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What do you do for a bike that will only shift well in one direction?

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What do you do for a bike that will only shift well in one direction?

Old 05-06-15, 08:27 AM
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What do you do for a bike that will only shift well in one direction?

My bike doesn't shift well while riding backwards down a hill. No I'm kidding.

I've experienced this problem a few times. I was fixing up a bike last night and I couldn't figure it out. I could get the rear derailleur to shift well only in one direction. Either it shifted to larger gears well, or it shifted to smaller gears well. Never both. Regardless of how much I tweaked the barrel adjuster. I fixed the small bend in the hanger, the chain wasn't stretched, the cable was new, the derailleur was clean. I ran out of things to try with it. In the end I just set it up so it shifted to "larger" gears better (to smaller cogs.) because it's easier to "shift" it a bit more going the other way. Both the shifter and the freewheel were for 7 speeds, so that wasn't the issue.

Regardless of how much trouble it had, it always shifted to the highest and lowest gears with no problem. If, for some reason, the back wheel had gotten switched with another bikes and the freewheel had slightly different spacers in it, I'd expect it to not want to shift into the extreme gears. I don't know. What do you guys do when you run across something like this?
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Old 05-06-15, 08:30 AM
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Check for wear and/or sloppiness and excess float in the upper pulley.
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Old 05-06-15, 08:52 AM
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Since you have a 7-speed, this probably means low end components (like Tourney). Maybe the spring that pushes the derailleur towards the small cog is too weak or the cable is snagging on something (too much friction because of worn cable housing for instance). You wrote that the cable is new, but the cable housing is also new?

Another possibility is that something is still bent.

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Old 05-06-15, 09:03 AM
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replace cables and housing .. there may be rust inside making drag .. Test: un hook cable and see if return spring works better, then..
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Old 05-06-15, 09:26 AM
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The bike was a... "cheaper" Trek? The older treks you always see on craigslist for a bit more than $100. So yes, it probably was cheap components. It wasn't my bike, I was fixing it for a friend. The cable had very little drag. I had disconnected it and pulled it back and forth, so I don't think that was the issue. It's possible that the spring was weak, the bike was certainly not new.

I'll probably never see the bike again. I'm just asking for future reference whenever I see a bike with the same problem.
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Old 05-06-15, 12:37 PM
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Shifting well one way but not the other (and being able to shift between the two by adjustment) is usually caused by cable friction. I'll bet that there was rust or crud in the lower loop of shift housing going into the RD. You might not notice it when moving a loose shift wire back and forth, but under tension it is a different story. If the RD spring was weak (doesn't happen often IME) it would not have shifted well to the smaller cogs no matter how you tweaked the barrel adjuster.
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Old 05-06-15, 02:51 PM
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How old is this bike? Are you keeping in mind that older bikes don't have teeth on the chainrings and rear cogs that are twisted or shaped to promote/assist shifting, and chainrings do not have the step pin that make shifting rings easier?

Old bikes do not shift the same way that modern drivetrains do. Someone familiar with older drivetrains may not see any problems with the shifting on this older Trek.
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Old 05-06-15, 03:54 PM
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Originally Posted by dsbrantjr
Shifting well one way but not the other (and being able to shift between the two by adjustment) is usually caused by cable friction. ....
+1, I skipped that when steering to a sloppy pulley earlier because you said "new cable" which I took to mean both wire and housing. If the housing isn't also new, replace the rear loop and see if it makes a difference.

BTW- one diagnostic trick that may point you in the right direction is to swap the upper and lower pulleys. Using a non-floating pulley on top should increase shift response and address then issue if it's not elsewhere. Unfortunately, it also makes trim more finicky, so if the pulley switch works and makes a big difference, consider buying the correct upper pulley (or a pair).
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