I'm adapting a derailleur to a bike by welding it onto the dropout.

By looking at other bikes it seems to me there is not a very strict specification for where the derailleur hanger goes. It seems to vary.

Is there a range of placement with respect to the axle that I should observe? Is it ok to have the hanger in front of the axle, instead of behind it?

  • 3
    Could you post photos of these different derailleur positions? I have never seen them, except the direct mount MTB derailleurs that usually come with an adapter for the standard mount point.
    – ojs
    Mar 13 '20 at 8:43
  • 3
    What kind of dropout do you have/why are you doing this? Is the bike frame spaced for a multi-speed wheel?
    – Batman
    Mar 13 '20 at 9:59
  • If you just go to derailleurhanger.com, you can see hundreds of derailleur hangers, and they don't all look like they put the derailleur bolt exactly the same place. If there is a spec that says the DH bolt is supposed to be X cm down and Y cm back from the axle, that's what I want to know. It looks like they are in approximately the same place but not precisely the same place. Plus there are generic hangers where the position of the DH bolt depends on the dropout slot angle, which can vary. Mar 13 '20 at 13:40
  • Here's the official spec from Campagnolo: campagnolo.com/media/files/… I couldn't find the official specs for other manufacturers, but historically Shimano copied the mount from Campagnolo and everyone else has been copying them. The direct mount has similar history, but originated from Shimano.
    – ojs
    Mar 13 '20 at 13:51
  • 2
    The campy spec is what I was looking for. For future reference: Distance from end of the cassette to the outer face of the DR: 10-12mm. Max radious of the DH boss 8.8mm. Length from the axle to the DR bolt 24-28mm. Rearward distance from axle to DH bolt 4-8mm. Angle of the hook thing from vertical 25-25 degrees. So there is some tolerance, but not whole cm of tolerance, at least to follow this spec. Mar 13 '20 at 18:35

Welding is probably a bad idea - for several reasons.

  1. Heat changes things - no doubt you have welding skills, but you'll know that relatively thin metals warp and bend and change their temper. The thin metal of a derailleur will not take nicely to significant heat. You also risk upsetting the thin-walled tube of your chain and seat stays.

1b. Many aluminium bikes are of a grade that can't be safely re-welded. Eg, The 7005 grade is heat-treated after building, and re-heating it later will remove that leaving a softer section.

  1. You're going to be limited to like-metals. A modern derailleur will have aluminium construction with steel pins/axles. An older or cheap/nasty derailleur will have more/all steel contstruction. I'm no expert, but welding aluminium to steel isn't going to work, so at that point you'd need to braze instead.

  2. Replacement - derailleurs aren't really high wear items and generally last tens of thousands of kilometres, but they do suffer damage in falls, and simple rough handling can whack the mech against things like rocks, other pedals, etc. Having to grind off a derailleur to replace it would be a lot of work

  3. Mechanical Fuse - in theory the hanger will bend before the frame breaks - this should allow damage to be reduced if there's something caught in the chain. For me a hanger saved my frame when the chain snagged the derailleur and pulled it around the wheel and forward to the front derailleur. Without a separate hanger, the frame would suffer more in an accident.

  4. Function - almost every rear derailleur in the last 50 years uses a series of pivots to get the jockey wheels close to the rear cogs.

From https://cyclingtips.com/2019/01/do-clutch-rear-derailleurs-add-drivetrain-friction/ with extra lines added

There are Seven total... four pivots in the parallelogram, and one in each jockey wheel. But the extra one is in the middle of the hanger's retaining bolt, and its not just a simple bolt

Heck its not even a simple pivot, instead its a spring-loaded pivot and I suspect this is the bit you want to weld to your frame. This pivot point allows the entire rear mech to rotate down-and-aft as the chain climbs up to bigger cogs. Without, the jockey wheel cage will start to hit the cassette, OR you have to weld it on so far backward that the chain will be extra long and slack.


Instead of welding permanently, look to making a hanger. Depending on the age of your frame, it could be a generic $5 item, or if its custom then they start getting silly-expensive.

