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How can I stop my bike from shifting on its own? After I ride for a couple of miles the bike starts to shift up one then down one, up one, down one. I just had the bike in for a tune-up and it still does it.
 

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NeoRetroGrouch
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seattle said:
How can I stop my bike from shifting on its own? After I ride for a couple of miles the bike starts to shift up one then down one, up one, down one. I just had the bike in for a tune-up and it still does it.
Very hard to say from internet land. Can you get it to do it the stand? What are you doing when it happens? Can you make it happen consistently?

First try a simple rear derailleur adjustment. Turn the barrel adjuster 1/4 turn in one direction. If it's fixed, your done. If it's better, try another 1/4 turn. If it's worse, try 1/4 turn the other direction. Repeat.

Take a look at the bike. Did they change out the cables and housing? Are they cruddy or kinked? Does the rear derailleur look straight? Is the guide under the bottom bracket worn or crusty?

Assuming you don't find anything obvious, you could try to adjust the rear derailleur. Check for instructions on the Park web site (www.parktool.com).

TF
 

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Resident Curmudgeon
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You didn't say....

What kind of shifters do you have?
 

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Sounds like

seattle said:
How can I stop my bike from shifting on its own? After I ride for a couple of miles the bike starts to shift up one then down one, up one, down one. I just had the bike in for a tune-up and it still does it.
It sounds like the derailleur is not centered on the cog. You can see this easily, if it is the case. Use the adjustment suggested by TurboTurtle to correct the problem. Other possibilities are kinked/frayed/dirty cables/housing or a bent derailleur hanger. If this only happens when you pedal hard, it could be frame flex, but that's not very likely.
 

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Roadbike Rider
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Check cables

Kerry Irons said:
It sounds like the derailleur is not centered on the cog. You can see this easily, if it is the case. Use the adjustment suggested by TurboTurtle to correct the problem. Other possibilities are kinked/frayed/dirty cables/housing or a bent derailleur hanger. If this only happens when you pedal hard, it could be frame flex, but that's not very likely.
Make sure all cables are running free, no kinks, no binds, no pinch points and no fraying insde the housing or shift levers
 
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