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If I pedal hard, i.e. while standing/taking off/going uphill, the chain skips over the cogs making a load screeching noise and my foot just sinks to the bottom position.

I attempted to fix this problem so I replaced the crankset, cassette and chain with brand new Shimano Drore M590, Shimano Alivio HG400 and KMC X9 93 chain but the problem remains. I needed to replace these anyway since they are 2 years old and I had low quality components (i.e. drive side crank arm becomes vertical with pedal in bottom position).

I feel like either the free wheel or tension in the back derailleur is causing the problem. I removed the front derailleur because I only use the middle chainring.

I read all the other questions on this issue and they talk about the chain or the cogs being old but this does not apply since mine are brand new.

What is the likely cause of the problem in my situation? How can I make sure?

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    1) Your chain is too long. 2) The ratchet mechanism in the rear hub is failing. 3) Your rear derailer is misadjusted or the spring is too weak. Take your pick.
    – Daniel R Hicks
    Commented Jul 10, 2016 at 12:39
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    Did you remove any links from the new chain, or did you fit the whole thing as it came in the box? Normal process is to count and match the number of links in the old chain (not the physical length)
    – Criggie ♦
    Commented Jul 10, 2016 at 19:54
  • The chain could be slipping due to natural weakening of the chain. Over time, the chain will stretch out and a loose chain means more slipping when pedaling hard. The chain could also be slipping if your cogs are word down. The cogs/crank also wear over time and can leave the chain with not much to "grab" on to.
    – love2phish
    Commented Nov 20, 2019 at 15:37
  • If you look at your cogs/crank and see that they are short or very sharp, this can tell you something. This happened on one of my mountain bikes and the chain or cogs could need replacing. If these are really not the problem and they were installed correctly, you could have to large of a chain as said below. Hope this helps! Good luck!
    – love2phish
    Commented Nov 20, 2019 at 15:37

3 Answers 3

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Since the rings, cogs and chain are new, and decent components we can safely eliminate the possibility that there is a problem with the chain grabbing the rings or cogs.

Here is what I suspect might be happening: it's your derailleur: due to wear, damage or perhaps weakened springs.

Here is how: as you pedal, the chain temporarily catches in the derailleur cage. The next thing that happens is that, since the chain is not moving through the derailleur, the entire derailleur is pulled by the chain and rotated backwards, which basically peels/unwraps the chain from the cassette until it loses grip.

You only notice this when there is significant load on the chain, like going uphill. Under light load, you just need one or two teeth of grip, if the chain and cassette are new.

There is no substitute for having someone watch what is going on. Or attaching a camera to a chain stay or seat stay to record what is going on.

If the derailleur is the culprit, you probably wasted money on the new crankset. Two-year old rings are probably still okay, and in any case, individual rings can be replaced, not the entire crankset.

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    Thank you for the insightful answer. The left crank arm screw was stripped so I thought it was best to replace the whole crankset because replacing the crank arm alone costs $30-40. My bike is known for having great frame, wheels, breaks etc for the price but a very cheap drive train, especially the derailleurs, they are the cheapest shimano derailleurs so I decided to slowly replace the whole drive train since the cheap factory components served me well for the past 2 years.
    – user6005857
    Commented Jul 11, 2016 at 2:23
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Had this so so many times and so far it’s always came down to the mech hanger being slightly bent, delivering the chain to the cassette slightly off causing it to be exaggerated under pressure. Spent over 400 trying to fix it, then as a last resort just by chance changed the mech hanger (30 euro), it was SLIGHTLY bent like a few millimeters and that’s what was causing the whole thing.

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On the assumption that everything was fitted properly, my first guess would be chain is too long, my second would be that derailleur adjustment is required, but this guess doesn't specifically fit your description of "hard" pedalling. But these are only guesses, remember, maybe your LBS can help you some more.

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