Salmon Falls:
This ride is 100% legal for MT biking. It is a bit bumpy and technical, not a good beginners trail. There are other versions of this ride, the below GPS tracks are for the pretty standard out-n-back, including the Sweetwater trail.
Download the GPS tracks for these four trails HERE.

Hazel Loop:
This is a full loop around Lake Natoma with 90% off road. Keep in mind ONLY THE SOUTH SIDE is legal for MT bikes.

Brown's Ravine:
I believe that technically it is only legal from Dyke 8 to the Marina... although many people ride out to the "Spanish House" and back. Check with CA State Parks.

 

Auburn area

Forest Hill Divide:
This is 100% legal for MT bikes, it is a multiuse trail so watch for horses. This is a great intermediate+ loop about 10.5 miles long. Great single track, good parking. To get there go I-80 to the Forest Hill road exit, head East approx. 4.5 miles to the parking area. you will head out and about the .6 mile mark you will hit a split. The "normal" direction is counter-clock wise, so go right. GPS tracks HERE.

Stagecoach, Manzenita and Mossy Rock coming soon