The park service calls this a road, but that is generous, especially the northern half. They claim it's passable in a high clearance 4×4, but they also don't maintain the northern half of the road at all. So it's perfect for a mountain bike!
From Glenn Springs Road
, you'll immediately cross a creek, possibly dry or maybe torrential if it's just rained (in which case think twice about crossing it). The next mile climbs 100 ft on a rocky, rutted road to an overlook of Talley Mountain and the beginning of a steep descent. Enjoy the descent and then you'll ride about two miles through badlands with little vegetation. Then you begin a climb up the shoulder of Talley Mountain.
After you reach the Elephant Tusk campground near the top of this climb, you'll enjoy an easy seven-mile continuous descent down to the junction with Talley Road
. The part of Black Gap Road from Talley Road
to the Elephant Tusk campground gets maintenance by NPS.
Both the start and finish of this ride are accessible by 4×4 high clearance vehicles (check with the park service for current conditions - 4×4 may not be required if they've recently graded the roads). It wouldn't be bad as an out-and-back or do it is part of the Glenn Springs to Hot Springs
Keep in mind this is a desert and it can be very hot most months of the year. There is no potable water available, so you'll want to carry more than you think you might need, and there is no shade available. Winter is a great time for this ride, but the desert can get surprisingly cold in the winter, and the sun can still be intense. If you're unsure of conditions, check with a ranger.