Quick disclaimer: You only need a chain keeper if you are certain that your front derailleur is adjusted perfectly, yet the chain still comes off the chainring occasionally when shifting to the smallest chainring. If your chain never comes off, you don't need a keeper. If your chain comes off and you're not sure whether or not your front derailleur is adjusted correctly, have it checked by a pro before adding a chain keeper.
Chain keepers keep the chain on
A chain keeper (they can also be called "chain catchers" and "chain guides" - here's a selection of commercial models) is a little device that rests inboard of the small chainring. Should the chain start to come off the small chainring, the chain keeper bumps it and pushes it back on so it can't drop off.
Chain keepers have become popular because compact cranksets have gotten popular. Compacts have a larger difference between the small and large chainring sizes, which can cause shifting glitches, including throwing the chain off when you shift onto the small chainring. This shouldn't happen on a properly adjusted matched system - in other words, on a Shimano compact crankset with a Shimano derailleur and shifter.
It provides peace of mind that your chain won't fall off
However, a lot of riders choose to mix and match components. Add that fact to the difficulty of properly adjusting front derailleurs, and it's easy to understand why chains can come off. Now, when a chain does happen to come off, it's usually a simple matter to just shift it back on, soft pedaling until it engages (you don't even need to get off your bike).
But, most riders would rather not have it come off in the first place. Hence the profusion of chain keepers. These days, besides on bikes with compact cranksets, you'll see them on bikes with standard doubles and triples sometimes, too.
George Argiris's ingenious chain keeper
|George Argiris' home-made chain keeper|
Plus, it looks far better than having a separate chain keeper attached beneath the front derailleur - especially a plastic one. It also won't collect crud and make the area harder to clean. And it weighs next to nothing. Interestingly, there have been wire chain keepers of various designs since the dawn of the derailleur so it's a proven way to control the chain.
What you need to make your chain keeper
1) From your local hobby shop get a #507 (or 1/8-inch) diameter steel rod (get stainless if you can so it won't rust).
2) The first bend is about 1 1/2 inches from the end, and about a 1-inch bent section at a 30-degree angle, or whatever it takes to reach over to the chainring.
3) The second bend is 1 inch from the bottom of that bend to where you'll cut it off.
* If the new hinge pin (your chain keeper) doesn't go in easy, drill out the hinge hole with a 1/8-inch drill bit.
* Don't worry if the new pin feels loose, because once you tighten the clamp onto the seat tube of you bicycle frame, it will hold the pin tight.
* You can adjust your new chain keeper by just nudging it one way or the other before the final tightening.
If you have a braze-on derailleur, RoadBikeRider.com owner/cyclist John Marsh sells an e-article on how to make one that resembles George's quite a bit, but mounts beneath the derailleur mounting bolt. The e-article goes for $4.25 and is available here.
Here's to no more dropped chains! Many thanks to George for sharing this trick bike mod!