How can I tell if my derailleur hanger is bent?

I have that problem too. It was bent, but I bent it back. Does it look straight to you. I took two pictures, one holding the derailleur out of the way, and the other one not.

http://picasaweb.google.com/ChrisWheeler...

http://picasaweb.google.com/ChrisWheeler...

Oh there we go! I can add them to the message.

The derailer hanger has an M10x1 thread, the same as the axle on most quick release rear wheels, with an axle and some nuts stripped from an old wheels sourced at your local bicycle recycle centre (or buy a axle&cone set at your LBS), some square tube, a drill and ruler is all you need.

Drill a 10mm hole into the tube put the axle through the hole and bolt it up tight enough that it doesn't wobble but not so tight you can't rotate it, (large flange washers and some UHMW plastic can help here). Use two nuts and lock them together so that is doesn't come loose). leave about 20cm of axle sticking out at one end.

Spin one of the remaining nuts onto the short end of the axle and screw this end of the axle into the derailer hanger and lock it tight with the nut.

With the rear wheel in place (the wheel has to be true, if it isn't fix that first!, also don't do this without a wheel in the bike you'll mess up the dropout alignment) measure the distance from the rim to the edge of the tube is several places.

Find where the difference is the greatest and gently (really very very gently aluminium derailer hangers are made out of cheese grade metal, and if you snap off a steel one it is expensive to repair), bend it, measure and tweak again if needed.

Expect to break aluminium hanger if you try to straighten them more then once or twice.

Absolutely. Every offset is bad for gears and chain and doesn't help clean shifting either.

Put a photo up and we will tell you :)

If you find that you cannot find a balance between perfect upshifting and perfect downshifting through cable tension then your derailleur hanger may be bent. There is a specific tool found at most bike shops that allows you to diagnose and fix a bent derailleur hanger. If you would rather buy the tool yourself, you can purchase it from any bike tool supplier. I would suggest ParkTools.