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I have a normal ranger bike without gears, I've had this bike for quite a while now, been 8 years. But recently it's chain has started slipping off. I have maintained it well, changed the chain a couple of times and got new tyres twice.

What could be the reason of the chain slipping? I always tell my bike guy to fix it permanently, on which he bluntly says, "there ain't a permanent fix". I ride it to work everyday so I can't afford these intermittent slip offs. Recently I went to a new repair shop where he told me I should get the rear wheel axle changed. Could it be the axle or the hub? Is there any way to find out. Coz everytime I go to the guy complaining that the chain is slipping off, he just aligns the wheel and says it won't happen now. LOL

I have used SO for a lot of other queries related to other walks of life. Hoping for a positive response.

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When was the last time you replaced that chain? –  hillsons Oct 19 '12 at 18:01
    
a couple of years ago. –  Pavitar Oct 19 '12 at 18:04

4 Answers 4

Possible causes:

  1. Worn, "stiff" or bent chain
  2. Too much slack in chain
  3. Worn sprockets or missing teeth, front or rear
  4. Bent sprocket (more likely on front than rear)
  5. Chain line misalignment -- frame is bent or something (misplaced spacers, etc) has caused the crank or rear axle to move "sideways". Vaguely possible that the axle could get cocked sideways (due to loose nuts) and cause this, but usually tire rub would become intolerable first.
  6. Some object (heel, shoestring, fender, etc) getting in the way of the chain.
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I'd vote for 4 or 5. First inspect the front sprocket by rotating it with the chain off and inspecting whether it wobbles left-right during the full circle. –  Mladen Jablanović Oct 20 '12 at 5:46

It sounds like your Rear wheel is sliding around in the rear dropouts and you wind up loosing tension on the chain. It's really a common thing on single speed and fixed gear bikes, you should make sure the tension is set properly when you ride.

If you have rear facing horizontal dropouts (track dropouts) that go straight back, then the axle can slide in the dropouts sometimes. One solution is to get a Chain Tensioner or Surly Tugnut which will help prevent axle slips in the dropouts.

Unfortunately, if your bike has front facing horizontal dropouts, then you can't use a chain tensioner like this.

Really, you should just get a wrench and learn how to set the proper tension on your chain and center it between the dropouts and then you won't have to go to your bike guy every time this happens.

Why would the mechanic ask you to change your axle? Maybe the threads on your axle are warped, making the bolts on the rear axle come loose faster, causing the wheel to slide in the dropouts more easily. If this is the case, then changing the axle and getting new axle bolts may help a bit with the frequency of adjustment.

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Because you said you have changed the chain a couple of times, I guess the problem is not again in the chain, because you would have it replaced.

Hence my guess is that you have to replace the rear sprocket.

The drive train wears in all its parts, but because of different mechanical functions and characteristics, some parts wear faster than others. After the chain, the next in the list of a single speed bicycle is the sprocket. Because of it size it wears faster than its front brother the crankset.

A good tip would be to look at the shape of the sprocket teeth. If they look very asymmetrical, it's probably time to change it.

I am not sure what the rear wheel axle would have to do with a chain slipping, and still have you ride the bicycle.

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So, chains aren't very different from tires in the sense that, they have to be replaced regularly, especially when you are commuting regularly. During summer and fall when I commute and race the most it's not unusual for me to replace my chain every other month. Chains get worn out, they stretch, they start to skip and behave poorly. If you haven't replaced that chain in two years, I'll bet my hat that it's falling off because it needs a replacement.

A new axle couldn't hurt either if you're axle is looking worn and starting to slip. You might have to trust the word of the repairman that pointed out the worn axle.

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Should I change the chain and axle both in one go, or change the chain,test it a few days and the go for an axle change. Also could it be the hub? –  Pavitar Oct 19 '12 at 18:17
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Indeed, always rule out one suspicious part at a time. First try the chain, that will likely solve most of your problem. If not, try the axle, wait a few days, try the sprocket, a few days later the hub, and so on. –  hillsons Oct 19 '12 at 18:31
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If you replace the chain after two years, you may have to replace the rear cog as well. As the chain starts to stretch, it doesn't sit perfectly between the teeth, and will reshape them over time. This may be less of an issue with single speed bikes, since they can thicker cogs than geared bikes. But it's worth checking to see if your cog is worn as well, particularly after such a long period between changes. –  Stephen Touset Oct 19 '12 at 20:30
    
I never heard of a worn-out chain and/or cog causing chain falling off. A typical symptom would be skipping, not falling off. –  Mladen Jablanović Oct 20 '12 at 5:42
    
You must not own a single speed. Single speeds rarely skip. –  hillsons Oct 20 '12 at 23:24

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