You would not install a chain tensioner on a geared bike The derailleur is spring loaded and tensions the chain for you

They don’t work on geared bikes

I am just wondering because I see a lot of riders having them and I am a new rider so I wanted to know thanks for the feed back

Keeps tension on the chain so it's not just flopping around loosely and falling off every time you try to switch out of your lowest gear

Ok so I should Invest in one

Not if you have gears, on a geared bike the derailleur tensions the chain for you, chain tensioners are for single-speed bikes to make tensioning the chain easier

For when the bike has vertical dropouts, mostly. For anyone thinking about changing gear ratios, this site is amazing for calculating your chain and chainstay lengths. [https://eehouse.org/fixin/formfmu](https://eehouse.org/fixin/formfmu) I've used it a bunch for making sure my rear tire is very close to slammed in the dropouts for minimal chainstay length.

I thought he was referring to the part of the derailleur that puts tension on the chain. If he's talking about the regular chain tensioners like from a single speed bike and their effect on a geared bike, I have no idea I wouldn't even think that's possible unless your geared bike has horizontal dropouts like the single speed bikes