train from Zagreb to Karlovac and from there I continued by bike. If you push pedal a bit you could catch afternoon train from Rijeka to Zagreb, but check train schedule before ride.
Note: My English ain’t perfect, but we understand each other, don’t we 😉
Fužine, Rijeka (Trsat castle)