It will prevent the chain from dropping off and reduce the feeling of the unsmoothness in the gear teeth. And it's suitable for the single-speed bicycles like for Dahon, FNHON, and etc..
.Name: Folding Bicycle Guide Wheel Oval Aluminum Alloy Cycling Single Speed Rear Derailleur Chain Tensioner with Sprocket MTB Bike Chain Tensioner Replacement
2.Color: black, red, silver and gold
3.Material: aluminum alloy
.Applicable for: outdoor bicycles, mountain bikes and etc.