gear change

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barbelfisher
Posts: 258
Joined: 5 Sep 2009, 6:54pm

gear change

Postby barbelfisher » 30 Mar 2011, 7:48pm

Hi folks, I am having problems with my bike gearing and need some advice please: It has a single 39 teeth crank with aon 11/34 tooth rear cog. I have put a new chain on and it keeps alternating between the top and second to top gears. Any adviced please?

Cheers barbelfisher.

herzog
Posts: 64
Joined: 10 Nov 2009, 10:55pm

Re: gear change

Postby herzog » 30 Mar 2011, 9:22pm

On the rear derailleur (where the cable enters) there may be a cable adjustment barrel. If the chain is trying to skip to a larger sprocket, give it turn clockwise and see if it's better. See this image http://bike.blogware.com/blog/Derailleur

Herzog

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CJ
Posts: 3010
Joined: 15 Jan 2007, 9:55pm

Re: gear change

Postby CJ » 31 Mar 2011, 10:46am

If you used either of those gears a lot previously, one of them may be worn by the old chain so that the new chain does not mesh perfectly. Under hard pedalling, the chain pulls tight into the slightly hooked teeth, so that where it runs on at the bottom, sometimes a chain roller will catch on a tooth tip, causing the next few links to rest on top of the teeth, from which position they can easily derail onto the next sprocket. This may cause not only a jump but also a shift, when that lump of un-meshed chain comes around to the top.

Improving the adjustment of the mech may cure the "ghost shifting" but if there is also some jumping, and if that persists, you need a new cassette. (Or maybe you can replace just the worn sprocket, which is not possible in Britain, but they are available separately in Germany and can be got by mail e.g. from Roseversand - not all sizes but you might get lucky!)

If you replace a chain early enough, when it is only 0.5% worn (i.e. 20 links measure 10.05 inches) you can usually avoid also replacing the rear sprockets, but people usually let their chains get more worn than that, so bike shops routinely replace both together.
Chris Juden
One lady owner, never raced or jumped.