[Brainstorm] Open Source Hanger for Framebuilding

Custom dropouts have always been a cool way to express yourself and solve problems. Unfortunately thru axles have made custom dropouts harder to design and manufacture. Low-cost metal 3D printing and garage CNC machines are putting custom dropouts back on the map, but there is still one big barrier: derailleur hanger standards. You only have a few options:

  • Do you use UDH (only standard at the moment) and end up with a bulky dropout?
  • Do you try to make a Paragon hanger work?
  • Do you bootstrap a big brands hanger and hope they don’t go out of business or sue you?

If we had our own open-source standard, we don’t need to worry about that!

Arguments for an open source hanger standard:

  • more reliable hanger and axle supply
  • design specifically for our needs (metal custom framebuilding)
  • no legal grey areas to tip toe around
  • freedom to re-produce and remix the design
  • if widely adopted, it guarantees the longevity of the standard
  • no one company owns the right to repair your bike!

Survey of Hanger Styles:

Sandwich Style Through Bolt Style Snapring Barnicle Nut Style Modular Style
Axle Faceing? Yes Yes no no no Yes
Field Servicable? Yes Maybe No yes yes yes
Bore? yes yes kidna no maybe no
Complexity 2 3 1 2 2 4
Size 1 2 2 1 1 2
Plate Dropout Compatability 1 1 3 4 1 2
Round Dropout Compatability 2 1 1 4 1 4
Printed Dropout Compatability 2 1 3 3 1 3
Total 8 8 10 14 6 15

A lower score is better, but these scores are subjective!

Sandwich Style:

The inside part contains the hanger and the outside part is a captive nut. Some designs rely on the axle threading directly into the dropout.


Through bolt:

The hanger has a threaded shaft that is inserted through the dropout bore and secured with a nut. Most designs have wrench flats to secure the hanger. The UDH is the best example of this design


Snapring Style:

The hanger is on the outside of the dropout and is held captive with a precision bore and snapping. As far as I know, only paragon uses this style. The snapping prevents the hanger from being field serviceable.


The Barnicle:

The hanger is totaly separate from the axle. It just hangs off by itself.


Nut Style:

The hanger sits on the outside of the dropout and is held in place with tiny screws. The axle provides the final clamping force.


Modular Dropout Style:

The entire hanger and axle retention is one large unit


Proposed Open Source Design:

I suggest we take the best of both worlds, the load-bearing of the through bolt with the simplicity of the nut style. This design is very similar to the Cannondale and Scott hanger, but adapted to be compatible with all styles of dropouts that framebuilders use: plates, sliders, large (1.5in) and small (1.125in) round dropouts


The hanger is designed to be as versatile as possible so it can be adapted and remixed. I think the only standard is (in the diagram above) A, B, C, D, and E. The other clearances can be customized to your dropout with just a slightly different toolpath. In a pinch, you could fit a generic hanger or file one down. However, I know that approach could also open up a can of worms.

the design is not without compromise:

  • there will be a gap between the hanger and the dropout lip
    • the hanger is less stiff and more prone to bending
  • the M2 flat head sits on the face of the dropout, where the axle
    • if it backs out, it will not prevent the wheel from seating properly
  • the hanger prevents the axle from resting 100% on the dropout face


An open source standard requires the adoption and acceptance of a community.

  • What are your thoughts?
  • What problems do you have with existing hanger solutions?
  • Will this solve some problems for you?
  • Do you think the design compromises are worth it?
  • Would you use it?

This is just a draft, so I want your honest opinions and feedback

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I like the idea overall! Only small thing is that I’ve found the M2 screw on the Paragon BoBI dropouts to be a bit lacking. It also locks you in to a pretty small hex head size which I don’t love. I think increasing that to M3 would be real neat if it’s feasible.

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I couldn’t resist :rofl:

IMO, based on our experience, it’s not a good design for 3D printing either. The nature of that feature causes the print to warp inwards. It requires reaming the interface after printing, and even then, it’s not perfect. The good news is the warping allows you to get rid of the snapring!

Also, that bore is also really difficult to mask for paint… On overpriced titanium gravel bikes these extra steps don’t matter, but I’m trying to make framebuilding more accessible and easier!

I know No22 and PVD use the snapring hanger for their 3D printed dropouts, but I personally think there are better solutions out there.


Good suggestion. I was worried about the M2 screw… Let me see if an M2.5 or M3 will fit.

