8

When I am pedaling harder on a slope, the chain sometimes skips or gets detached from the front chainring.

I have several videos shot with high speed camera in order to record what was wrong here:

Video 1

Video 2

Video 3

4k 60FPS

Photographs of chainring: enter image description here

enter image description here

Can anyone tell me what is wrong with my bike?



Update April 21th, I have replaced the chain with new chain today. My chain doesn't skip on chainring anymore. However,

my chain seemed to skip on rear cassette. Here are three videos for demo.

Video 4

Video 5

Video 6

My picture of my cog.

enter image description here

16
  • Does the chain sit properly on the chainring when you are not pedaling?
    – ojs
    Apr 20 at 18:19
  • @ojs I think so, please take a look at new 4k 60 FPS (youtube.com/watch?v=zwlobrWtd8A) for references.
    – kile
    Apr 20 at 18:32
  • 4
    Then most likely the cassette is worn as well (it is a common reason why you sometimes have to change both cassette and chain at the same time). Or you could have a poorly adjusted rear deraileur. It is hard to tell the amount of wear from the video, a good close up high-res pic is better. Worn cassettes/cogs have sharper/narrower teeth often with burrs. It is often usefull to compare the different cogs as people usually have some favourite gears/cogs that are much more worn. You could also try to see if there is no skipping in other gears/cogs. If so you very likely have some worn out cogs.
    – WornChain
    Apr 21 at 21:35
  • 2
    You can also gauge the cassette/chain wear/meshing by having the entire bike standing completely still/fixed. Change into the gear that is under suspicion. Backpedal a few degrees, and then pedal forwards while the bike stays put. If parts are worn you will see the chain rise/slide up on the cog teeth before catching/stabilizing when the freehub is "engaged". If you put down lots of torque when riding the chain might slip completely before catching new teeth.
    – WornChain
    Apr 22 at 0:13
  • 4
    @kile looking at the video, my conclusions are that you are using autofocus to focus on the derailleur and the cassette is blurry, and that you are also really overestimating people's willingness to watch yet another video. The cassette is also worn out and covered in rust. No wonder it skips.
    – ojs
    Apr 22 at 6:06

3 Answers 3

13

Looking at the last update, it looks like the chain is worn out. The chain should mesh with the chainring all over its length, but in the video I see gap between the chain and teeth even before the person starts pedaling. This indicates that the chain stretched so badly that it does not fit the teeth any more.

The middle ring is also quite worn, so there is the risk that after replacing the chain you need to replace the ring and cassette too.

2
  • I replaced with new chain. But the chain is skipping on cassette. Please take a look at my latest update videos
    – kile
    Apr 21 at 20:24
  • 4
    @kile I think I already mentioned it in the answer. When your whole drivetrain is worn out, you need to replace everything.
    – ojs
    Apr 22 at 5:48
11

From the videos, it appears that your chain is not seating on the teeth of the largest chainring.

enter image description here

When the chain is not fully engaged/seated on the chainring, the behavior you are showing in the videos will happen. As to why it is not seating, the most likely reason would be that the front derailleur is not moving the chain over far enough to the right, which keeps the chain from moving far enough right to drop into/engage the teeth of the chainring. This possibility is confirmed somewhat because when the chain falls off the chainring, it falls to the left (inside) rather than right (away from the bike).

The solution has two parts.

  1. Make sure that the high-limit screw is not turned in too far, which would limit the travel of the front derailleur to the right. It should be adjusted so that the front derailleur can travel further right than it currently is limited to. The high-limit screw should have the letter H next to it (the low-limit screw should be next to the H screw, but has an L next to it - leave this one alone - not your problem). One way to check if the high-limit screw is at least part of your problem is to first put the front derailleur in the position you have in the videos. Next, using your hands, see if you can move the front derailleur further right/away from the bike. If it moves more (the shift cable may show some slack if it moves), then the problem is not the high-limit screw, it is the cable adjustment.
  2. The problem could also involve the cable length adjustment for the front derailleur. In your case, the cable is too loose. There should be a barrel adjuster on the front shifter that if you unscrew a turn or two will effectively lengthen the cable housing, which tightens up the cable (your front derailleur will move to the right assuming it is not already stopped by the high-limit screw mentioned above). It may take a few turns to get the front derailleur moved over enough to allow the chain to drop into the teeth of the largest chainring, but it will be very apparent when it does (an you will stop dropping your chain too).

Other potential issues if the above does not resolve the problem:

  • If your front derailleur cable has slipped where it attaches to the front derailleur, a readjustment of that may be needed.
  • If your front derailleur adjustment using the barrel adjuster does not move the derailleur enough and runs out of threads to adjust it, then a readjustment of your cable would also be needed for that. This would be to bring the adjuster into a range that would allow you to adjust it correctly.
  • If the front derailleur has been bent or if it has moved on the attachment to the bike frame, then it would be out of alignment, and that is something that would need to be addressed (a more specific question than this one).

The video is very revealing. Great input to help work towards a solution.

enter image description here

2
  • Can you take a look at my new video 4K 60FPS (youtube.com/watch?v=zwlobrWtd8A)
    – kile
    Apr 20 at 18:35
  • I don't think it's because of front derailleur. Anyway, thank you for your advice. I have checked the front derailleur and it's around 3mm away from the chain. The front is okay now. Today I replaced with new chain. But the chain is skipping on cassette. Please take a look at my latest update videos
    – kile
    Apr 21 at 20:25
1

This is an easy fix albeit I will caution you, what I refer to as 'chain jumping' is one of the more dangerous scenarios when riding a bicycle. Each time it jumps, from pushing hard on the pedal, the worse it becomes.

For the moderator, my experience comes from building over 400 bicycles, 2/3 of these vintage mountain bike conversions to road bike. Nearly all bicycles I received I later gave away free, yes the Cannondales also.

I took a look at your first video which showed me the chain jumping on the same link each and everytime. This is simply a bent link, or rusted, or too tight. Lube it, check alignment closely, is it the same width as the other links? If it is okay your fix is NOT the front derailluer but rather, the barrel adjuster on the rear derailluer. Only turn this 1/2 turn per test. It will solve your issue. Having said that, please check this prior to doing anything. Flip the bike over, get on one knee behind the bike and look at the alignment of the rear derailluer. Are the cogs - wheels on derailluer -straight? That is to say at both 180 degree angles. Straight up and down with each other and straight - lined up with chain travelling to front derailleur. You can get away with bending them by hand but he careful. At this point also check for looseness or typically with your issue, over tightening of one, usually the lower, cog/wheel. The sleeve inside the wheel allows travel/spinning so clean them! WD40 is a good cleaning agent however, it is not a lubricant and will attract dust.

Finally, I see you've purchased and installed a new chain? Is it exactly the same length as the original? If so the jumping on the rear cassette should have stopped. However, the chain falling off the front cassette is worrisome. You've either too long of a chain or, you should start by readjusting the rear derailleur from scratch.

Want to prove my response? Go to this link for the complete guide and trust me, for mountain biking the front derailleur is an engineering overkill (unneeded).

Good luck!

https://www.bicycling.com/repair/a33969991/how-to-adjust-rear-derailleur/

1
  • Nope, the chain is wider than the original.
    – kile
    Apr 24 at 9:45

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