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On my commuter bike I decided to rid myself of the small inner chain ring (which is almost never used) and front dérailleur (now redundant). However, I have come across an unusual problem. Whilst cycling the chain is coming off of the outer chain ring and falling onto the crank arm. This usually coincides with a shift of the rear dérailleur onto a smaller sprocket, although notably not onto the highest gearing - it has happened shifting from the 5th to 6th scrocket on the 12-27 cassette for example. Furthermore, I am not exerting abnormal force on the crank, just sitting and riding as normal. This just happens out of the blue and as you can imagine is quite disconcerting/dangerous.

I have considered and checked the following so far:

  • Chain length - tension is good so no obvious problems there...
  • Chain dirty - it most certainly is not, it has only seen 200 miles and its lubed up a treat ;-)
  • Chain ring to cassette alignment - Hollowtech II BB and Ultegra 53T 6600 crank in 68mm shell, standard setup afaict, no obvious problems there...
  • Lack of front-dérailleur - I removed this since there is only one chain ring - could this be a factor that prevented me noticing the problem before (surely not...)
  • Chain ring worn - the chain ring looks in pretty good shape to me

I experimented briefly, turning the crank and applying a very slight outward pull (away from frame) on the chain and it promptly fell off onto the crank. Frankly, I was surprised how such a small misalignment could cause the chain the fall off...

My question: Has anyone else encountered this problem before?


Any comments on the things I have checked (why I may be mistaken in my assessment of the problem)?

Other possible causes I have missed would be gratefully received - this is spoiling an otherwise nice bike and I really do not want to go back to using the double crank configuration if possible.

Ultegra crank adapted to use only 53T outer chainring

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landstatic, welcome to the site. Interesting question! Please consider continuing to update your question text as you resolve this problem. I'm looking forward to seeing how this gets resolved. Will share this with a mechanic friend of mine not on this network. –  Neil Fein Jul 18 '11 at 23:51
    
I really appreciate that, I will certainly update the ticket so those interested know the outcome. Thanks again! –  codeitagile Jul 19 '11 at 10:06
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4 Answers 4

up vote 11 down vote accepted

Yeah. A modern double chainring is ramped and pinned, in order to be more easily shifted from one ring to the next. Simply put, the chainring "wants" to pass off the chain to the next cog. Without a derailleur to keep it in place, the lateral pull of the chain shifting across the rear, combined with some road jostling, can make the chain fall off the front.

A couple solutions:

  1. install a singlespeed chainring without ramps or pins, with "straight" teeth.
  2. use an outer chainring guard and inner "dog tooth" chain keeper
  3. combine the two (which is the best solution).

The free and easy way to keep it on while looking at other solutions is to reinstall the front derailler and adjust the limit screws so it keeps the chain in place.

This is a pretty common concern with cyclocross. Racers often use a single-chainring setup for the sake of reliability, and either or a dual chainguard or chainguard-and-chain watcher setup for just such reasons (also because they keep picking the bike up and putting it down whilst running and jumping over stuff), so you get stuff like this: http://www.bikeman.com/CC-SRAMS300CX.html.

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Another solution I've seen is the Jump Stop. Site: n-gear.com. Ebay: cgi.ebay.com.au/ws/… –  Mac Jul 19 '11 at 6:48
    
Thanks for your very interesting observations and practical suggestions for how to resolve this; a great response to my original question! –  codeitagile Jul 19 '11 at 10:12
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This is a perfect answer. –  zenbike Jul 19 '11 at 11:44
    
@Matt, I am going to move to a single speed chainring and get a dog tooth chain keeper for good measure. –  codeitagile Jul 21 '11 at 14:04
    
Totally agree. +1 –  heltonbiker Nov 4 '11 at 11:55
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My bike slip has badly worn components and the chain slips off the front when I am in the big cogs at the back. I know this problem is a combination of chain wear plus alignment between the sprockets and chainring.

For single chain-ring bikes this is a common occurrence and normally you can get one of these to go on the seat-tube to prevent it from happening:

enter image description here

Your bike sounds like it has different chain-alignment to mine, yet the the symptom is the same. A new chain will almost certainly rectify the situation. To check the chain wear, see how far you can pull it at the point on the chainring nearest the front of the bike. There should only be a millimetre or two of movement, if it is a lot more than that then you know it need replacing.

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I agree, this could be a worn chain. And a new chain rarely hurts. What is the name of this device? I've seen them on bikes from time to time, and there's a similar one used for double-chainring bikes. (Also: How to know when to change a chain might be useful.) –  Neil Fein Jul 19 '11 at 1:22
    
@Neil Fein it looks like a Deda Dog Fang wiggle.co.uk/deda-dog-fang –  Mac Jul 19 '11 at 6:56
    
I agree its most likely down to chain alignment. Concerning chain wear, its only been ridden around 200 miles so far so I am not expect much, if any, wear. –  codeitagile Jul 19 '11 at 10:11
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How many miles on the chainring? A worn chainring could be part of the problem.

In addition, the rear derailer tension arm may be sticking a bit and not maintaining good tension (though from the looks of your bike in that picture the derailer is probably shiny clean).

But I suspect the missing derailer is a big part of it. When the chain "jumps" onto a new cog it whips around quite a bit, and the front derailer would help contain things.

Have you tried cranking it and shifting while it's on a service stand, so you can observe the derailer action in detail?

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A noted before and now in the updated question text, the chain is almost new, having only been ridden for approx. 200 miles. Thanks for your response, I will double check the dérailleur arms action and let you know the outcome should it be presenting a problem. –  codeitagile Jul 19 '11 at 10:13
    
But keep in mind Matt's point -- the ring is DESIGNED to "help" the chain come off. It's not intended to be used without the derailer. (And note I was wondering about your miles on the chainRING, not the chain.) –  Daniel R Hicks Jul 19 '11 at 11:31
    
I beg your pardon, the chain ring is a few years old but not overly worn and its got another year in it I reckon. This could be the root of the problem, and I will update you if this is the case once I have eliminated other possibilities. The last thing I shall do is restore the front mechanism, that would be most undesirable. –  codeitagile Jul 19 '11 at 19:23
    
But you don't have any idea how many miles you have on the ring? –  Daniel R Hicks Jul 20 '11 at 0:23
    
And have you tried cranking and shifting with it in a service stand (or otherwise supported)? –  Daniel R Hicks Jul 20 '11 at 0:24
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You do not mention how many cogs you have on your cassette. The reason I ask is your chain line will be affected by removing the inner chain ring as it will be permanently moved when on the big ring. I feel that this will give you some cross gear issues when you travel up and down the cassette. The chain line for a double chain ring is measured from the bike centre line to the middle of the two chain rings when bolted together.

A single chain ring and crank with a shorter Hollowtech 11 bottom bracket spindle should solve the problem. Hope that helps.

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