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I have a 7 speed Mongoose Bike. The chain often falls off the front sprocket. There is only one chain ring in the front. Any ideas what to check for?

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Thanks, Paul

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    Is there anything guiding the chain onto the chainring ? Like a chain guide or keeper? How worn is everything? A clear photo of your chainring area might help. – Criggie Jan 9 at 3:07
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    If you grab the end of your pedal crank and dry to move it sideways (ie toward and away from the centerline of the bike) then is there any discernable play at all? This would mean your chainring wobbles during riding. – Criggie Jan 9 at 3:09
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    There is not play on the pedal. I attached a picture to the question. – Paul Jan 9 at 14:05
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Worn chain or chainring can cause this, as can a very dirty or un-lubricated chain, especially is there is a stiff link pin. A bent chainring can also buck the chain off.

Another possibility is the derailleur is not providing enough chain tension. This can happen if one of the tensioning springs has broken or has become weak. Check the 'B' spring (at the attachment bolt) or the 'A' spring (where the cage pivots).

  • The bike hasn't been used that much. The chain has plenty of lube on it. The spring on the derailleur seems to work but not sure how much tension there should be. Is there a way to adjust the tension? – Paul Jan 9 at 14:07
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    Does this happen more when in the higher gears (smaller sprockets at the back) or going over bumpy ground? – Argenti Apparatus Jan 9 at 14:24
  • I happens when shifting or hitting a bump – Paul Jan 9 at 21:55
  • If it jumps while shifting, the cause might be there is too much of a bent in the "chain line" between the rear cogs and the chain ring. If it jumps when hitting a bump, that is usually because there is too much slack in the chain aka the derailleur is providing not enough chain tension. – Baratier ErebusDuHalm Jan 15 at 20:39

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