Chain tensioners and guides are often used in "extreme" mountain biking such as downhill, freeride, and fourcross. They are used to stop the chain from bouncing around and falling off. However, they are very expensive for a simple item, generally ranging from 30 to over 100 euros. This is a simple way to make one that costs almost nothing and you probably already have the materials! Also, chain tensioners can usually only be used with one or sometimes two front chain rings. This one works with three!
Step 1: Materials
1) Rear Bike reflector
2) Small piece of garden hose
4) Scissors or something else to cut with
5) Phillips Head Screwdriver