The entrance to the Grubb Run Trail is just south of the elementary school on the west side of Ferrell Rd.
Start with a nice downhill followed by an narrow bridge crossing into a short steep climb. Turn left at the top and across a drainage pipe then head left back into the woods. Stay on the singletrack all the way down to Grubb Run Creek, then turn left. This will take you to the high narrow bridge across the creek.
Follow the trail on the other side of the creek until it turns left away from the creek and begins to switchback up the hill. When you can go no farther, turn around and ride it back out.