Damage to bicycle sprocket - any thoughts on how it happened?

[roger2]

As can be seen in the pic, one or more teeth bent one direction. Another tooth bent the other direction and its tip broken off.

I have no clue how this happened. My riding style is fairly gentle (usually) and I rarely go though gravel or off of the pavement. There was no "event" that I am aware of that could have caused this. No falls, no crashes, etc. I don't even shift the chain off of the middle sprocket in the front (the middle sprocket is the one that is damaged). I ride my bike several times a week and am aware of any changes as they occur. Yesterday I noticed a clicking noise, and could see that the chain was riding up on the sprocket on each revolution. Further inspection revealed the damaged teeth.

For the sake of my own safety going forward, I would like to understand how this happened. Had the teeth been bent slightly more, the chain would likely have come off, and I don't want that to happen at a time when I am pushing hard on the pedals.

Anybody know, or have a theory on, how this damage might have happened? It is a mystery to me...

 

[El Pee]

Often times the teeth are angled to make shifting easier... Honestly, it looks ok to me. If you're noticing clicking or the chain riding funny try cleaning/lubricating the chain and all gears and adjust the derailleur cables (front and rear) - an out-of-adjust derailleur will be putting lateral tension on the chain, preventing it from sitting properly.

 

[pingpongcomp]

I would replace that center chainring and probably get a new chain. The tooth with the top arrowing pointing at it appears to be broken off at the tip, in addition to the chainring itself being warped. I would guess the center position on your front derailleur has been wrong for some time. Depending on how much you want to invest, a new crankset entirely is a good idea. Even a cheap($75) new crankset will out preform what is currently on there.

 

[roger2]

Thanks for posting. The first thing I checked after I heard the clicking noise was the derailleur position, it was not putting any lateral pressure on the chain. And my pic doesn't show it but the angle of these teeth are out of proportion to the rest.

I bent them back into line with needle nose pliers, and the chain now rides on the sprocket as it should.

But I am still trying to figure out what could have caused this.

 

[whoaru99]

Not sure what the hardness of that chainring should be, but being able to bend it with a needle nose plier seems a bit odd to me. That said, I've never tried to do that so maybe it's normal.

 

[roger2]

It is a cheap bike...don't know if that means the metal in the sprocket is softer than that in higher quality sprockets. But something bent those teeth before I bent them back.

Curious to hear more theories on how this type of damage could happen. IMO, a mis-positioned derailleur seems unlikely to cause bending of just two or three teeth, and breaking the tip on one of them. Even if a rock got caught between the sprocket and chain, the spring tension on the chain doesn't seem great enough to create enough force to do this type of damage.

 

[usedto]

Not sure what the hardness of that chainring should be, but being able to bend it with a needle nose plier seems a bit odd to me.

I agree. When shifting gears on a bicycle, the chain and sprocket are forced to do something that they weren't originally intended to do. Looks to me like the chain jammed when shifting and bent the sprocket that (in my opinion) shouldn't have bent that easily.

You say it's a cheap bike - that's probably one reason why.

 

[pingpongcomp]

lateral force of the chain over time could have broken the initial tooth, then tip of tooth stuck in chain mangles a few more teeth before it's dislodged?

 

[dr*audio]

It sounds like a cheaply made gear cluster. If you tried to bend a Shimano, Campagnolo, Suntour or Huret cluster with a needle nose pliers I don't think you could do it. It would probably break before it bent. Replace the cluster.

 

[mkane]

And on some inexpensive bike those chaiwheels are not replaceable. You seem to enjoy riding. Go find yourself a decent used bike if you have the $$. Were down to 8 bikes and do all the work ourselves. Nothing lasts forever except maybe copper.

 

[roger2]

OK thanks everybody.

Jammed chain when shifting seems like a plausible explanation. Possibly the broken tooth tip contributed as well.

The bike is a Kent brand made in Indonesia. Pretty sure some of its parts are non-standard sizes. But it has served me well for 20 years or more. When I first got it I disassembled it completely and re-greased everything. And I have it set up just right for my body size, preferred riding position, and riding style with a special seat, handlebars, and tires. No doubt some cleaning, lube, and maintenance are due, as well as a close inspection of the chain. Going forward I will be more aware of chain position after a shift. And will be vigilant in monitoring for any further weirdness. If there are any more issues with the sprocket I will look into replacing it.

 

[roger2]

To clarify further, and now that I have had more time to think about it, I recall that I recently had to shift while going up a hill. I unintentionally hit the lever causing the front derailler to pop the chain down to the smallest sprocket. When I moved it back to the middle sprocket I was putting more than usual pressure on the pedals going up that hill. I think that had to be what did it. Cheap materials in the sprocket, and even possibly some metal fatigue setting in, likely contributed.

Thanks again everyone. I am satisfied that the cause of the damage has been sorted and will keep a close watch on it going forward.

 

[KentTeffeteller]

It sounds like a cheaply made gear cluster. If you tried to bend a Shimano, Campagnolo, Suntour or Huret cluster with a needle nose pliers I don't think you could do it. It would probably break before it bent. Replace the cluster.

Very good and valid point here. The good to great gear clusters are not going to bend easily. They'll break first. Yes, I'd replace clusters. And get a new chain while you've got it apart.

 

[mkane]

They look like steel chainwheels