How To Repair A Rear Derailleur Hanger

When something terrible suddenly happens to a possession one dearly loves, it is quite natural to utter WHAT THE… and then top it off with a globally recognised expletive associated with horizontal entertainment.

To find out how my beloved celeste Bianchi Via Nirone 7 bike (Shimano 105 R7000), and the Shimano SH-RC7 cycling shoes got damaged during a recent accident, please read my previous two articles:

Getting Hit By A Motorcyclist

How To Repair Cycling Shoes

QUIZ: Can you spot any differences in the hangers shown above? Find your answer at the END of this article.

Now let us get on with repairing a rear derailleur hanger that looks like the hunchback of Notre Dame.

Drop(out)dead gorgeous

A hanger is a metal bracket that connects a bike’s frame to its rear derailleur. During the post-accident inspection, my rear derailleur emerged unscathed but the hanger got damaged. I sat cross-legged under a Banyan-tree, researched the subject, read articles and watched YouTube videos until a solution unveiled itself.

I took off the rear derailleur for the first time since buying the bike. With a wire I tied it to the chain so that the two would not hang lose and interfere with tinkering.

The derailleur and hanger specifications for Shimano WH-RS100 aluminium wheels are as follows:

1. Mounting Bolt Thread: M10 × 1.0 (10 mm diameter with a 1.0 mm fine thread pitch)
2. Required Tool: 5 mm hex (Allen) key
3. Recommended Tightening Torque: 8 to 10 Nm (using a torque wrench)

Hang ‘em high

I used three tools to get the two hex bolts out:

1. Hex (Allen) 5 mm bit
2. MXITA digital torque adapter
3. Stanley ½” drive ratchet (set 2-85-586)

Unscrewing them revealed a weakness: both were only torqued to 4 Nm (or only half of Shimano-recommended 8-10 Nm).

Bolts and nuts do tend to become lose due to road vibrations. Besides, the manufacturers are not very particular about correctly torqueing the hardware of lower and middle tier bikes. Whether you buy a new or a used bike, do correctly torque all your bolts and nuts before riding.

I recall how after a few rides on my new bike, I found a calliper brake-pad that was ready to fall like the Roman Empire. From that day on, I worked to create a master-list of all the hex bolts, their Metric sizes and torque (Nm) settings. The data was painstakingly collected from official Shimano catalogues.

It does not hurt to be a nutcase. I now maintain a precise inspection schedule based on the mileage and the number of riding hours.

Alignment, but why?

Not until I got into serious maintenance did I realise that bicycles, much like cars, required alignments of important parts. Looking at my deformed hanger I realised that a derailleur hanger alignment tool was needed to straighten it. Disappointingly, neither any repair shop owned one nor was it available locally for purchase.

To read about professional alignment tools, click HERE and then HERE, but rejoice, cheaper options are also available from other manufacturers. Currently I am working on making the tool myself—you guessed it—to save money and learn new things.

Standing at the rear and slightly to the right side of the rear wheel, I eyeballed the hanger. Vertically, its front edge was twisted inward and the bottom end bent inward towards the wheel.

Without removing the hanger from the frame, and using very small and careful corrections with an adjustable 10-inch wrench, I unbent and untwisted the hanger back into position. The chain could now be easily slipped over the cassette and the chain-ring.

Off to the mechanic

Finding and then importing a hanger from a foreign on-line shopping platform seemed like an uphill task. I called several cycling stores but none had the exact same hanger for sale.

I headed to a friendly bike mechanic. His verdict was, “Since we don’t have a new hanger of this shape, we’ll repair the old one”.

When he tried to remove the wheel from the vertical drop-out, it was stuck. Earlier, I had failed to notice that due to the accident’s impact, the claw-shaped opening of the hanger had narrowed down from 12 mm to 9 mm, which was insufficient for a 10 mm axle bolt to boldly go where no axle bolt had gone before.

Open your mouth and say ahhh…

I suggested using the edge of a chisel and hammer to gradually open the claw to an acceptable degree. This worked.

The mechanic noticed a wrinkle in the hanger’s aluminium. First he recommended heating and then beating the hanger at a welder’s workshop but I instantly rejected the idea.

