Bicycle Mechanics - Loosing sprocket hold
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01-31-09, 04:04 PM
I have a buddy who rides his vintage road bike on centuries and he has a problem where he'll be riding along really fast and he looses the chain/sprocket connection. It's not while shifting either? I think he needs to put a new chain on, but more expert advice is needed, what should we watch out for when we are repairing it?
01-31-09, 04:08 PM
What do you mean "loses"? The chain skips on the cogs? The chain shifts to a different cog?
The problem could be a worn chain, worn freewheel/cassette or a bad shifter. What kind of shifter does he have; friction, indexing? Where are the shifters mounted?
More info is needed.
01-31-09, 04:12 PM
His drive train is probably worn out.
As bicycle chains wear they get a little longer. Then they wear the sprockets to fit the longer, worn chain. Eventually they'll start to "skip" usually at bad times.
Chains are easy to check. Hold an accurate ruler against the taut chain. On a new chain you'll find 2 rivets that are exactly 12" apart. I replace just the chain when 2 rivets are 12 1/16" apart. If the 2 rivets are 12 1/8" apart you need a new chain and freewheel/cassette.
Chainrings are also easy to check. New front chainrings are rounded over the tops of the teeth. If your chainrings are pointy, you need new ones. Sometimes it's cheaper to replace the whole crankset.
Rear cogs take a kind of shark fin look when they wear, but it might take a practiced eye to spot it.
01-31-09, 06:33 PM
It may be that the cogs are slipping because the pawls disengage?
02-02-09, 11:25 AM
Worn chain and/or teeth is/are far more likely than worn pawls. The former occurs commonly. The latter is very rare, though it does happen.
Replace the chain first, as it's cheap and easy. If the teeth are badly worn, the problem will then get worse. At that point, replace the affected sprockets.
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