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
Step 2: Take Apart the Reflector
Take out all of the nuts and bolts from the reflector. You can discard the reflecting piece but keep the rest as you will need to put some of it back together.
Step 3: Add the hose
Take the piece of hose and put it on to the round part of the reflector. It will not fit on easily so I recommend either putting it in boiling water or holding it under a small flame (quickly - don't burn it!) to make the process easier. It only needs to cover the plastic circle so trim the hose if it is too big.. I had already made my tensioner so I didn't put the hose on the whole way because I still needed that reflector.