15

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?

4
  • Do the chain just skips (like jumping over the cog's teeth) or you mean it goes to another gear? 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. 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
14

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.

5
  • 1
    ...or a stiff chain or a flexing crank. 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. 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... Feb 4 '12 at 8:37
  • Crack in the frame is a real thing. This was me, 4 months ago. Nov 12 '20 at 5:14
7

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)

1
  • 2
    And a slightly sticky cable can produce a similar effect. Jan 31 '12 at 23:23
2

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
4
  • 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
  • 1
    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. Oct 8 '13 at 7:30
0

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.

0

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.

0

Change the chain, cassette and/or chain rings

If your shifting works perfectly on a bike stand, and only slips when pedaling strongly, these components being worn out are overwhelmingly likely the cause of the problem.

When you start riding bicycles you think: hey, the chain/cassette/chain ring are made of metal, they will last as long as my frame, right?

But no, wrong. Those parts are basically disposable components of the bicycle. For example, I have to change chains more often than I change brake pads!

The chain stretches with usage. And the chain always eats away the metal of the cassette/chain ring. And if the chain is overstreched, it very quickly eats away those parts. More precisely, the cassette/chain ring are eaten up to adjust to your stretched chain, and then when you put a new chain on, they start to slip.

You can then often very clearly notice that the gears you use the most are the only ones that are slipping. E.g. on my 8 rear + 3 front gear bike, the 6th on the rear and the 2nd on the front are the most used ones by far.

When you start to notice this, you also start to more consciously try to use the gears more evenly to try and increase the lifespan of those components.

What you have to do is to learn how to identify which of those three parts need to be changed, here's how.

Is the chain worn out?

This one is easy: you MUST buy and a chain stretch checker and use it regularly

This tool is a must because out of the three parts, the chain is the one that goes bad first. And once over-stretched, it very quickly destroys the other two parts, which are more expensive and harder to replace.

I got myself a Park Tool CC-2 CHAIN CHECKER for example, and it is definitely worth it.

Here's me measuring a new chain with it to test it out. It reads 0.25, so the chain is new.

When you measure the chain and it reads 0.75 or more, you rebuy it. It is mathematical.

enter image description here

Another thing is that you start to learn the frequency at which you normally need to change your chain.

For example, this is my cadence for my Shimano HG40 8 speed chain according to Strava kilometrage:

  • replace 2021-07 after 3000km
  • replace 2021-04 after 2000km

If you don't have the checker tool, which you must obtain sooner or later, you can try to check the chain by simply pulling on it on the large chainring with your hand. If a visible space appears, the chain is worn. Here is an extreme example of when I started riding and didn't maintain my chain. A well maintained drivetrain should never reach this point:

enter image description here

Are the cassette and chainring worn out?

These are a bit harder.

There are a few options:

  • visual inspection of the teeth
  • sync with the chain replacement
  • the chain is new, but some gears are skipping

By visual inspection, you can look out for two tell signs:

  • symmetry of the teeth. New teeth are symmetric, old teeth are worn out in the direction of pull.
  • width of the teeth. Old teeth are narrower/pointier than newer teeth.

Since those are not so easy to measure, once you reach a steady state of bicycle usage and maintenance, it might be easier to just replace them in sync with the chain. Do I really have to replace the cassette when I replace the chain, or is this a scam? says one cassette every two chains, and it sound reasonable according to my experience. TODO chain ring, possibly one every two or three cassettes.

Another more lazy approach that I'm de-facto using is to just inspect the chainring and cassette on every chain swap. If they look fine, give it a try. If the chain skips a bit, buy a new one whichever one of the two seems worse. This can be determined by feeling in which gears the slip happens. With this approach, you will have a few days of slippage, but you will use your parts closer to their limit.

Here are some side-by-side of identical models of new vs old Shimano CS-HG50-8 cassette and a Shimano Shimano FC-M311 chainring that I wore to oblivion before I knew about any of this. The chainring was especially worn on the 2nd gear. My bike cleaning skills also improve a bit since.

enter image description here

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Related: While pedaling standing up, bike crank gives in as if shifting, what is causing this?

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