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I have a road bike that I use to commute ~6 miles to work every day. Some of the portions of the ride are at a steep incline, and I've noticed that when I get up to stand on some of these hills, my gear slips.

I'm not sure if "gear slips" is the right term, but basically after I stand up my gear switches without me triggering it. It's very annoying and sometimes painful when I completely lose control of pedaling for half a second, not to mention the different gear setting.

What could be the cause of this?

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Do the chain just skips (like jumping over the cog's teeth) or you mean it goes to another gear? – heltonbiker Jan 31 '12 at 20:05
    
It goes to another gear, because after it happens it's usually harder to pedal. – John Jan 31 '12 at 20:40
1  
I would go with a misadjusted derailer, or a worn rear cluster. It's vaguely possible that a too-loose chain could cause this, but that's more likely to stay in the same gear. – Daniel R Hicks Jan 31 '12 at 23:25
    
What kind of bike is it? Do you know what kind of shifters and cassette or freewheel it has? – Eric Feb 6 '15 at 17:42
up vote 11 down vote accepted

In order of least expensive to most expensive, you either have

  1. a misadjusted derailleur,
  2. a very worn out cassette, or
  3. a crack in your frame.

If you're not noticing any problems shifting under normal circumstances, I'd odds are good that your cassette is worn out and your chain no longer meshes with the cogs correctly.

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1  
...or a stiff chain or a flexing crank. – Mere Development Jan 31 '12 at 20:56
    
A stiff chain would be skipping besides just on hills, and a flexing crank by far a long shot. – Stephen Touset Jan 31 '12 at 21:30
1  
I'm having a similar problem, and I'm pretty sure it's due to a worn out cassette. – Kibbee Feb 2 '12 at 15:50
    
My money is on the worn cluster. However I did have a "road" bike 30 years ago that had shifters on the neck stem. If I was standing on an uphill my knee would tap the changer and cause a world of hurt for me. Stupid place for shifters... – Chef Flambe Feb 4 '12 at 8:37

I'd go with the following hypotheses, beyond that already proposed:

  1. Your derailer is a bit misadjusted, so the gear is already almost shifting down. When you pedal hard, your frame flexes, thus releasing some cable, and the gear shifts "automatically". This is specially true if you have a brazeon on front part of downtube, then the cable runs outside of the housings, probably passing through an under-bottom-bracket guide, and then enters housing again in a braze-on close to the rear shifter (I'm doing some paranormal guess, here, but you got the idea);
  2. You have friction shifter, and the shifter is slowly slipping down, so that when you pedal hard it actually shifts.

Two hypotheses are wild guesses, but maybe one of them is true (tell me if so ;o)

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2  
And a slightly sticky cable can produce a similar effect. – Daniel R Hicks Jan 31 '12 at 23:23

I had this trouble, and after checking the usual suspects (Replaced chain, cassette, cables, housing, and derailleur), it turned out my frame was failing near the bottom bracket.

Although it's the most expensive to fix: before you start replacing components, do a quick visual inspection of the frame. Look for cracks/separations around the rear triangle. This is a rare problem, but could be the most serious.

If you determine your frame is fine:

  1. Make sure your drivetrain is in good shape (not overly worn)
  2. Your cables move with ease and don't need replaced
  3. Your derailleur hanger is straight.
  4. your cassette is tight
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While a frame issue cannot be ruled out, it is a lot rarer, and more expensive to fix, than worn cassette, derailer needing adjustment, worn shifter cable, or worn chain. TL;DR frame issue should only be determined after eliminating derailer, shifter, cassette, chain, or cranks, as the source of issue. – moshbear Oct 8 '13 at 3:41
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Yes, but it's easy to look for. And could be a serious concern. In my case, i found out only after having replaced cables, housing, derailer, chain, and cassette. I wish i had known that was the root before i replaced all that other stuff! – mrsoltys Oct 8 '13 at 3:46
    
The part about replacing the cables & housings, derailer, chain, shifter and cassette should precede the part about frame failure, otherwise it can give an impression of jumping to conclusions before exhausting the usual culprits, for those who don't read comments. – moshbear Oct 8 '13 at 3:48
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I think the point is that it's a lot cheaper to check the frame FIRST, rather than to replace all those components and THEN discover that the frame is cracked. Not only check its integrity, but whether it has somehow gotten bent. Include the frame check in the general check for bent or obviously loose/worn parts. – Daniel R Hicks Oct 8 '13 at 7:30

I had this problem on an old mountain bike.

I changed the derailleur and it doesn't do it now.

I noticed the old one was not screwed in very tightly when swapping out so this could have caused it.

I had a broken spoke and thought this caused it, re-spoked wheel with all new spokes. I also swapped out the freewheel as I thought maybe the pawls had worn out.

Finally changed the derailleur and it worked.

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As an addendum to other answers, this was happening to me (not ghost shifting but the chain jumping forward). This started on a new chain, so I changed the cassette. It still happened. A test ride demonstrated that it only jumped in the middle chainring (by far the most used) even though it didn't look much more worn than the other rings. So in this case changing the crankset was the solution.

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