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Consolidating and responding to this from the UDH thread:


I don’t have much to say about the UDH since I don’t know enough about it, I won’t adopt it until it’s proven necessary. I just wish if “we” are going to start a new standard that everyone that makes parts has to be on board. Otherwise we’ll be making Shimano specific MTB frames or SRAM UDH MTB frames?

This is one of my concerns as well. If Shimano suddenly releases a direct-mount derailleur in the next 2 years, that would definitely drive all mountain bike dropouts to UDH designs. I think it’s unlikely, but I definitely have that on my radar.

Even if that happens, I think this hanger design will still survive on gravel, road, and commuter bikes. That is also where the tubes are smaller and the aesthetics and weight are more important.

So it should be open source like you say @Daniel . I really don’t want it to go the way of what happened with BB shells several years ago. That was nuts. Everyone manufacturer had their own and expected everyone to follow since they thought theirs was the best.

Agreed. This is exactly how it is right now with hangers. Every custom frame parts manufacturer is using their proprietary hanger for their dropout design.

Best case, an open-source hanger gets traction and we reach some sort of standardization in 3-4 years. Worst case, we end up with another dropout with yet another hanger design.

So to me, this is really a zero risk, high reward endeavor

All in for the new CFF new standard drop out

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The snap ring drop out without the snap ring installed is probably the best dropout and hanger I have worked with (both as a builder and mechanic).

  • does not fall out even without snap ring installed
  • dead easy to swap if you bend it mid ride
  • no bolts to loose or strip

imo small retention bolts should be avoided - easy to strip, easy to loose or have they have a tendency to back off into drivetrains.




M3 is a good size for that

I like the idea of having a hanger that can be used for all the different styles of dropout. Your design looks very nice. But unless you’re going to get Paragon on board, it’s going to be a hard sell.

I’m really happy with the snap ring dropout. I think your decision matrix should be altered. It is field serviceable since the snap ring is optional. The size and weight are better than any of the other options barring a fancy 3d print, so that should be 1 not 2. They’re cheap and easily available from Paragon. If your painter/powdercoater isn’t a bicycle specialist, then you can use the stainless version, so they can mask the bore instead of clogging it. They work great for any style of bike. They don’t require tiny bolts to keep them in place.

I’ve had maybe three or four requests in the last couple of years from people with Niner steel bikes to fix their cracked dropouts. All the dropouts cracked at the hole for the hanger fixing bolts. The position of the bolt holes is likely the culprit, but no holes is better.

Despite not being a fan, I think UDH will become the standard hanger for off-road bikes and maybe eventually road bikes too. A lot of the big name brands are already on board with UDH. If we pool our talents to making a good UDH dropout for metal bikes, that might be a better direction to take. Until then, the snap ring dropouts seem to be the best thing going.


When I see those 250+ hangers, I see zero standards. The only standard at the moment is UDH. Everything is proprietary with no documentation. For one or two builds, you could buy and reverse engineer a hanger, but the biggest problem with these hangers is that they are not open-source.

Open source is the right to reuse, redistribute, and remix. But more importantly, it is about the culture of collaboration.

This may not matter to an individual, but as a community, I think the lack of shared resources is setting us back. The status quo is to accept what Paragon makes, which tool makers are in business, or what the bike industry decides to do. Everything just comes and goes. Open source is a platform that allows people to build upon each other’s work.

In other hobbyist communities, a young kid can jump in and find free, well-documented resources, and a community that encourages collaboration. In the frame-building world, there are no community resources that ground the art. There are just bits and pieces of hearsay and conflicting opinions.

This open hanger and the open fixture are my efforts to shift the culture towards well-documented, accessible, and shared resources. I spend more time on documentation and community engagement than it takes for me to crank out designs and find suppliers.

Open source also solves this problem. Having the files, documentation, and right to reproduce empowers our members with garage CNC’s. I don’t think economics is not the blocker, its community adoption.

I am going to chat with Mark at Paragon about open-sourcing their existing hanger designs (and joining the forum). I know they care more about supporting the community than their bottom line, and I think open-sourcing some aspects of framebuilding will build a bigger community in 3-5 years.

That is a good datapoint. Doing some internet sleuthing, it seems like this is the mode of failure:

The hanger bolt is definitely the initiator of the failure, but I think the root cause is that the steel is really thin (2mm?) and in the load path of the stays. Still, it is a valid concern. I’ll do some FEA on the bolt location to mitigate the risks.

