Fun ride through the Mt. Sutro forest. Technical signal track that has some big drops if you slip on "Historic Trail". A lot of cutbacks going down to Stanyan St. exit. This trail is well maintained by volunteers at http://sutrostewards.org.
Directions: Fun downhill in San Francisco. Go up to the top of Mt. Sutro by way of Clarendon Ave. and Johnston Dr. Drop down from 800 ft to 400ft on this singletrack. All the way to Stanyan Ave/ 17th St.