Derailleur Hanger: The Move to a Circuit Breaker

Have you ever broken  a derailleur hanger?  I mean really, really broken one?  I mean broken one so hard that the derailleur goes into your rear wheel, breaking a bunch of spokes and taking you out of a 200 mile gravel endurance race?  No?

Me neither.  Who would do something like that?

But some people have.  Some very nice people.  And those people started thinking about how dumb derailleur hangers are.

You see, years ago, cars included these things called fusible links.  I had a mid-70s Jeep CJ and it was full of them.  I spent untold hours hacking them out of the wiring harness.

In the world of wiring, you have to accept that from time to time, something will draw too much current.  When it draws too much current, it can result in the wiring harness getting hot and/or burning up–literally melting and/or catching fire.  For a variety of reasons, that’s not a good thing.  And hence, cars include circuit protection.  The basic idea of circuit protection is providing a weak link in each wiring circuit–something that will ‘give’ in the event of an overdraw in the circuit, before the wiring harness (or an electrical component) blows up or catches fire.  That’s a very good thing.

Circuit protection in cars comes in three typical forms.  There are fusible links, which are essentially a failure point built into a length of wire, directly wired into the wiring harness.  The fusible link will burn up before the rest of the harness, but once it goes, replacing it involves hacking and splicing a new fusible link into the harness.  The second type of circuit protection is a fuse.  A fuse is a little circuit protector that goes in a fuse holder or fusebox and serves the same purpose; it blows before the wiring harness burns up.  When a fuse blows, you can pull the fuse out and just stick a new one into the fuse holder (assuming you fixed the underlying issue).  No drama, no hacking, no wiring.  Much more eloquent.

The third general type of circuit protection in cars is a circuit breaker.  A circuit breaker is an automatic device; it will trip when there’s an overload, and once the overload condition goes away, it will reset.  The power windows in your car are likely wired to a circuit breaker (unless they’re on a computer-controlled circuit).  If you hold the ‘window up’ button for too long, the motor will start to heat up and the circuit breaker will interrupt the circuit, allowing the motor to cool down.  Thereafter, it automatically resets.  (Sure, there are manual-reset circuit breakers, but let’s get back to bikes here, some time).

The general thought I have is that in most applications, a circuit breaker is superior to a fusible link.  It doesn’t require tools to fix, and can be reused and reset.  You want the protection that it offers, but when it trips, it’s a lot easier to deal with a circuit breaker (or for that matter, a blown fuse) than it is to deal with a burned out fusible link.  And that brings us back to bikes.

Derailleur hangers are fusible links.  They’re designed to be a weak spot in the system–a spot that will break before the derailleur explodes (hopefully).  But when they break, they break.  You have to have a spare with you, unbolt the old one, bolt a new one in.  Not simple.  Not eloquent.  Necessary, but inconvenient.  And when they do break, the derailleur is fully cut-loose, ready to wreak havoc on unsuspecting rear wheels.  That leads to my underlying suggestion: derailleur hangers need to become more like circuit breakers.

Imagine a derailleur hanger that, instead of being firmly bolted on, was held on by high-powered magnet and perhaps a pinch-bolt.  Under normal conditions, it would be totally rigid.  Under ‘something’s about to break’ conditions, the power of the magnet and friction of the pinch bolt would be overcome, and the derailleur would pop off the frame, serving the purpose of derailleur circuit protection…but after you stopped, you’d be able to loosen the pinch bolt, pop the hanger/derailleur back on, tighten the pinch bolt, and pedal off.  No need for extra parts, no heartache.

The one missing piece of that setup is something to prevent the derailleur from blowing into the rear wheel and taking it out.  I’m still working on that part…but I’m picturing either a limiting cable that would attach the derailleur to the frame and prevent it from moving laterally into the wheel, or a tension device of some form (spring?  elastic?) that would pull the derailleur and hanger away from the wheel and out towards the chainstay, in the event that the hanger released from the frame.

Is this needed on every bike?  Certainly not.  There’s a narrow niche market of bikes that could use this technology.  Fat bikes, gravel bikes, mountain bikes…there are some potential applications.  There are certainly other solutions as well..singlespeed, internal geared hubs, carrying a spare derailleur hanger, etc.  But the current system seems to inelegant.  This–this seems like a more intelligent approach.

I may be an embittered veteran who suffered a broken hanger.  Ok, I am that person.  But regardless, there’s still room for improvement in the design of bikes.


One thought on “Derailleur Hanger: The Move to a Circuit Breaker

  1. Sounds like you need to do some mtb races in the mud. Teaches you how to conserve your drivetrain when your shifting goes to hell.

    It’s like knowing how to crash.

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