do I need chain tensioner??
 

I ride xc hardtail bike with single ring crankset(104mm/32tooth/truvativ stylo) but, no front derailleur. It's been running almost hassle free expect chain fall off from crankset when I pass through rough rock garden or flat down section.. even with shorten new chain installed it's not getting any better... is there cost effective way to prevent falling chain or do I need to buy chain tensioner?? how about use front derailleur as chain guide???

It's called a front deraillieur,