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What are the failure modes (symptoms) of a failing or failed freewheel?

My understanding of normal free wheeling means the rear tire should rotate when the bike is moving and the pedals are or are not rotating.

Failure would mean the bike and the pedals always move together, like a fixed or locked hub, or the pedals and cassette move but the rear tire seems disconnected.

I intermittenly have a 1/4 - 1/2 crank turn 'slip' in the pedals before the wheel begins to move. The condition is very obvious on my trainer, not noticeable on the road. Is it a free wheel failure or something else?

If the freewheel is indeed the problem as I suspect, is it repairable or am I required to replace the freewheel and cassette together.

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    Failed == it does not work. Simples! Care to elaborate your question? Do you have a specific problem with your freewheel, or that is a pure theoretical question?
    – trailmax
    Feb 22, 2013 at 21:58
  • Yeah, basically 2 failure modes: 1) It doesn't freewheel when it should. 2) It does freewheel when it shouldn't. Of course there are degrees of each -- there could be significant drag while freewheeling, or there could be "slipping" when a driving force is applied. There should be none of either.
    – Daniel R Hicks
    Feb 22, 2013 at 22:39
  • A freewheel could also fail when its teeth are too worn, I suppose.
    – WTHarper
    Feb 23, 2013 at 2:18
  • Also note that you either use a freewheel or a freehub/cassette, not both.
    – WTHarper
    Feb 23, 2013 at 2:19
  • There is a certain amount of distance the sprocket must turn relative to the hub before the ratchet engages. This varies with the design of the ratchet mechanism -- some move very little, others maybe 1/4 revolution worst case. Note that you can also get what seems like slip with a freewheel, when a cluster has just been installed and has not yet screwed itself on tight. But this only occurs the first 3-4 times you put force on it. I can't think of any reason that any effect would be more obvious on the trainer, though.
    – Daniel R Hicks
    Feb 23, 2013 at 3:10

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You are correct about the two failure modes. Try lubricating your freewheel with a light oil. If it is dry or dirty that can cause it to not engage. If that does not fix it then you will need to replace it.

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