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I have two bicycles that suffer this problem, both pretty heavy frames (one a ladies, Hollander bike, and the other a man's commuter) - both have correctly adjusted Derailleurs which don't produce any noise or rub on the chain during normal riding.

However with both, on flat ground, at nominal speed - once every 3-10 revolutions of the chain it will simply skip; there's no drive for a quarter of a turn and then everything is OK.

I had originally assumed that this was a misalignment, or normal chain-skip (as I had only noticed it under heady load, cycling hard out of the saddle. However noticing the same happens on my girlfriend's bike, and on mine when I'm taking it easy disconcerts me, and leaves me with little confidence in the bike.

I'm not sure where to start diagnosing, however I suspect that stripping the drive train might be a start… perhaps also examining the chain for nipped-links, and the sprockets for damaged teeth (although, both bikes have a bash-guard on the front sprocket)

Any help or advice greatly appreciated, I'm a pretty handy motorcycle mechanic, but I always found working on my Human powered bikes a lot more fiddly :)

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2 Answers 2

It turns out that it was a stiff link, stripping the chain off the bike, dousing it in brake cleaner, then oil and leaving it to bathe a few minutes, then repeating the process left the chain rust and dirt free, looking like new.

When I took off the chain, I broke it at the stuck link (which was clearly evident going through the derailleur when pedalling the bike backwards) -- cleaned everythingh whilst I had the opportunity and reassembled.

Whilst the chain was off, I did measure it, and things were within tolerance, although (I assume naturally) a little stretched.

Thanks for the help, I now have again a reliable bike!

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3  
Stiff links can often be fixed by flexing the chain from side to side a little. –  markdrayton Sep 27 '10 at 6:41
    
You can fix a stiff link with inner teeth on a chain tool (nearest the pin). –  Paul Ruane May 9 '13 at 14:28

You might want to check the chains for "stretch" if they've been on the bikes for any length of time as this can cause the cause the chain to skip (as can worn sprockets resulting from running with a worn chain for too long.

The easiest way is to get a ruler and measure 12 double links worth of chain. If the chain has "stretched" (in other words worn) it will measure more than 30cm (12 inches) and may also have damaged the sprockets on the rear cassette or chainring too. The degree of wear will indicate whether the chain needs replacing or the sprockets too. Sheldon Brown's site gives the following indicators:

This gives a direct measurement of the wear to the chain, and an indirect measurement of the wear to the sprockets:

  • If the rivet is less than 1/16" past the mark, all is well.
  • If the rivet is 1/16" past the mark, you should replace the chain, but the sprockets are probably undamaged.
  • If the rivet is 1/8" past the mark, you have left it too long, and the sprockets (at least the favorite ones) will be too badly worn. If you replace a chain at the 1/8" point, without replacing the sprockets, it may run OK and not skip, but the worn sprockets will cause the new chain to wear much faster than it should, until it catches up with the wear state of the sprockets.
  • If the rivet is past the 1/8" mark, a new chain will almost certainly skip on the worn sprockets, especially the smaller ones.

I left mine to long and had to buy a new chain, rear cassette and chain rings.

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Thanks for the tip, I also just found this which points to a stiff link - which is more likely as the bike has only been used about 15 times, it's seen less than 50Km in it's whole life… I'm headed downstairs to work on it now, I'll post back when I have an answer, thanks for the suggestion! athenscyclepath.com/wconnchainskips.html –  Lee Hambley Sep 12 '10 at 15:00

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