Not singletrack, but in the vein of the Leadville, the Crusher in the Tushar, etc. A couple years ago, Vicious Cycle put on a Gran Fondo in Ephrata WA, with 30 of the 80 miles being gravel. It's a great early season event. Lots of pavement, but generally more roadie than mtb.
This year, two more were added in Leavenworth and Winthrop. These two had drastically more climbing (~8K in 90 miles, and ~10K in 80 miles), a lot more gravel, and much rougher roads. For those who like unique, challenging events with beautiful scenery (especially Winthrop), I'd recommend looking into these next year. Good post ride food and nice event t-shirts too.
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Ephrata: way more pavement than gravel, and the gravel is pretty smooth. Cross bike with wider/tougher slicks works great. I've used 24mm vittoria pave's.
Leavenworth: much rougher road and longer climbs, and the roughest downhills of all three. I used a cross bike with cantis, 30mm slicks, and 34/28 low gear. I wouldn't do it again without disc brakes, wider tires and lower gears.
Winthrop: really long climbs, lots of gravel, but smoother than Leavenworth. I used a HT MTB with 2 x 9, 650 wheels (stuck in a 26" frame I had) with 2.25 racing ralphs and a rigid fork. It worked well for me. A little faster tires would have been even better.
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