mattijones1979

Hi all

Any ideas please?

Bought new bike for my boy, but chain keeps coming off the front when on the biggest cog on the rear. It then sits on top of the teeth rather than on the teeth and doesn’t catch. Thanks in advance.

matt

BTinNYC 

Hello Matt,
It sounds like the derailleurs need adjusting, and I'm assuming the bike didn't come from a local bike shop. Links to video help below, but you have to get the bike off the ground to do this. Need to pedal at about 15-20 rpm during adjustment and testing. You can buy a bike stand, hang the bike on a car rack, or rope it up to a tree or rafters. Come back with questions, derailleur adjustment seems finicky the first couple times. You could take it to a bike shop too, but I think you should learn a bit of fixing, become enchanted with cycling, get yourself a bike, a repair stand, and skintight lycra outfits and...well, yeah, welcome to BikeForums.


Park Tool videos
Intro Rear Front

JohnDThompson 

If you bought the bike new from a bike shop, most include a post-purchase tune-up at no additional cost. Check this first. Failing that, all you may need is a derailleur adjustment; the videos @BTinNYC referenced should help with that.

You don't mention whether the crank has only one chainring, or perhaps two or three chainrings. If the latter, "cross-chaining" can cause inadvertent derailment. Cross-chaining happens when you use the extreme gears: large ring in front, large sprocket in the rear and small ring in front with small ring in the rear. These gear combinations put the chain at its most extreme chainline mismatch, and are best avoided, both to minimize derailment and to minimize chain wear.

BobbyG

Those Park Tool videos really helped me a couple of years ago!

Iride01 

Does it happen any time or is it just after you've done something else? One thing that might cause a chain to come off is if you quit pedaling before the shift is completed. Then when you start pedaling, it will sometimes come off. But all sorts of other things that you'll be advised of by others here can also be an issue.

With new bikes, going back to the shop is a good suggestion. Especially if you are new to riding a bike or a bike of that type and drivetrain. It might be a mis adjustment, Or it might be something you do and don't realize it's not correct. Bike shops want you to come back to them. They'll be glad to help.