This is the Jump Stop chain guide.
It is designed to make chain derailment past the small chainring virtually impossible by covering the entire engagement zone with a sturdy stainless steel guide plate. With a Jump Stop properly installed, you can safely adjust your front derailleur for maximum performance downshifts, and the Jump Stop will ensure that your chain lands and threads perfectly every time.
Why is chain derailment bad?
At its very best, chain dive is still a messy nuisance. It usually happens when you're climbing a hill, so you lose all your momentum, then you have to dismount, get your hands greasy putting the chain back on, and then get started again on an upslope. But when a chain derails, it can also jam between the crank and bottom bracket assembly -- sometimes so hard that the only way to extract it is to remove the crank. This sort of jam can damage the crank or chain and sometime even do serious damage to the frame.
If you race, chain derailments will cost you time, and maybe the race. If you go on long rides or ride in remote areas, a jammed or damaged chain can leave you stranded. If you drop your chain while riding out of the saddle, the abrupt loss of pedal resistance can be very destabilizing and might cause a fall. And of course, any of these problems are worse on a tandem.
Do I need a Jump Stop?
Maybe not. Take a look at how close your front derailleur comes to your smallest chainring. On a road bike with double chainrings, the derailleur might be close enough to serve as a fairly acceptable guide. On most bikes with triple chainrings however, the front derailleur has to stand off at some distance from the smallest chainring and employ the toss-and-hope strategy. Even so, this works most of the time on clean, new, well-adjusted drivetrains under ideal riding conditions. However, if "most of the time" isn't good enough for you, if you don't ride under ideal conditions, if you plan to keep your bike long enough to put a little wear on the drivetrain, if your front derailleur adjustment is a compromise between safe and fast downshifts, or if you simply think chain derailment is a bad enough problem that you never want to have to deal with it, the Jump Stop could be for you.
Will the Jump Stop fit every bicycle?
No. Most, but not all. The Jump Stop mounts on the seat tube (or derailleur post) alongside the top of the smallest chainring (see the illustration at top). This tube needs to have a standard outside diameter of 1" (needs shim), 1-1/8", 1-1/4", or 1-3/8" (26, 29, 32, or 35 mm.) and should not be strongly ovalized. The Jump Stop requires about 3/4" (18 mm) of tube length to mount on, at about the same height or a little higher than the top of the smallest chainring. The Jump Stop will fit almost any bike that meets these basic requirements.