I set my old Peter Mooney up to ride the promised gravel of Cycle Oregon last year and did a trial run on the Trask River Trail over the Oregon coast range. Ran a 38c Pasela in front, 35c in back. Three chainrings, 44, 42 and 36. Flip-flop hub, fixed both sides, starting with a 17-21 on one side, 14 on the other. (The three cogs line up perfectly with the three chainrings. This is a 3-speed, not a 9-speed. All bulletproof 1/8". On the way up, I changed out the 17-21 for a 24. (Still too high a gear, I finally bogged down on an 18% stretch, rear tire spinning and me going over. Lots of walking the rest of the way up.) Coming down on the 14 and those big tires was a blast! Yes, just as much 18% coming down! Loved that big front tire. I could just lay into completely blind corners on the inside tire track of deep 1 1/2 to 2" stones and just trust the bike to do its thing. The grins were ear -to-ear.
If you can mount a tiny cog and get the gear ratio up, steep, loose downhills are far more secure feeling than anything you can do on a (non-suspension) freewheeled bike. (With good brakes, of course. This isn't about skidding or showing off, It's about control.) And a minor rant - this is where I think track ends suck. Wheel flips are so much easier and faster with road (horizontal) dropouts that open to the front (or even better, down from front like a vertical dropout where, once in, you can slide the wheel back).
Ben