  1. Cheap generic claw hanger for a generic bike of the `80s. enter image description here

These are cheap, and readily available. You only need one, and it slides into the drive-side dropout that looks like a long slot, like this :

From pintrest who says it came from https://bike.bikegremlin.com/2015/07/22/rear-derailleur/rear-dropout-without-hanger/ but that's a 404 now.

1b. If your frame had an integrated hanger like this then just use it to mount the derailleur. Easy!

From https://bicycles.stackexchange.com/questions/12412/

If someone has cut off an integrated hanger like this, then just use the claw adapter as per the first option.

OR if you're a great welder, consider reinstating the hanger and fabbing the missing metal, hole and threads to attach to standard hanger bolts. Do notice the wee lip at the bottom, that is for a screw to push on.

  1. If your frame is more modern and doesn't have long slotty dropouts, like this:

From https://forums.bikeride.com/thread-3524.html

Then you will have to either buy the right hanger, or engineer somethign that fits the interface in your bike's frame. I needed a Cannondale one and it cost me $40 NZ (roughly 30 USD/ 25 euro) There are silly amounts of different styles!

enter image description here

  1. Does your bike have track-ends like this: From https://www.flickr.com/photos/waterfordbikes/8403223012

If so, your only option to add gears is to use an internally geared hub like a Shimano nexus 3/7/8 or Alfine 8/11 or a rohloff 14 speed, or one of the smaller 3 speed gear hubs.

A frame with trackends tends to be too narrow for a modern wheel with cassette, so only has space for one rear cog.

I guess you could try designing and fabbing a hanger for a bike with trackends, but its a lot more than straight welding.

An IGH is a great solution here, but again they're not cheap and they look like motors. But you can even get wireless DI2 capable ones for a sleek appearance and "I've got electronic gears" bragging rights.

From https://www.montaguebikes.com/folding-bikes-blog/2011/05/why-use-an-internal-gear-hub/

  1. Emergency hanger - these aren't great because the derailleur is held on by the wheel skewer/nut/QR, so its prone to move around.

However this would have saved me a long long walk had I carried one.

From https://www.wiggle.com/lifeline-emergency-rear-derailleur-hanger/ but is now sold out

Again notice it has the same large hole with threads to take the main pivot bolt on a derailleur, and it has the step for the B tension screw to land on.

  1. Last option is the easiest, but doesn't get you gears. It is possible to run a bike with exactly one rear cog, and to pick the size based on your pedal cadence at full speed. This is called a singlespeed and might be as simple as fitting a cassette with one cog and a lot of spacers, like this:


Chain tensioning might be fiddly if you lack long slots and can't mount a chain tensioner where the derailleur would go, and that loops us back around to the start.

In short: Welding on a derailleur is bad, so don't.

  • 1
    Some hanger-porn that didn't find a place in the answer but is too pretty to discard. Hot pink anodised aluminium through-axle hanger for a ~clutched~ duraace mech from a TdF bike. cdn-cyclingtips.pressidium.com/wp-content/uploads/2019/07/…
    – Criggie
    Mar 13 '20 at 10:36
  • Minor point of information: the Dura Ace RD in question doesn’t have a clutch (which prevents the derailer cage from moving forward, reducing chain slap off road). At this time, only SRAM puts clutches on their road RDs. Shimano has one Ultegra model with a clutch, plus its GRX groups have clutches. The hangar in question is probably best described as a direct mount RD hangar. It’s exceedingly unlikely that the OP is in need of a direct mount hangar, so this is purely a tangential comment.
    – Weiwen Ng
    Mar 13 '20 at 12:06
  • 2
    @BetterSense It would be wonderful to see a picture.
    – David D
    Mar 13 '20 at 13:43
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    The chain tugs are super cool and I will remember them. It's a good idea. Since I have vertical dropouts they won't work for my immediate problem. I'm fabbing a whole new back end for this bike, so I can put the derailleur hangar wherever I want. I just need to know where to put it. I will review the Campagnolo spec but I kind of wanted to know how far I can deviate from it. Mar 13 '20 at 18:34
  • 1
    @amb nice - I didn't consider that option. Feel free to add that as an answer.
    – Criggie
    Mar 13 '20 at 19:23

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