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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.
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