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How to straighten your bike's bent derailleur hanger

Saving the frame from damage, derailleur hangers can be prone to bending. Find out how to straighten your bent mech hanger with this guide

The derailleur hanger is the mounting point for your rear derailleur and almost all are designed to be sacrificial parts that will bend or snap in a crash, or sometimes even if your bike just falls over while stationary.

Its purpose is to act as a firewall absorbing damage that might otherwise write off your frame.

As such they are made of fairly soft aluminium leaving them somewhat prone to bending, which in turn will throw your gears out of alignment.

If the surface of the hanger is visibly fatigued, or it’s so twisted as to be noticeable even without looking closely, it’s time for a replacement.

However, a slightly misaligned hanger doesn’t need to be replaced, it can simply be straightened. Often the undiagnosed cause of many shifting woes, checking and straightening yours is an easy job, although it requires a specialist tool.

Read on to find out how to check yours…

How to fix a derailleur hanger with and without an alignment tool

1. Take a look

Looking from the back of the bike, a bent hanger can sometimes be spotted by eye. More often it will be pushed in towards the cassette, as this is the most likely impact direction. If it looks crooked, then you’ll need to take remedial action.

However, small bends can be hard to spot, yet will still mess up your gears. If you’re having trouble setting up yours, this can be another clue something is amiss.

2. Remove your derailleur

The alignment tool fits in place of your rear derailleur, so this will have to come off first. Normally it's undone with a 5mm allen key, an anticlockwise turn will slacken the bolt that holds it on.

An upwards lift of the derailleur body will help relieve pressure on the b-tension screw and make the whole assembly pop off more easily.

3. Fit the alignment tool

With your bike in a stand, screw the alignment tool onto the derailleur hanger. Let the arm drop down to the six o’clock position (pointing straight down). Manoeuvre the gauge at the far end of the tool so it lightly contacts the rim. This is your first data point.

4. Work around the clock

Rotate the arm of the alignment tool to the 12 o’clock position (pointing straight up). If your hanger is straight, the gauge arm should sit in the same position relative to the rim.

If it’s not straight, this will indicate the direction in which the hanger is bent. A gap suggests it’s bent in, while a lack of space suggests it’s bent out.

5. Gather more data

Repeat the process at the 9 o’clock (horizontal left) and 3 o’clock (horizontal right) positions. Gaps here will suggest the hanger is also bent on a second plane, i.e. twisted slightly inboard or outboard.

By now, you should now have a complete idea of how to pull the derailleur hanger back into shape.

6. Horizontal alignment

The hanger alignment tool has dual purpose; first is obviously to check the alignment and then secondly to act as a long lever to aid straigtening.

Start by adjusting the horizontal plane. Pull or push on the hanger alignment tool to bend in the direction that's required. Remember, the hanger is soft – so go gently – and little and often with continual re-checking is the best method. 

7. Vertical Alignment

Repeat the same process working in the vertical plane. Make small adjustments. You’re aiming to be within a couple of millimetres at the rim. Small differences here will equate to fractions of a millimetre at the derailleur, so it’s not necessary to be absolutely perfect.

8. Sweep up

Sweep the gauge arm all the way around the rim to check the consistency of your work. If the gauge follows the arc of the rim, you’re good to remove the tool and pop back on your derailleur.

Be careful to properly position the b-tension tab on the back. This will need to sit behind the small lip on the back of the hanger. Also be careful not to cross-thread anything.

If the derailleur bolt doesn't screw in easily then it's a sign you may be cross-threading, which is another sure way to wreck the hanger. 

9. Adjust your gears

The reason you started messing with your derailleur hanger is probably that your gears weren’t working. Now would be a good time to give them a fettle. You can find a video on how to do so here: How to adjust a rear derailleur

If you're in the market for the correct tool, you can buy a LifeLine hanger alignment tool here for £45

Alternative tool-free methods

If you don’t have a hanger alignment tool, an adjustable spanner might do in an emergency. You’ll have to rely on your eyes for alignment, so it'll never make a perfect job like the proper tool, plus it’s far easier to break the hanger, but it might get you out of a jam.

Simply dial down the jaws of the spanner onto the end of the hanger and then it can be used as a lever to bend the hanger. Again, go carefully, and as this is not an exact science aim for something that looks ok by eye, and leave it at that.

Don't be tempted to keep on waggling the hanger back and forth, as that will weaken it.

If you’re out and about, and without any tools, then it’s also possible to pull the derailleur into place following a mishap. Again, not recommended if avoidable, but it might still get you home.

First judge how you believe the hanger to be bent. Then take hold of the upper part of the derailleur with both hands, cupping it right around the back to give the derailleur as much support as possible, so as not to damage it.

Use reasonable force to yank it back towards straight. A slightly more finessed alternative if you have one is it is sometimes possible to use a 5mm allen key in the derailleur’s mounting bolt as a lever.

When attempting either of these methods, you should weigh the possibility of snapping the hanger against the benefit of potentially improving your gearing.

If you have resorted to these roadside methods, it is advised to get the hanger either replaced or checked properly at the first opportunity.

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