The (possibly) hard-to-find part you should have on hand
Find the right derailleur hanger with a bit of sleuthing
The rear derailleur hanger might be the most important part on a geared bicycle. Most riders don’t give it a thought in the world, until it breaks. Not only does this part attach the rear derailleur to your bike, it also acts as a sacrificial component. In a crash or fall, this little aluminum part is actually designed to bend or break to save the rear dropout of the frame from damage. It is much easier to replace a $40 part than an expensive carbon frame. Since it’s such a common component, you’d think your local shop should have plenty in stock. But, no. There are literally hundreds of derailleur hangers out there. The shop is unlikely to have your exact hanger in stock. That said, the first rule of derailleur hangers is always have a spare on hand for your specific frame. If a mishap should occur, you can bring your bike to the shop with the spare (in case they don’t have one) for them to install and get you riding again.
Since that little hanger is so important, you would also think that it would be easy to find. But again, no. There are two configurations of hanger designs: standard mount and direct mount. A direct mount positions the main pivot of the derailleur farther back and beneath the cassette when compared with the standard mount. The benefits of the former include quicker wheel changes and more options for frame manufacturers when adopting thru-axle designs. Also, moving the derailleur farther down increases its damage protection. This standard was released by Shimano with the introduction of its Shadow derailleurs for mountain bikes, then later within their road-bike line. Direct-mount hangers are designed for direct-mount rear derailleurs, but you can still use a direct-mount derailleur on a standard-mount hanger with a B-link. So what does all this mean? Both a standard-mount and direct-mount hanger could possibly be available for your frame.
How to choose your hanger
Figure out if you need a direct- or standard-mount hanger for your derailleur. Next, determine whether you need it to work with thru-axle or quick-release. How many fasteners does it have? As many as four bolts can be used to attach it to your frame. Sometimes there are unique features. A threaded post could be used to fasten the hanger. The part can be mounted to the inside or outside of the dropout. Most of the time, the hanger itself is threaded but sometimes the dropout is threaded.
Once you have the above information, you will be in a much better situation when going to look for a hanger at your shop, if they don’t deal with your specific brand of bike. Ideally, you’d get your hanger from the dealer of your exact make and model of frame. But if that is not possible, you have to turn to aftermarket parts. Aftermarket hangers are labelled with a specific number (No. 163, for example). There are a few good websites where you can crossreference your frame with a specific hanger number. This process, however, is not always 100 per cent reliable as there are so many different types of frames and hangers. If you arm yourself all the information I’ve covered, you can then better compare your old hanger with the suggested aftermarket part.
Finally, once you have the correct hanger and after you attach it to your frame, you must ensure it’s aligned properly with the rear wheel. This is a crucial step that you must do before reinstalling the rear mech. You’ll need a special tool. If you don’t have it, entrust the repair to a professional.
“It is much easier to replace a $40 part than an expensive carbon frame.”