Peter wants to know: my bike fell over on the drive side and bent my derailleur hanger. Do you have any tips on how to straighten it back out?
Having a spare derailleur hanger on hand is always a good idea and it's something that I make a point to purchase with every new bike. With that said, even with a new hanger, they're rarely in perfect alignment.
Quick release skewer tension can often pull a derailleur hanger out of alignment once the wheel is mounted in the frame. The only way to really dial in derailleur hanger alignment is with the hanger alignment tool. Using your wheel as a guide, you can see exactly where you are in the alignment process. Checking the alignment at the top, bottom, and back of the wheel, you can get it perfect every time, but this dedicated tool is expensive.
Another less accurate option is to use a crescent wrench to straighten the bent hanger. The problem with the crescent wrench is that you just need to be careful that you tighten the wrench down tight on the hanger, and make sure that the jaws of the wrench extend past the mounting hole for the derailleur. If the bolt for the derailleur mount gets bent, you won't be able to thread your tderailleur back into the hanger. To check your work, eyeball the alignment first and then thread your derailleur back into the hanger and use the policing cage as a guide that you can compare to the cogs above it. If everything lines up visually, you're probably close enough that your derailleur will start shifting well again.
Whatever you do, don't grab on to your derailleur and pull on it to straighten out the derailleur hanger. This often leads to bending the derailleur itself, and while it may look in allignment in the 11 tooth cog, when shifted into a larger cog, it'll be out of alignment. So don't do it.