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While standing up and pedaling, the cranks give in as if shifting.

It seams like I am shifting down, but my hands are off the shifter, almost like the chain is skipping a link.

I took it to a bike repair shop that charged me 30 bucks to replace a chain. However, the problem still exists. What are the likely causes of this?

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Welcome to the site! I've taken the liberty of editing your question for readability. I would further suggest that more information about the bike - age, type of components - would be helpful or us to give you a more specific answer. –  Neil Fein Dec 25 '11 at 22:02
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I understand that your are not touching the shifter lever. However, does the chain skip from one gear to the other when it happens? –  AndreyT Dec 26 '11 at 8:40
    
I am having the same problem. I took a close look at my chainring and noticed it was worn pretty badly, as in rounding of the teeth on the big gear. I'm going to replace the chain and the chainring, since my back gears still look pretty good. Will let you all know if it works! –  Eric Hanson Mar 7 at 20:07
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6 Answers 6

My guess is that your bike simply needs a tune-up.

A worn chain can indeed cause the problems you describe, and replacing a chain is fairly inexpensive, compared to other repairs to address this.

If a worn chain is left on a bike for too long, these problems can become permanent, requiring the replacement of drivetrain gears. How long was this going on before you brought the bike in?

Is your chain too loose? A chain at the incorrect tension can cause problems like this. A good

Also, proper shifting technique can go a long way towards helping this. Are you shifting before you need to change gears? Shifting under too heavy a load will cause the chain to skip on even the best of bikes.

The next step would be to give the bike a tune-up, focusing on adjusting the rear and front derailers (or the internal hub, if your bike has one). Also, performing normal maintenance like checking that your shifting cables are at the correct tension and that the drivetrain is clean and properly lubricated is a good idea.

(It's worth noting that cheaper bikes from big box stores can sometimes simply not be adjusted to run smoothly. If you have one of these "department store"/"big box" bikes, I wouldn't spend too much on tuning it.)

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I would have started by noting which gear (front or rear) is slipping and adjusting the tension on the corresponding shifter/derailer to bring it closer to correct adjustment. A rule of thumb is, make the adjustment and if it gets worse then you are going the wrong way :-)

If the shop changed your chain did they also change the cassette (rear gears). The chain and gears usually wear together and cause the slipping problem so it is a good idea to change them both at the same time.

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You can have either of two situations:

  1. The chain is actually shifting from one sprocket to another.
  2. The chain is slipping on the sprocket, without shifting to another sprocket.

The first case may simply indicate that the derailers are not properly adjusted (or may be due to worn components). The second case indicates that either the rear derailer does not have enough tension for the selected gear combo (maybe the chain is simply too long) or the chain/sprockets are badly worn.

A chain is good for roughly 1000-2000 miles, and a rear cluster for 5000-10000 miles. And running a worn chain for too long will cause the rear cluster to wear out faster.

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I think it could be a few things.

  1. Loose / damaged bottom bracket. The play in the crank would cause some ghost shifting.
  2. Frame flex or damage. As you stand steel bikes flex, and can cause alignment issues (but should be within reason, unless there is damage
  3. loose rear derailleur

Check for flex or looseness in the cranks and bottom bracket. Does it wobble? Try just shifting weight from pedal to pedal without spinning. It should be stiff.

Cranks fine? check for frame cracks or a loose rear derailleur. Does it wobble a lot when you wiggle it with your hand?

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Could be a loose crank arm, as well. –  Stephen Touset Dec 28 '11 at 15:36
    
Indeed! i cant count how many times a truvativ non chainring side hase come loose on me. Always seemed no amount of loctitie or torque would keep them tight. Seems to be an issue they fixed as of the newer models. –  Matt Adams Dec 28 '11 at 16:23
    
Even a loosely clamped back wheel, although that's unlikely back from the bike shop. –  Karl Feb 6 '12 at 6:03
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Since it only happens when you stand I would say it is definitely a combination of a worn chain and worn chainring, and nothing to do with shifters or derailleurs.

As a chain wears, it 'stretches', meaning the distance between the pins in the chain grows. This is a result of the side plates wearing into the pin. Once the chain starts to 'stretch', it starts to wear on the teeth of the chainring. If you look at it close you will likely notice that the valley's between the teeth on the chainring are not uniform, and the the tips of teeth are wearing down, compared to a new chain ring.

When you apply torque to the crank, each tooth of the chainring pushes against the roller on the pins of the chain. On a bike with a rear derailleur, there is a spring which keeps tension on the chain. As the chainring teeth wear out, and you apply a certain level of torque on the cranks, it becomes possible to exceed the tension applied by the derailleur and the chain will simply 'roll' up and over the teeth on the chainring, effectively skipping a tooth. This usually happens very suddenly.

The only right solution is to buy a new chain and new chainring (or chainrings). Putting a new chain on a worn chainring or a worn chain on a new chainring will only accelerate the wear on the new item. The bicycle shop should have sold you both.

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I had a similar problem on my mountain bike. My bike is a 2008 Giant trance x2. It would "ghost shift" under load usually going uphill. It would even miss shift locking the back wheel. Very frustrating. I took it to my local bike shop and they did some adjustments and replace the chain. Resulting in a little improvement but still basically unrideable.

I decided to change out everything having anythging to do with the back derailer. I went a little on the high end by buying a SRAM XO rear derailer and shifters, a SRAM 1070 cassette, and a KMC chain. I also replaced the cables & installed a Clarks cable kit which protects the exposed cables.

Result - Unbelievably good shifting!! Better then when the bike was new. The SRAM XO is so easy to adjust, I did all the work myself (had to buy a few tools) but its like a new bike now.

Hope this helps

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