Chain skip := chain advances one tooth under load on a sprocket and
re-engages with a sharp report.
Chain skip is a problem not only for those who ride in dirty terrain
but primarily for riders who replace their chains to achieve better
shifting and preventing undue wear on chainwheels and sprockets. A
worn chain can bend laterally more easily than a new one and,
therefor, tends to shift to an adjacent sprocket less easily, being
able to make a zig zag path. Just the same, the most used sprockets
of the gear cluster often skip when a new chain is installed and must
be replaced with new ones. This is the classic chain skip that occurs
mainly when pedaling hard.
On road bicycles a chain can be ridden until it is so out of pitch
that it doesn't show on a ruler. The method of testing is to hold a
ruler along the side of the chain aligning an inch mark with the
center of a link pin and noting how far off the inch mark the pin a
foot farther down the chain lies. Most bicycle chains are 1/2" pitch
and, therefore, one foot of chain has 24 link pins. Being off by 1/8"
or 0.125" is approximately 1% of 12" and that is probably the outer
limit of wear one should allow before replacing a chain.
In this assessment, the 1% wear is actually 2% because only half the
links are affected by pin-and-sleeve wear, the effect being measured.
Therefore the pitch error occurs once per inch of chain in spite of
there being 24 pins. Inner links, the ones in which the rollers are
held, do not change length.
If chain wear is greater than 4% then the half inch mark a foot
farther down the chain will be right on the mark and obscure that
there is enormous wear. Riders have believed that their chain doesn't
wear because they were 1/2 inch off per foot. Of course when a new
chain was installed, it skipped with even slight pedal force in most
gears.
Skipping is caused by wear pockets made by a worn chain. A chain that
has a larger pitch than the one for which the sprocket was designed
will not mesh properly. Therefore, as the chain wears and increases
its pitch, it climbs up the face of sprocket teeth and concentrates
all its load on a larger contact pitch diameter. A new chain, in
contrast, lands its rollers on the sprocket base circle and bears on
all the teeth at their base.
A new (in-pitch) chain may fit on a worn sprocket but under forward
pull on the sprocket, the chain roller that bears the load, the last
one to engage the sprocket, engages the wear pocket of the old
elongated pitch chain (a point that is behind the engagement it would
have at the base circle) so the next link of the chain cannot engage
the following tooth because it is already behind the following tooth
corner that prevents entry. The result is that all successive links
also fail to engage and when the leading engaged link disengages the
sprocket the chain skips ahead.
So much for new chains on old driven sprockets.
Driving sprockets, on the other hand, are forcefully engaged, the
incoming chain being under high tension from pedaling. Idling
presents no problem because the chain can fall to the base circle,
having no preferred spot on the teeth. That a worn (elongated) chain
rides high on sprocket teeth under load is visible by observing that
there is light between the chain and chainwheel's base circle even
when light pedaling.
A new chain on such a chainwheel, rides on the base circle because
already engaged links force it to do so. Therefore, there is no skip
with a new chain. If a chain is allowed to run long enough to have
substantial wear, it can ride high enough on a chainwheel that hard
pedaling rounds off tooth tips and causes slip-skip, essentially a
continuous riding over the tops of chainwheel teeth, these generally
being aluminum that is readily deformed when chain load is
concentrated on a small area.
Faces of chainwheel teeth are often flared to both sides when their
teeth become narrow from running in skewed chain lines. That way an
out of pitch chain loads primarily one tooth and that tooth yields.
The smaller the chainwheel the faster it wears its fewer teeth and the
sooner an out of pitch chain can cause it to skip. However this does
not occur with 1% wear but with greater chain elongation or abrasive
chainwheel wear, and effect seen mainly from riding on gritty dirt,
especially mud.
Jobst Brandt