Fixing Bent Derailleur Hangers Part 2: Using a Professional Hanger Alignment Tool

By Jim Langley

Jim’s Tech Talk

This is the second article on repairing bent derailleur hangers. In part one we covered common ways they’re bent and the shifting problems caused by bent hangers. And we recommended derailleurhanger.com as a good source for finding the correct replacement hanger for frames with bolt-on hangers (most frames today).

We also went into different methods to fix a bent hanger on the road. You can read the whole story here: https://www.roadbikerider.com/fixing-bent-derailleur-hangers/. For even more helpful tips, don’t miss the comments at the end of the article.

This article is about using a professional hanger alignment tool to fix bent hangers.

Why Use a Professional Tool?

The main reason to buy and use a pro-level alignment tool is to be able to align hangers accurately. Even minor bends can cause shifting issues such as hesitation or missed shifts. More serious ones can cause the chain to come off or the derailleur to shift catastrophically into the rear wheel.

You can use some of the workarounds that we showed last week to improve a bent hanger on the road and hopefully be able to get home. But, for precision alignment a good tool does a much better job. They serve two purposes, too. They are a gauge for determining how bent a hanger is and in which direction. And they are the lever used for straightening the hanger, too.

Should You Straighten or Replace?

Before breaking out your alignment tool, you evaluate the hanger to see if it’s fixable. If you have a steel hanger, as long as the hanger is just bent, it can usually be straightened. But, if the derailleur went into the spokes and that twisted the hanger and derailleur (or worse, tore the derailleur off or partially off) then you might be out of luck.

Still, it’s surprising how malleable steel hangers and dropouts are (the dropout is the part of the frame the hanger is built into). So you can sometimes straighten them even when they look ruined. Hanger alignment tools aren’t made for this. You use bending tools such as adjustable wrenches, Vise-Grip locking pliers, etc. You have to end up with a round hole, threads that still accept the alignment tool (and derailleur bolt) and a flat outside face on the hanger.

With a steel hanger that can’t be fixed, your best option is to have a framebuilder install a new one.

TIP: When trying to save a badly bent hanger be sure to install the rear wheel in the frame. That will reinforce the dropout so that all the straightening force is directed to the hanger and can’t affect the dropout.

Replaceable Hangers

Most replaceable derailleur hangers are made of aluminum. They are held on to the right/drive-side dropout with bolts and nuts or screws, the axle or a combination of these things.

Aluminum doesn’t like to be bent and it may break when attempting to bend it back – especially if it’s bent badly. So, in most cases, rather than trying to straighten an aluminum hanger, you would replace it with the correct one. It’s smart to keep the correct replacement hanger in your seat bag for this purpose on the road and at home.

However, and this is where the pro hanger alignment tool comes in, how do you know if your new aluminum hanger is correctly aligned? It may look good to the eye, but is it as perfect as it should be?

The only way to check it is with a pro alignment tool and that’s one of the best reasons to own one. They show whether your replaceable hanger is straight and they’ll also verify that when you swap out a bent one for a new one, the new one is straight (not always the case).

If you find that a new aluminum replaceable hanger is bent, it’ll probably be barely bent. But that can still cause shifting issues. And you can align these minor bends with the professional tool.

Using Professional Hanger Alignment Tools

My photo shows the basic setup for using the most common type of hanger alignment tools. The tool here is Park Tool’s newest aligner the DAG-3, my favorite hanger fixer because it addresses all the shortcomings of most of the other tools out there. 

These types of tools screw into the hanger and feature a 360-degree swinging arm that lets you compare the surface of the hanger to the face of the rim.

Notice that the rear wheel is in the frame. It should be fully tightened in the frame and centered, too.

The first step is to remove the rear derailleur. It’s easier to do this if you lift the chain off the chainring so that the chain is slack and there’s no tension on the derailleur pulleys. Then, with replaceable hangers, be sure to check any screws, bolts and nuts to ensure the hanger is attached tightly to the frame.

Installing the Tool – Don’t Strip the Hanger Threads!

To check hanger alignment, carefully screw the tool into the hanger. Traumatized hangers might have bad threads. Don’t force the tool in if it doesn’t start easily. Instead try saving the hanger threads by tapping them from the backside (closest to the wheel). The threads are deep enough that even if a few of the first front ones are bad, running a tap through will “chase” them and allow threading the tool in. (It’s a 10mm tap https://amzn.to/3P20qny.)

If you don’t have and don’t want to buy a tap, you can screw your tool in from the backside but the worse the hanger threads are the more chance you’ll dull your tool’s threads. Alternatively, if you have a threaded quick release rear axle, they’re usually 10mm thread and you could use that instead of the tool or tap.

Checking and Fixing Alignment

Once the tool’s in the hanger, let it hang as shown. Notice the label on the rim is at 6 o’clock and there’s a pointer on the tool that’s also there. That pointer moves in and out and it’s set here in the photo so that it barely clears the surface of the rim (it’s almost touching). 

Once set the pointer won’t lose its in/out position unless it’s moved. The pointer is held in a sliding piece that moves up and down on the tool. This lets you move the pointer away from the rim so the pointer doesn’t bump into the rim and lose its position relative to the rim face.

Next, the rim and tool arm are rotated together to bring the label of the rim and the pointer on the tool to the 12 o’clock position. Because the derailleur hanger hole is offset below the wheel axle, the alignment tool pointer must be raised so that it’s next to the rim at the same place on the rim as it was at 6 o’clock.

Moving the tool pointer from the bottom to top of the wheel like this reveals if the derailleur hanger is bent in the vertical plane. If the hanger is straight the distance from the rim to the pointer will be the same at 6 and 12 o’clock.

If it’s not the same, you use the lever on the tool to gently align the hanger until the pointer gaps are equal.

Once the vertical alignment is spot on, you can check the horizontal alignment by placing the tool at 9 o’clock and then 3 o’clock the same way you did at 6 and 12. And you can use the tool to straighten as necessary. When you have the same gaps at 3 points on the wheel, such as 12, 9 and 6, you know the hanger is back where it should be.

More on Park Tool’s New Hanger Alignment Tool

Here’s my full review of Park’s DAG-3 Derailleur Hanger Alignment Gauge, which I consider the easiest and most accurate tool I’ve used to date.

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Jim Langley is RBR’s Technical Editor. A pro mechanic & cycling writer for more than 40 years, he’s the author of Your Home Bicycle Workshop in the RBR eBookstore. Tune in to Jim’s popular YouTube channel for wheel building & bike repair how-to’s. Jim’s also known for his cycling streak that ended in February 2022 with a total of 10,269 consecutive daily rides (28 years, 1 month and 11 days of never missing a ride). Click to read Jim’s full bio.