Running a single up front? Wondering about wide/narrow? Here's a way to find out cheaply.
After tossing chains constantly, I went with this. Thus far, seems pretty solid. Ideally they'd be run with a clutch rear derailleur, but I just survived a VERY bumpy Westwood CX w/no problems. (Other than a self-imposed cornering failure.) I ran the 40t at Charm City and Nittany, but used a Paul's Chainkeeper for reassurance. No issues.
I downsized to a 38t as the 40t wasn't playing well with my chainline.
Used hard for three weeks. (Well, as hard as I can go anyway...)
$50 shipped. I can be reached at seanrunnette at gmail dot com.