“Sourdough Trail (South of Brainard Lake)”
— Alan Doak
From the Peak To Peak Highway, take the exit for the CU Alpine Research Center and park at the trailhead on the left. An outhouse is available at the parking lot.
The trail climbs up singletrack to the Brainard Lake road, which splits the southern and northern halves of the Sourdough Trail. The southern half of Sourdough is less technical than the northern half, and travels through splendid forest that rarely opens up enough for a vista.
This first two miles is a climb up fairly loose rocks and will keep you on your toes, though it's all rideable. The grade isn't too steep, but there's not much of a break until you get to a pretty bridge over a stream crossing at 2.5 miles.
The trail becomes smoother and faster at this point, but keeps climbing for a while with occasional views east through the trees. After about 3.5 miles, the trail becomes flatter and much faster to the end.
To return to the trailhead, either ride back on the trail or along the Peak To Peak Highway.
The trail is typically rideable (or mostly rideable) by the first week of June. Any sooner than that and you'll quite possibly be pushing through snow.