It's not mountain biking but FYI;

USA Cycling’s premier track event of the year is in Rock Hill on August 12–17, 2014, competing for national titles in all track cycling disciplines. 
Some of the “must see” racing events during this 6-day event are the Miss and Out races on Thursday evening, the Match Sprints on Friday evening, the Keirin and Madison races on Saturday evening, and the Points Races on Sunday morning.

Admission for spectators is free and seating is available on a first come first served basis. On Tuesday–Saturday, race sessions will begin each day at 9:00 am and 6:30 pm. On Sunday, August 17, there will only be a morning race session at 9:00 am. Visit the Race Expo on Friday and Saturday, with free giveaways. Food and drink will be available for purchase at the velodrome on Thursday, Friday and Saturday evenings.

While there you can ride the 7 miles of mountain bike trails and also check out the BMX track that in October will host the 2014 Gold Cup and 2017 World Championship.