Your transmission is shot. You probably need a new chain and rear cassette/freewheel.
The best way to determine whether a chain is worn is
by measuring its length. A new chain has a half-inch
pitch with a pin at exactly every half-inch. As the pins
and sleeves wear, this spacing increases and this
concentrates more load on the last tooth of engagement,
changing the tooth profile. When the chain pitch grows
over one half percent, it is time for a new chain. At one
percent, sprocket wear progresses rapidly because this
length change occurs only between pin and sleeve so
that it is concentrated on every second pitch; the pitch
of the inner link containing the rollers remaining
constant. By holding a ruler along the chain on the
bicycle, align an inch mark with a pin and see how far
off the mark the pin is at twelve inches. An eighth of
an inch (0.125) is a little over the one percent limit
while more than a sixteenth is a prudent time to get a
new chain.
prockets do not change pitch when they wear, only
their tooth form changes. The number of teeth and base
circle remain unchanged by normal sprocket wear.
A new chain often will not freely engage a worn rear
sprocket under load, even though it has the same pitch
as the chain. This occurs because the previous (worn
and elongated) chain formed pockets in the teeth by
exiting under load. A chain with correct pitch cannot
enter the pockets when its previous roller bears the
previous tooth, because the pocket has an overhang that
prevents entry.
Without a strong chain tensioner or a non-derailleur
bicycle, the chain has insufficient force on its slack run
to engage a driven sprocket. In contrast, engagement of
a driving sprocket, the crank sprocket, generally
succeeds even with substantial tooth wear, because
The drive tension forces engagement.
However, worn teeth on a driving sprocket cause
"chainsuck", the failure of the chain to disengage the
chainwheel. This occurs more easily with a long arm
derailleur, common to most MTB's, that is one
Reason this occurs less with road racing bicycles, that
experience a noisy disengagement instead.
In contrast a worn chain will not run on a new driving
sprocket. This is less apparent because new
chainwheels are not often used with an old chain. In
contrast to a driven sprocket (rear) the chain enters
The driving sprocket under tension, where the previous
chain links pull it into engagement. However, because
a used chain has a longer pitch than the sprocket,
previous rollers bear almost no load and allow the
incoming chain link to climb the ramp of the tooth,
each successive link riding higher than the previous
until the chain jumps. The pockets in a used sprocket
are small but they change the pressure angle of the teeth
enough to overcoming this problem.
Jobst Brandt <jobst.brandt@stanfordalumni.org>
Beo