Electric bike chain jumping off the sprocket

[Comedian]

We have a bosch e-bike with 10 speed deore on the back. It's got an 18t sprocket on the motor. The chain and front sprocket are new and everything else is relatively new. A handful of times we've had the chain jump off the front sprocket when shifting. When I replaced the front sprocket and chain I hoped I had it fixed.

My daughter rides it to school some days and yesterday I get the call "Dad the bike isn't going". I tried to talk her through putting the chain back on the sprocket - but it's got a chain guard that can only be removed by removing the pedal which requires a puller!. A teacher (cyclist) also tried and he couldn't get it on either as it's really hard to get to with that guard.

The bike is this one here (can't link the photo directly).

http://www.newfarmbikes.com.au/index.ph ... ugh-detail

Any ideas?

[twowheels]

When changing which rear gears does the chain jump off the front sprocket? May need attention to chainline, change bottom bracket spindle length. Is the front sprocket "true"?

Adapt/modify the chainguard to effect remedy without need to remove the pedal (crank?). That is poor design & a PITA.

[uart]

I know it's hard when it only happens very rarely, but have you been able to establish any pattern in where it happens. I mean like, only in high gears or only in low gear, and on upshifts or downshift. This might give some more clues.

[Comedian]

So I got the bike back from school. I managed to get the chain back on. It had gotten wedged on the outside of the sprocket in between the chain guard. It took four hands to do it. My two to fight it and another two to tip the bike onto the stand and hold the derailleur cage forward to take the tension off.

When changing which rear gears does the chain jump off the front sprocket? May need attention to chainline, change bottom bracket spindle length. Is the front sprocket "true"?

Adapt/modify the chainguard to effect remedy without need to remove the pedal (crank?). That is poor design & a PITA.

The sprocket is straight. I can't really see how you could install it crooked. I have considered modifying the chain guard so it can be removed if this happens again. You'd have to cut the bottom of the guard somehow.

I know it's hard when it only happens very rarely, but have you been able to establish any pattern in where it happens. I mean like, only in high gears or only in low gear, and on upshifts or downshift. This might give some more clues.

Yes - difficult to ascertain. The one time it's happened for me was shifting down (mid cog) to go up a steep hill. I asked the daughter what she was doing and she said "changing gears but not .. like badly".

I rode it to work and it's fine. I looked at the chain length.. it looks pretty good.

I could take the chain guard off and fashion a guide of some description - but unlike a normal bike there isn't much or anything to mount one off. Plus we can't get grease on those nice socks. She's enough of a lepper being forced to ride to school rather than being driven like all the cool kids.

[twowheels]

So I got the bike back from school. I managed to get the chain back on. It had gotten wedged on the outside of the sprocket in between the chain guard. It took four hands to do it. My two to fight it and another two to tip the bike onto the stand and hold the derailleur cage forward to take the tension off.

When changing which rear gears does the chain jump off the front sprocket? May need attention to chainline, change bottom bracket spindle length. Is the front sprocket "true"?

Adapt/modify the chainguard to effect remedy without need to remove the pedal (crank?). That is poor design & a PITA.

The sprocket is straight. I can't really see how you could install it crooked. I have considered modifying the chain guard so it can be removed if this happens again. You'd have to cut the bottom of the guard somehow.

I know it's hard when it only happens very rarely, but have you been able to establish any pattern in where it happens. I mean like, only in high gears or only in low gear, and on upshifts or downshift. This might give some more clues.

Yes - difficult to ascertain. The one time it's happened for me was shifting down (mid cog) to go up a steep hill. I asked the daughter what she was doing and she said "changing gears but not .. like badly".

I rode it to work and it's fine. I looked at the chain length.. it looks pretty good.

I could take the chain guard off and fashion a guide of some description - but unlike a normal bike there isn't much or anything to mount one off. Plus we can't get grease on those nice socks. She's enough of a lepper being forced to ride to school rather than being driven like all the cool kids.

I meant it may not be true after an incident, for example after a chain wedges between a sprocket & a chain guard. I've also heard of crank centres being off-centre, ie causing the sprocket to run out left to right.

[Comedian]

So I got the bike back from school. I managed to get the chain back on. It had gotten wedged on the outside of the sprocket in between the chain guard. It took four hands to do it. My two to fight it and another two to tip the bike onto the stand and hold the derailleur cage forward to take the tension off.

I meant it may not be true after an incident, for example after a chain wedges between a sprocket & a chain guard. I've also heard of crank centres being off-centre, ie causing the sprocket to run out left to right.

Aaah.. got you. I'll take a look at it.