Halfway between Hønefoss and Vikersund, you'll find a dirt road (near the 'Haga' house) that leads into a forest and gradually gets smaller. The forest climbs steeply. At the top, there's a gate, but it's easy to open. After that, there's another barrier (near the 'Klavåsen' house) that's also unlocked.
In midsummer, the grass is already very high—only recommended for mountain bikes.
If it's on your way, it's OK—but it's not worth driving all the way here.