I was riding up a steep road when I shifted to a lower gear and I was pushing really hard on the pedals. Then I checked my freewheel, one the cogs was slightly bent I don't know if it got bent because of that or it was like that from the start (used bike but new). so can pushing hard while shifting bend the teeth or a cog on a freewheel?

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    Unlikely. Maybe if it's one of the largest couple of cogs, but more likely something banged into the freewheel. – Daniel R Hicks Nov 14 '20 at 18:57
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    You would normally break the chain earlier than the cogs. – Vladimir F Nov 14 '20 at 19:08
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    Could you add a photo of the damaged cog? Curious if its folded vs if its twisted, and whether it has gone to the inside or outside. – Criggie Nov 14 '20 at 22:15
  • The bent cog is very hard to notice but you have to pedal and look at it to see the bend. You wouldn't be able to tell if I sent a photo. So since the bend is very small and barely noticeable and I am not having shifting problems, I am just going to live with it and replace it when it wears out since it still has a lot of life in it. Thanks to everyone who helped. I didn't know that when shifting you have to ease up on the pedals so I am glad this happened before any real damage occured. – Mike Nov 15 '20 at 9:24
  • The cog is likely to be made of extreme "cost-saving" material, see question related: bicycles.stackexchange.com/questions/65110/… (you said in a comment it is a shimano tz-20 6 speeds cog, right?) – EarlGrey Nov 16 '20 at 8:44

So the damage happened at the same time you shifted to a lower gear ? I'm guessing you were pushing hard because its a steep grade, and the chain was not fully engaged in the bigger cog when you pushed hard.

enter image description here
Notice the top 6 teeth are not engaged on the biggest sprocket, but are also not meshing with the next-smallest sprocket?

Image shows a 28 and 32 tooth, so this could be exacerbated by a larger tooth difference, like it the final cog is a "megadrive" than can be a 10 tooth difference from the next cog.

enter image description here
Trying to show a view from the rear - the chain veers to the right-hand side at about the 11 oclock position. Both photos are the same chain lay.

So this chain lay, combined with

  1. a lot of leg power because you're going up a hill
  2. probably the rider is out of the saddle and leaning forwards for more power,
  3. a smaller front chainring working to multiply the leverage
  4. a chain that has no particular weak spots

could twist a part of a cog. And it would look like a twist too. The other option is a fold, which would have some other kind of cause.

Also, what "groupset" or level is the cassette? If its non-groupset shimano, or some pot-metal quality cheese-grade steel cog then that increases the likelyhood it failed.

As for fixing it, you may be able to straighten the cog. Likely its all secured together into one block so you're working around the other parts. Be aware that if you do get it straight, the metal will be permanently weakened and more likely to do this again.

The best option is to replace the damaged freewheel, which can be galling if there's significant life left in it.

  • Please make it clear: Are these photos of your bike, with a hypothetical chain misalignment simulated, or did you somehow obtain them from Opie? – Daniel R Hicks Nov 15 '20 at 1:04
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    @DanielRHicks the former - I have damaged a cog in the past by shifting under high power, but it was over 30% and I wasn't in the lowest gear soon enough. I don't have the old megadrive cassette anymore, so tried to replicate what happened. That's also why I've drawn the distinction between this "twisting" damage vs a "fold" damage. – Criggie Nov 15 '20 at 1:08
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    Thanks for the help and insight it's a shimano MF-TZ20 6 speed freewheel. Since the damage is not major I am just going to live with it for now. – Mike Nov 15 '20 at 9:27

This exact scenario recently happened to me. I was powering up a grass bank on my cargo bike but it was softer than I thought and I slowed down fast. I quickly grabbed several gears on the rear derailleur to get up it, and I didn't have time to let off the pedals. It went fine, but the next day my bike had a shifting problem in low gears. When I tried to adjust the rear derailleur, I found the second gear was bent. I am positive it was from shifting under load.

I was able to pound the gear straight enough to mostly work until a new cassette came in the mail. The chain was not damaged and I'm still using it. This was a stock Mongoose Envoy with a 8 speed cassette. The purpose of this story is to illustrate that this can indeed happen when shifting under load.

  • Thanks for sharing your story. I used to shift underload all the time thinking it won't do any damage but never paid attention to how shifting actually happens for a sec or 2 all the power is only on a couple of teeth and the action of shifting pulls them out. Luckily it was minor and I am not having problems with chain skiping or shifting so I just gonna live with it . – Mike Nov 15 '20 at 9:32

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