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?

enter image description here

enter image description here

Thanks, Paul

  • 1
    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 '19 at 3:07
  • 1
    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 '19 at 3:09
  • 1
    There is not play on the pedal. I attached a picture to the question. – Paul Jan 9 '19 at 14:05
  • It would be good to have a better photo of the actual chainring, to see how worn are the teeth. It may be necessary to do several shots as it must be done from a closer distance and without the chain. How old is your chain, do you check its wear? – Vladimir F Jan 15 at 14:01

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).

Unfortunately the tendency to drop the chain may just be a 'feature' of an inexpensive bike. You might look into fitting a chain tensioner to keep the chain on. Modern 'one-by' drive-trains with a single chainring use 'double-wide' chainring teeth profiles which are much better at retaining the chain. However, this is not an option on inexpensive bikes where the chainring is integral to the cranks.

  • 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 '19 at 14:07
  • 1
    Does this happen more when in the higher gears (smaller sprockets at the back) or going over bumpy ground? – Argenti Apparatus Jan 9 '19 at 14:24
  • I happens when shifting or hitting a bump – Paul Jan 9 '19 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 '19 at 20:39

You might also want to check if your front chainring or the crank spider (the crank arms) is/are bent. Check this by rotating the cranks and measuring the distance from a fixed point (eg the frame) to the teeth of the chainring. If this distance changes significantly (more than 1mm) whilst rotating the cranks this might be causing the issue. If so you might need to replace your chainring or right side crank (depending on the severity). In some cases (if it's not too bad) the chainring can be straightened/trued (similar to how disc brake rotors can be trued). Similar to this:


Check the chain ring (front sprocket) for damaged teeth .

  • 1
    Welcome,you do not need to sign your posts and a date is automatically appended to all posts here with a complete dated edit history. Could you tell us more about your suggestion? – Vladimir F Jan 15 at 13:58
  • 2
    Also, this answer duplicates an existing one. We do strongly prefer answers that offer at least some new information. – Weiwen Ng Jan 15 at 19:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.