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.
Step 4: Attach the DIY Chain Tensioner
Place your new chain tensioner so that your chain runs through the circle and reattach the nut and bolt. You can choose to attach the second piece of the reflector. If you do this, you can attach the tensioner more securely but your chain will not be pulled up as much. Now all you need to do is ziptie it on through the remaining hole or holes in the reflector. The picture shows where to put the zipties through. Ziptie the tensioner to the chainstay on your bike (the bar above the chain). Make sure that you do not put it too close or too far away from the front chainrings. Use the pictures of my bike as a guide.
Step 5: Finished!
You are now finished with your DIY chain tensioner - Go try it out! It should help your chain stay in place for a long time. When the hose eventually wears out, all you have to do is stick on another piece and ziptie it back on!
Thanks for reading my Instructable; I hope you liked it!