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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?
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.
I would have started by noting which gear (front or rear) is slipping and adjusting the tension on the corresponding shifter/derailleur 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.
UPDATE (2018)
I've referred to these Park Tool videos many times to understand and adjust my drivetrain.
I think it could be a few things.
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?
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.)
You can have either of two situations:
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.
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
A comment by @user10512 diagnosed the problem and provided the right solution exactly (see below). It was a shot rear wheel hub causing the chain to slip on front chainrings. I didn’t think to troubleshoot the rear wheel hub until after I’d already swapped out the bottom bracket, chain, cassette and front chain rings. Now that I have everything new and a rear wheel hub working, it feels like a new bike!
Bike: 2005(ish) Carbon Trek 5200, Shimano Ultegra components.
I have the same problem and the pedals slips when going up hill. I don't think it's at the crank or bottom end anymore. I'm pretty sure it's at the other end of the bike the Freewheel or Freehub needs replacing.
I had a similar symptom and it ended up being a couple of worn out freehub pawl springs. The pawls weren't fully engaging and would sometimes release then catch the next ratchet tooth when under load, usually when accelerating from a stop.
Pedaling while stand up is not something you should or should not do. It is harder for your body because it increases heart rate, but gives you instant power and is even necessary for some obstacles of technical climbs on MTB. Sometimes you need to go over that rock, sometimes you don't have the time to shift and just want to accelerate right away.
Every bike should be able to sustain pedaling while standing simply because it is part of going for a bike ride, road or mountain.
The only thing you should keep in mind is that you should avoid shifting while pedaling hard, and this is true both standing or sitting down. Even so sometimes you're on a sprint with a friend and you just want to win it and that's ok. Think of it as revving a car. You should avoid it but sometimes you need that instant power and a well maintained car should always cope with it.
Good answers above. Most likely a worn sprocket, but could also be that your B-tension on the rear deraileur is insufficient, causing the chain to not wrap as far around the cassette cogs as it should, which would make the problem worse. Increasing B-tension will cause the deraileur's guide pulley to come closer to the cassette, thereby engaging more chain on the cassette cog.
Here's an interesting thought. If you're tall like me 6.4 or higher and you also have a lot of power, you could end up like me, where the only bikes you can ride are single speeds which have bigger chains and cogs with bigger teeth.
If I take a geared bike and pedal it as hard as I can, usually in 32/18 or 32/20 the torque from my long legs will be enough to spin and skip the chainring right out from under the chain.
That's why I need the single gear drive, because the chain is always snug with the gears.
It's a shame since there were some very fun things about geared bikes.
I had the exact same problem and it was due to broken teeth on one of the chain rings. Inspect your chain rings and look for worn or frankly broken cogs. Replace the chain ring.
