I had my gear cable replaced by my local shop and they also adjusted my gears as they wasn’t shifting correctly. In 2 weeks I have snapped 2 chains since they had the bike in. Anybody have an explanation for this? Could they have done something to put tension on my chain? I’m doing nothing different and have properly rode about 10 miles between each chain breaking.
When I take it back to the shop and have a moan, anything I should say or do to get this resolved?
Thanks in advance:)
Do bikes with derailleurs have tension on their chains? Is that even possible? Yes it pulls on top, but I think it has to always be loose on the bottom. Sounds more likely that something is maybe happening when you shift. Are you shifting while doing hard pedaling? That’s probably the easiest way to break a chain. Or there could have been a defect or problem from the previous repair. Who is fixing the chain? One break might be considered normal. But a second break right away suggests there is a problem beyond normal wear & tear.
So your first question should be “Why did my just-repaired chain break again?” If it was that shop that fixed it, add that bit to the question.
Annoying, but I’ll say it so someone else doesn’t feel they have to: If you’d done your riding on a non-geared unicycle, this wouldn’t be happening… 
Yes, there is tension in a typical geared bike drivetrain provided by the spring in the derailleur.
A chain that is too long can prevent ideal shifting. Maybe they shortened the chain but too much? Or maybe they put a different chain on that’s the wrong width?
Did you break two new chains or the same one twice? If it’s the same one and they did shorten it, then maybe they put it back together poorly?