While mountain biking on BBA land is restricted to only some of their trails there is some nice riding here. A map showing the permitted trail use can be found here: http://www.beaverbrook.org/wp-content/uploads/2011/05/BBA-Trail-Map2013web.gif . Most of the permitted trails are easy riding which makes this a great place for beginners. But the area north of Rt 130 does provide some nice singletracks that permit bikes.
One of my favorite rides is an about 10 mile loop that stats at the Worcester trail head (see GPS track, to be uploaded). That loop connects some of the best riding on BBA land. As you cross Rt 130 you may wonder where the trail continues. It goes left of the barn across the street and can be rather overgrown depending on the season.
While dogs are allowed on BBA land they need to be kept on a leash which doesn't work well for us bikers.
Please respect the leash law and that many trails are off limits to bikes in order to preserve access to this area. There are lots of other nearby places that are not as restrictive which provide more challenging riding.