The section of singletrack between the San Juan Water District (just after Hunkle Reservoir) and the Beals Point entrance from Auburn-Folsom Road is off limits to bikes (Pioneer Trail) per Cal State Parks Folsom Lake SRA superintendents order - see park map for detail.
Starting from Folsom Bike, follow the paved American River Bike Trail towards Granite Bay. Stay on the paved American River Bike Trail until you get to the park entrance road for Beal's Point - at the stop sign. Turn left (downhill), ride past the ranger office, and just before the park exit out to Folsom-Auburn Road you'll see a singletrack trail start at a break in the split-rail fence on the right side of the road. There should be a trail sign here....this begins the bike legal segment of the Pioneer Trail for this part of Folsom Lake SRA.
The two and a half miles of this trail is a winding singletrack. There are a few short climbs. Watch out for horses and joggers.
The trail then rejoins with the fire road. Just before the 4 mile mark there is a turn off to the left and a challenging climb up to the water tower. Once at the top there is an option of following a fire road down or continuing straight ahead for a steep descent. Either way, the trail rejoins the fire road which leads to the Granite Bay trail.