1x dropping chain

[JHZR2]

My oldest has a Trek 24” bike. We took it on trails last weekend, no issues. Cleaned it up, lubed it up, took it on a test drive going through all the gears, and when in the largest rear gear, it would continuously drop the chain. It would go around about 2-3x, then snag somehow and the chain would fall off.

Turning in reverse would drop the chain in about one rotation.

On the smaller rear gears it seems to stay on, which is perhaps why we never noticed it before. I don’t think the largest couple rear cogs get much use.

I can observe the chain lifting up and slipping off.

I do notice that the front gear is slightly bent. Maybe it was always that way, not sure. It wasn’t an issue riding, and I don’t think any of the obstacles we cleared on the weekend hit it. But who knows.

What’s the best way to diagnose and fix? This bike is too small, and so I don’t necessarily want to drop a lot of $$ into new parts. But I do want it to be reliable.

Thanks!

 

[AZjeff]

Mark the area with the bend and see if that's where it drops, bet it is. The chainline is most extreme on the largest cogs. You should be able to straighten a steel chain ring. There could be 1 bent tooth.

 

[slo town]

That chainring looks seriously bent. Fix that and see what happens. And while you're at it, use a gauge and check the chain stretch.

Scott

 

[Pablo]

Yes either the front chainring is bent or the spindle is bent

 

[Nessism1]

Check the derailleur hanger also.

 

[JHZR2]

That chainring looks seriously bent. Fix that and see what happens. And while you're at it, use a gauge and check the chain stretch.

Scott

Chain is ok I think. Had done this before.

I do think there may be some bend to it. Not sure how it happened though. Bent teeth is something I’ll need to check closer.

 

[JHZR2]

Check the derailleur hanger also.

For what? Ease of motion? Bent one way or another?

 

[Nessism1]

For what? Ease of motion? Bent one way or another?

Bent, causing a misalignment. They actually make derailleur hanger tools, so you can check alignment. The angle should be parallel to the middle of the chainline, or a little bit open (pointing right, when looking from the back)

 

[AZjeff]

For what? Ease of motion? Bent one way or another?

Stand behind the bike and see if the derailleur pulleys look like they're parallel to the cassette or tilting toward the wheel or away. That really wouldn't cause the chain dropping off the chainring but it makes it hard to get the shifting adjusted in every gear. I never heard being bent outward was acceptable.

 

[Nessism1]

Stand behind the bike and see if the derailleur pulleys look like they're parallel to the cassette or tilting toward the wheel or away. That really wouldn't cause the chain dropping off the chainring but it makes it hard to get the shifting adjusted in every gear. I never heard being bent outward was acceptable.

5mm open or less on a 700c wheel. I usually try for perfect, but it's not worth the risk of busting the hanger off for just a tiny bit open.

 

[NYEngineer]

When my son was on a 24" bike I put a 1x10 drivetrain on it. Only nine gears would work because of the short distance between the crankset and the rear axle. I shimmed the crankset so the chain ring was as far in as possible and then adjusted the rear derailleur to block out the smallest cog, the most outside one. Also, made sure the hanger was straight and if you can find one without spending a ton of money, a narrow wide chain ring will help immensely.

 

[MRC01]

I agree the chainring is bent and that is almost certainly the problem. That black plastic thing next to the chain is a guide that is supposed to prevent the chain from coming off the inside. The guide can only work when the chainrings are true. When the chainring bent, it probably pushed that guide over so can't do its job anymore.

 

[slo town]

Chain is ok I think. Had done this before.

View attachment 298722

I do think there may be some bend to it. Not sure how it happened though. Bent teeth is something I’ll need to check closer.

Chain looks good.

Scott