If we just accept the status quo, framebuilding will always be at the mercy of what other companies throw at us.

Headtubes is another example: most gravel and road bikes have 50mm beer cans for headtubes. It makes no sense from a strength, weight, and aesthetics standpoint. It is simply because the industry shifted to tapered 1.5 sterrers in the mid 2000’s in their “stiff is better” phase. Framebuilders had to react with the ZS44/EC44 solution. Today, most companies have scaled their road bikes back to 1.25 but framebuilders still stick with the oversized headtube from 12 years ago.

I applaud Pine cycles for questioning the norm and using the EC37 standard. I think it’s a great idea, but it has zero adoption: only one headset and no viable titanium headtubes.

If we don’t work together to challenge or question the things that are handed to us, things will never change. Open source designs, and more importantly the culture, give us power to develop and grow as a community.

Design update:

I got a M3 flat head into the design. Ill do some dumb FEA to find the best location of the bolt.

For reference, here is the research on bolt size and head shape:

Thread Head Diameter height hex
M2 Socket 3.8 2 1.5
M2 Socket Low 3.8 1.35 1.3
M2 Flat 4 1.3
M2.5 Socket 4.5 2.5 2
M2.5 Socket Low 4.5 2 1.5
M2.5 Flat 5 1.5
M3 Socket 5.5 3 2.5
M3 Socket Low 5.5 2 2
M3 Flat 6 2

If people are happy with existing round dropout solutions, then designing a hanger that focuses on a plate and 3D-printed dropouts gives more design freedom. We can move the retaining bolt out of the axle slot. This also allows you to use an m2.5 socket head (2mm hex).

I would still strongly prefer the hanger to be compatible with round dropouts. When I get some time I will print some prototypes to test how annoying the M3 flathead is.




Your replies seem overwhelmingly in favour of sticking to existing (largely proprietary) solutions. Out of interest what is your preffered hanger solution for the small builder and why? I was also wondering if you’re inherently opposed to an open source solution, and if so, why that might be?



Here’s some lunctime FEA, so take it with a grain of salt (and some Siracha).

The sim has 100N on the CS and 100 N on the SS trying to twist the dropout. No idea what real world forces are, I just wanted to visualize the flow of the stress.

The current design looks safe to me, but I will try rotating the bolt spot further back in the slot.

This could also be an argument against the Cannondale K33049 hanger, which has the bolt in front of the axle. Although it’s pretty low on the dropout, so it might not be an issue.

To me, I think the most interesting result is that the FEA predicted the crack of the niner dropout. Because the chainstay is a larger diameter, I think it transmits more force into the dropout, and that location is the fulcrum of the SS and CS forces!

I designed more than a hanger, I also designed four different dropouts to check if the architecture works across the different dropouts we use in framebuilding. If you try that with the Cannondale hanger, you will realize it’s not possible.

The millimeters really matter and the Cannondale hanger is not compatible with small round dropouts due to the bolt location. It must also interface with a very specific dropout edge.

To me, the open hanger is not about IP or economics. It’s about the culture and community. Right now, everyone who wants to design a dropout has to buy a hanger, reverse engineer it, and hack it together. An individual with CAD or fabrication skills can solve it one time, but that leaves nothing for others to build upon.

I also don’t think this is an economic problem. Here are some proposed solutions:

  • With my Taiwanese supplier, I can have a MOQ of 600 at <$8 each.
  • I have a meeting with Paragon this week to discuss this exact topic (open-sourcing their hangers)
  • Hobbyists can help each other by making hangers with desktop CNC
  • I can metal 3D print a hanger for <$15 with MOQ’s of 10


Yes. The time and knowledge I spend on this forum are worth way more than $5k. By the time we need 600 hangers, the project would have already succeeded. Before we reach that point, I know there are plenty of members of this community who would be happy to step up and produce prototypes and samples for the standard to gain traction.

To pull back the curtain a bit, I am contracted to engineer a 3D printed titanium FM dropout system from a big domestic manufacturer. I could just do what firefly or moots do, design their own hanger, and add yet another standard.

OR, I can give my engineering time for free and develop an open hanger that everyone can Hobbiests and smaller builders can use this resource, develop their own designs, and build a stronger trade and community.

That exact same argument could be said for framebuilding: why do we build frames when we can buy one for $300? Some people are passionate about CNC machines, might own one, or want to learn, or want to help people.



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