From the Beaver Reservoir Trailhead, ride the easily followed blue diamond blazed jeep road west. This section is rocky and has a couple of climbs, all rideable. At 1.3 miles you'll come to a junction with a sign directing vehicles and bikes to go left. Continue following the road, which improves after a bit to some pleasant riding.
Next comes a short but rough descent to meet back up with the trail at about 3 miles. Continue for another 1 mile to an open area with great views of the Indian Peaks. Cross a bridge and continue to follow the Cony Flats Road (at this point you'll be seeing lots of jeeps, 4x4's etc) which descends steeply with large rocks and drops to deal with for about 0.6 mile. This will deposit you onto the Middle St. Vrain Road.
At this point options are: turn around, not recommended, ride the St. Vrain Road to Sourdough Trail to Beaver Reservoir Cutoff Trail
back to car OR best option, head up the Middle St. Vrain Road for about a 1/4 mile to the Buchanan Pass Trail #910
and head down that to the Sourdough Trail - Southern Half to Beaver Reservoir Cutoff Trail
back to the car. This is a great adventure if you don't mind hike-a-bike some, and is a great introduction to the kind of riding around the Peak to Peak Highway. Get some!