My son has a 10-speed mountain bike. (Royce Union)
I have put the chain back on the rear wheel sprocket myself many times, also
the local bike shop has. But usually it comes off again the very first time
he rides it.
Is it just a matter of knowing how to adjust it, or is it my sons' shifting
technique that causes this?
Is it coming off on the spoke side?
Does it work ok when testing ( bike upside down or on a stand)?
It could be the result of one or more of several problems.
The rear derailleur low gear stop adjustment screw is set incorrectly and allows
the chain to shift off the lowest sprocket.
The rear sprocket set is not properly centered on the front sprocket pair.
The rear axle is not parallel with the crankset (pedal axle).
The rear wheel stays are bent.
The derailleur is bent or twisted out of position.
The real wheel slips out of position under load.
The chain is worn (stretched) and is too long for the tooth spacing on the
sprocket and or the sprocket teeth are worn excessively.
My guess is that the bike has been dropped and the derailleur is bent.
Take it to a real bike shop with your son and if necessary have him demonstate
the problem to a bike mechanic. This should be trivial for any experienced
mechanic to diagnose and fix.
Putting a spoke protector (pieplate) on the rear wheel will at least eliminate
your intervention of having to pry the chain out of the gap. Of course you
could adjust the low gear stop so that it cannot access the lowest gear and thus
cannot come off.
It is most likely at least one of the two following:
1) Rear derailleur has adjustable limit screws that prevent the chain from
traveling off either the high end or low end of the sprocket. If so, the
fix is easy. Buy any 101 bike repair book, or search the internet - or
maybe someone with more time can describe it here. However - it probably
didn't go out of adjustment by itself. More likely, see #2...
2) Rear derailleur is bent from a crash, or whatever, and won't function
properly. The 2 small traveler wheels should line up almost perfectly in a
plane with whatever gear on your rear sprocket you have shifted to. This is
a very common problem on children's bikes, and there are steel protectors
that you can buy to help prevent this. Every little rugged adventurer
should have one on their bike. Look for "derailleur guard" or similar.
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