In this article you’re going to learn how to fix a bent derailleur hanger from an aluminium frame. Aluminium differs from steel in that it’s more difficult to bend and easier to crack. That’s why extra care is in place to make sure we don’t ruin the aluminium frame with this procedure.
A bent derailleur hanger can be cause by a bike falling on the drive side. In my case, the brakes on my front wheel locked it, flipping me over the handlebars. Inspection of the bike afterwards saw relatively minor damage to the handlebar grip, the saddle and the point where the rear derailleur is mounted on the frame.
When I started moving the bike the rear derailleur was caught in the spokes. Further inspection revealed that indeed I had a bent derailleur hanger.
So now I had something to fix, so let’s get started.
Specialty tools to fix bent derailleur hangers
There are indeed tools which are specifically made to fix bent derailleur hangers. However they’re also incredibly expensive. Strictly speaking a Park Tool derailleur hanger alignment gauge costs more than my entire bike. And how many times will I be using this tool? Probably not all that much.
And I wasn’t too keen on using an adjustable wrench either. It seems a crude method that could easily cause damage to the frame. Or at least would severely scuff the paint job, which wasn’t all that great to begin with.
That’s when I decided to reuse the parts of my DIY bearing cup press I made for the headset of my Cannondale super V.
Step 1. Fixing the bent derailleur hanger with a DIY derailleur alignment tool
I don’t have a bicycle repair stand. So working on this bike means turning it over before removing the wheel to get to the hanger. With some repairs a bike repair stand is handy. this isn’t one of them.
After you’ve removed the wheel, you need to get to the hanger. To do so remove the derailleur by unscrewing the rear derailleur. You can simply leave it hanging. It only needs to be out of the way of the bent derailleur hanger.
What we’re going to do is tighten a 8mm threaded rod with washers and long hex nuts to the frame. The washers are going to go on each side of the bent derailleur hanger. This is to prevent tightening damage from the nuts and to have a larger area of the hanger receive the force I’m going to exert on the rod.
When you pull the rod through the hole in the derailleur hanger, be patient and move slow. The threads in the derailleur hanger are thin and easily damaged. Tighten the nuts with a adjustable wrench so there’s no play when your going to apply force to bend the derailleur hanger into the right position.
Step 2. Realigning the bent derailleur hanger
Realigning the hanger is done by grabbing the rod and bending it in the direction you want to. The length of the rod will guide you in knowing how far you need to bend and if the hanger is relatively straight again.
If you feel the hanger is straight enough you can perform the following checks. You can use a level tool to see of the horizontal position of the derailleur is correct. And you can use a ruler with a 90 degree angle to check the vertical position in relation to the seat stays. Both check can also be performed simply by looking at the rod in relation to the frame.
If you’re satisfied your derailleur hanger is bent back into it’s intended position. You can reassemble your bicycle and check again with the rear derailleur attached. By sight alone you can see if your efforts had the desired effect.
You will need to readjust the shifting and the check the limiter screws of your rear derailleur, but that is going to be covered in another blog.