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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
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

2 Answers

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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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...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
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
up vote 5 down vote

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

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