Chain keeps slipping on in-hub geared bike. Stretched chain?

Question

I have an Avanti Discovery 8, which is a mid-range hybrid bike with Shimano in-hub gears. I ride about 20klms a day on bike paths so whilst it gets a work out, it never really gets much bad abuse. I've owned it about a year.

Yesterday I found that the chain would slip and catch occasionally when I was applying torque, like going up a hill. When I got home I checked all the links in the chain were flexing, moved the rear wheel back a few millimeters to tighten the chain, and tightened the crank (it was a little loose and wiggling a few mil) and re-oiled the chain. I also ensured the gears were aligned.

Next morning the bike felt fine until halfway to work then the slipping started again and increased as I rode. It got so bad that by the time I reached work I could no longer ride. Every stroke the chain slipped.

I can't figure out why this could be. It's not the gears, as there is no derailleur. Could it be the chain is actually stretched?

Answer 1

Chains do not literally stretch, i.e., the metal links do not actually elongate due to the force placed on them. A chain will get slightly longer as it wears though, caused by the holes in the links the pins go through getting slightly bigger, allowing the individual links to move slightly further apart.

Here's a good article on chain wear: http://www.bikeradar.com/us/road/gear/article/bicycle-chain-wear-explained-46015/

If you ride a lot, your chain might be worn out and this has worn the rear sprocket as well, causing the chain to slip over the teeth.

See this question for diagnosing if you rear sprocket is worn out: How to tell when to replace cassette?

However, in your question you say that the problem went away when you adjusted the chain tension, then came back when you rode the bike. It may be that the wheel had slowly moved in the frame over a period of time until the chain started slipping, then you moved it back but did not tighten the wheel sufficiently allowing it to move again (and more than before) when you rode the bike once more.

If the wheel moved you will see that the chain tension is now too low again. If you do not feel happy tightening the wheel in the frame more, or don't want to risk stripping threads on the axle or nut, have a bike repair shop adjust the chain tension and tighten the nuts to the proper torque setting with a torque wrench, and check the chain and sprocket for wear at the same time.

Answer 2

20 km a day, 5 days a week for a year is 5,200 km. Your chain will be worn, but maybe not worn out. Definitely time to check it though. 1/8" chain should last longer than 3/32" chain in a bike like this.

Your sprocket's teeth may be worn also, but its hard to know without seeing how worn.

Aside, your IGH will likely be due for its annual/5000km maintenance service, which is normally an oil change. Best do this to extend the working life of your bike.