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I have a steel cyclocross bike which doubles as my weekend bike and my daily commuter. About a month ago I got in a wreck and bent the rear derailleur hanger and the rear derailleur. Got the hanger re-aligned and put on a new derailleur. Chain was not all that worn (checked it) so didn't replace it.

Fast-forward about three weeks. I was noticing significant "phantom shifting" which was primarily when putting a lot of torque on the pedals (I have 220mm cranks and go up a 10% grade on the way to work). Everything seemed to be lined up on the stand, but on inspection the chain had multiple links with cracks in them. New chain=problem solved... for about 3 days. Now it's started acting the same way. It frequently "skips" and occasionally tries to shift from the middle chainring to the small chainring if I'm riding uphill, which threatened to unhorse me once. It can also do this on the large chainring. I checked visually and there's no snaggletooth. The chainring does seem to have a very minor wobble -- less than 1mm left/right, but I've noticed that the phantom shift/skip is indepenendent of where my feet are (ie can happen with left foot up, right foot up, etc)

Any ideas on what else could be causing this / what should I check?

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Without see the bike and riding it around it is hard to guess. You said it was after a crash and you re-aligned the rear derailleur, but my first instinct was the front derailleur might be the issue. –  BillyNair Aug 15 '12 at 7:58
    
Hrm. Is it possible that your wheel is not aligned in the dropouts correctly? –  JohnP Aug 15 '12 at 17:42
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4 Answers

These are all good ideas but I like the idea of frame flexing due to a damaged frame. Phantom shifting can often be cause by poor chainline. Since you didn't have this problem before the accident I'd suspect something tweaked or cracked in the frame, especially in the bottom bracket or chainstays (this is where most torque is transmitted to the rear wheel). I'm guessing that as you pedal hard it causes the frame to flex more now that it did before the accident causing a poor chainline and the chain to drop to the smaller ring.

Besides the annoying phantom shifting, you may also have a safety issue with the frame.

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You didn't mention the bike's age or what it's made from. Older bikes will flex a lot more in general, but it would be more noticeable in the bottom bracket area. Steel frames will flex more than aluminum, which will flex more than carbon fiber.

If you're putting a lot of torque on a bike with an old steel frame, maybe it's time to look at a new frame.

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It might be wise to check for a hairline crack in the frame at the BB shell, etc. –  Daniel R Hicks Aug 15 '12 at 19:59
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One thing to check is whether the crank integrity is intact -- the shaft could be bent, there could be a cracked cup, or the axle/cartridge could simply be loose. Also, the crank arm may be bent or loose. If the crank shifts under load (due either to the axle shifting or a loose arm) it could easily account for your phantom shifts.

Next is the rings. You imply that one ring has a "slight wobble", which may indicate a bent arm or may indicate the ring needs replacing. Also, the rings may simply be worn to end of life -- from experience this can cause phantom shifting.

Another possibility is a "galloping" rear derailer. If for some reason the rear derailer is not taking up chain smoothly it can cause front ring phantom shifts.

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I've had this happen before too. "Skipping" while putting lots of torque down, although it wasn't skipping left/right it was skipping up/down on the sprocket.

It turned out that replacing the chain without replacing the gears was a waste. Because the chain had been so old and thus deformed/stretched this caused the all the teeth on the sprockets to be deformed as well. Normally when a chain gets old you replace it before it's deformation influences the rest of the drivetrain but it will if you let it go.

By replacing the chain and not the gears your new chain was aged almost instantly to the state of the old chain by the already deformed gears. the gears are much stronger than the chain and try to force the chain to conform to their shape. Before their deformations are in sync you will feel skipping.

The answer for me was to either

  • deal with the skipping until the new chain was just as bad as the old chain but at least there was no skipping because then the gears and chain would be in sync (this is at the cost of needing to replace the chain sooner because the wear is greater and faster and then deal with the skipping all over again) or
  • replace everything and start over fresh. the start over fresh option means you'll get more life from a new chain and you can pay better attention to it's wear and replace it in a more timely manner.
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Why the down vote? –  Brad Aug 15 '12 at 18:57
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