With no option left except to have the bike-doctor hammer the hanger, I became an apprehensive patient experiencing his 'piece of cake' procedure.

Later research showed that my 8-metres long gut-feeling was spot-on about the dangers involved in both the repair methods.

A minor diversion here. Interestingly, it stands proven beyond a shadow of a doubt that the word is not ALUMI-NUM but rather alu-min-ium. In 1990 The International Union of Pure and Applied Chemistry (IUPAC) accepted aluminium as the international standard.

The dangers of hammering and heating

Hammering changes the microscopic internal crystal structure (microstructure) of aluminium, but it does not change its chemical composition, basic molecular or atomic identity.

Although bikes usually use fairly strong high quality 6061 aluminium alloy, hard hammering will make it brittle and prone to sudden cracking or component failure (metal fatigue) during a ride.

Heating the aluminium is even worse as it causes thermal expansion, phase changes and chemical reactions. Therefore, keep hammers and blow-torches away from aluminium. I shall not delve into how irreparable the expensive carbon ('plastic') bikes become after a crash.

The mechanic’s third suggestion was to ‘have a hanger fabricated out of steel’, I again shot down an idea that was based on his personal preference but not backed by science.

Research later revealed that an aluminium hanger is designed to fail (bend) and acts like a cheap protector of other expensive components. Like all mechanics here, my man had learnt his trade by the roadside and without formal mechanic's training.

After the hanger was carefully hammered back to about 95% correct shape, minor re-adjustment of the rear derailleur using the cable tension barrel adjuster, gave us smooth and noise-free gear shifting.

A problem remained

After what seemed like a job done, we got stuck again. The hanger’s top inside edge was shaped to fit the vertical drop-out at an angle of almost 110°.

Since it did not fit perfectly, I suggested, “Let’s hammer this in”.

Fearfully, the mechanic put his hands in the air, “Sir, you do it; I won’t touch the bike.”

I wrapped a piece of cloth over the vertical drop-out to avoid scratches and used a spring-loaded locking grip wrench to hold the hanger firmly in position.

A piece of wood was then placed over the hanger’s curved rear end for further protection and then gently hammered.

The L-shape aligned but not 100%. I left it at that and tightened the 5 mm hex mounting bolts using a digital torque adapter atop a ½” drive ratchet.

Wait, one more problem!

The top hole of the hanger had become somewhat oval due to hard hammering. Its 5 mm hex mounting bolt had a retaining notched-nut on the back with two small holes in it. To properly tighten the bolt, a pin-wrench was used to hold the nut in place. I would later make the tool myself. Stay tuned for an entire article on the subject. 

Once at home, I noticed that the torque setting of the top hex mounting bolt had dropped by 50% (to 4 Nm). Furthermore, the hex bolt’s head had become round, hence unusable.

Spare hex nut and bolt

Another visit to the mechanic. He found me a new set of hex bolt and retaining nut. This was successfully torqued to 8.5 Nm (Shimano-recommended range: 8 to 10 Nm).

He even found me a small M10 bolt with 1 mm thread-pitch which I needed for making my own rear derailleur alignment tool. Stay tuned for an entire article on the subject.

Since the hammered hanger’s claw was still not wide enough to fit over the vertical drop-out, I again used a chisel’s edge and a hammer, then twisted the slight bend with a wrench. The rear derailleur’s gear shifting was re-checked and the cable tension re-adjusted with the barrel adjuster.

As a final bit of DIY, I used a permanent marker to paint the scratched hanger black. I thought I heard the hanger cry, “Please leave me alone!”

A final note

Incidentally, a 150 mm digital Vernier calliper is an absolutely essential tool to have in the toolkit. I use a Chinese copy of Mitutoyo (originally an excellent Japanese brand) which runs on a cheap LR44 coin battery.

After long sessions of precision, methodical tinkering and dedication, do enjoy your rides.

💚

Now here is your ANSWER to the quiz: Yes both the hangers are different in shape. The one on the right is the original shape with the correct angles and curves.

Read more CYCLING articles HERE.

©Tahir Gul Hasan, 2026