A while back I picked up a trashed Specialized Hotrock kids bike. The frame looked sound but everything else was pretty far gone.
This bike would need far more workshop time than it’s replacement value, so it’s owners rightly binned it and moved on. But it’s a decent size for my son’s next bike, giving me enough time to fix it up before he can ride it.
Among numerous other issues, the derailleur hanger was bent, chain broken and derailleur was in a sorry looking state.
I straightened the derailleur hanger with a pipe wrench plier and then set to the derailleur itself.
It’s pretty straightforward to service a derailleur, remove, clean, degrease and regrease jockey wheels then reinstall. But it’s a bit of a dirty job, so rarely gets to the top of the DIY service list.
After getting it completely wrong, I removed the jockey wheels again and correctly rerouted the chain, using the smallest cog and chainring to give enough slack to work with.
Total bench time, maybe 40mins on a rainy Sunday afternoon?