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My chain sometimes momentarily jumps gears when hitting bumps. Does anyone know what might cause this? Shifting is otherwise perfect. Thanks in advance!
 

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I assume it's always to the next smaller rear cog?

This might be a shifter problem.
 

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My chain sometimes momentarily jumps gears when hitting bumps. Does anyone know what might cause this? Shifting is otherwise perfect. Thanks in advance!
Assuming you're talking about the rear derailleur, the first thing that comes to mind is that you have to adjust the spacing between the upper pulley and the cassette, which means different things on different derailleurs but general means tweaking the "B" screw. Of course it could be a worn chain or cassette (check chain wear to be less than 1/16" elongation in 12" of chain). Does it do it in any cassette cog? Big chain ring or small?
 

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Assuming you're talking about the rear derailleur, the first thing that comes to mind is that you have to adjust the spacing between the upper pulley and the cassette, which means different things on different derailleurs but general means tweaking the "B" screw. Of course it could be a worn chain or cassette (check chain wear to be less than 1/16" elongation in 12" of chain). Does it do it in any cassette cog? Big chain ring or small?
How many miles are on the bike? If the bike has very high mileage, another thing that possibly comes to mind is RD play. Try to wiggle the RD pulley side to side. Is there a lot of movement?
 
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