As a ‘sacrificial’ part of the bike, the mech hanger breaks or bends in a crash to help protect the frame itself from damage (and sometimes, the mech too). It attaches to the driveside dropout and connects the rear mech to the frame, and if you're finding that you're shifting is poor after a recent knock or crash, then the hanger may be bent and need straightening. Here's how…
1. Check your mech
First, check your rear mech is done up tight against the hanger (turn the bolt clockwise with a 5mm Allen key).
With your bike in a workstand or upside down, shift into the middle of the cassette. Look down through the jockey wheels to see if they align with the chain.
2. Remove the mech
If the hanger is bent, the mech cage won’t line up with the chain. As long as it’s only a few degrees off, you should be able to straighten it.
Make a note of the direction it needs to be adjusted in. Shift into the smallest sprocket, undo the mounting bolt and remove the mech from the bike.
3. Align the hanger
At this stage, a hanger alignment tool is useful (if you can’t get hold of one, skip to step five).
Thread the tool clockwise into the hanger until snug. Adjust the arm until it sits flush against the rim in the 6 o’clock position, then rotate the wheel and tool through 180 degrees (12 o’clock).
4. Check alignment
If the tool has now separated from the rim, push it gently towards the wheel until the hanger is bent permanently back into line. Check again in the 6 o’clock position and repeat until the tool is flush in both places. Repeat this process at 3 o’clock and 9 o’clock.
5. Alignment with a spanner
If you don’t have an alignment tool, you can make do with a large adjustable spanner. Tighten the spanner over the hanger so that it covers the threaded hole and clamps the hanger snugly. Then twist the spanner gently until the hanger is back in line.
6. Reinstall the mech
Check the hanger looks straight, by eye. Avoid over-correcting the bend because this will weaken the hanger.
Reinstall the mech, shift into the middle of the cassette and check the alignment again, as in step one. If the hanger still isn’t straight, remove the mech and try again.
7. Check the shifting
Once you’ve got the hanger straight, shift through the gears to check the shifting. You’ll probably have to re-tune the cable tension and limit screws to get it working properly.
Straightening the hanger will have weakened it, so it’s canny to order a replacement just in case.