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Crank is slipping freely occassionally when pedaling; occasional "clonk" sound-not sure where sound is coming from-crank is tight, no movement or looseness

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By "slipping freely" I assume you mean that the entire crank assembly "slips" -- offers little resistance for an instant? The two crank arms aren't moving relative to each other, right? Is this a single gear bike, a derailer bike, or a bike with an internal gear rear hub? –  Daniel R Hicks Nov 11 '12 at 14:04
    
Check out the terminology index: bicycles.stackexchange.com/questions/244/terminology-index The more information the better! –  WTHarper Nov 11 '12 at 15:18
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2 Answers 2

More information would be helpful. What kind of bicycle? How old is it?

Assuming the bike has a freewheel or a freehub, there are a couple of things you can check.

For example, if the bicycle has a freewheel (in other words, if it is older), it may need lubrication inside the freewheel. Automatic Transmission Fluid (ATF) is a great lube for that application, or you can buy various oils. I would advise against using WD-40 for lubricating a freewheel.

A similar kind of skip/slip can happen occasionally (say, once in a few hundred miles of riding) with a freehub, and it doesn't necessarily mean anything is wrong. However if it is more frequent, you may want to service the hub, or have your local bicycle shop service it.

If the freewheel/freehub is not the source of the problem, the drivetrain may be worn. The chain can skip, or jolt forward, if the cogs in the rear are worn excessively.

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ATF is also one of the best cable/cable-housing lube I have tried! –  heltonbiker Nov 11 '12 at 17:18
    
Good answer! If the freewheel/freehub is failing, change it immediately. I've been previously hospitalised after standing up (working hard) on the bike and going fast. The freewheel/freehub teeth failed to lock and I lost my pedals, and fell off. There's a nice illustration on the wikipedia page on Freewheel mechanism en.wikipedia.org/wiki/Freewheel –  wonea Nov 14 '12 at 12:32
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I have seen this happen when the chain gets "trapped" between two chainrings, or more often between bigger chainring and chainguard (if the crank has a round chainguard type), or more often yet between two rear cogs.

If no other probable cause, this is something that should be looked for.

Hope this helps!

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