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I have a very nice bike that I built from a vintage new steel frame, threadless bottom bracket (because of the french thread in the frame) and 110BCD crankset. It is currently mounted as single speed, and I would like to add a rear derailleur.

The dropout spread is 125mm and the current chainline measurement is 45mm.

I have many questions and try to evaluate the feasibility. I would like to keep as many parts as possible, in particular not change the bottom bracket because of the odd standard (and the fact the thread is in part damaged).

I am contemplating the SRAM 1x solutions, but I don't know if 11 speed compatible wheels will fit the 125mm dropouts.

I suppose I could pick a 9 or 10 speed wheel (would they fit?), and add a 9 or 10 speed rear derailleur, but I am not sure of the chain line that is required.

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  • Have you considered using an internal gear hub instead? The problem to find one that fits your dropout width remains though, but you are spared from problems of mounting a derailleur. – Grigory Rechistov Sep 22 '18 at 13:11
  • Internal gear hubs look nice indeed, except that they don't seem to play well with drop bars. – takumar Sep 22 '18 at 13:51
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Steel frame can be set out to accommodate a 135 mm rear hub without substantial effort (bear in mind that the dropouts will need to be realigned). With a rear derailleur the chainline is not an issue (in my opinion) as long as it is not kept on the most outside cogs in the rear.

You can also play with the chainline by mounting another crankset (different offset) or changing the bottom bracket spindle (you mention that it is threadless, you don't say which type is it exactly, cartridge bearings vs cup and cone).

What is often an issue in such conversion (I personally dropped one frame and kept it as fixie because of that) is whether all the derailleur cable eyelets and shifter bosses are available. Also you'd need the derailleur hanger or a suitable adapter.

Have you considered internal gearing hubs? There are Sturmey-Archer or Shimano models widely available with up to 8 gears if I'm not mistaken.

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  • Thanks for the response. You mentioned adjusting the chain line, but I don't know to which value. I did some more research on the subject of 11 speed hubs which seems to be 131mm wide, and from what I understand 45mm seems pretty spot on the middle. I just don't know how sensitive the derailleur will be. – takumar Sep 22 '18 at 13:55
  • It's not that sensitive, it's not rocket science. And bear in mind that if you are riding a "normal" setup which is 2x10 or 2x11 you are in fact always couple of millimetres off the chainline. – Mike Sep 22 '18 at 14:51
  • “What is often an issue in such conversion (I personally dropped one frame and kept it as fixie because of that) is whether all the derailleur cable eyelets and shifter bosses are available.” With Nokons or other good cable housing you should be able to run continuous housing to the rear derailleur. You can attach the housing to the frame using cable guides (better than zip ties only). – Michael Sep 22 '18 at 18:31
  • @Michael - yes, this is a possibility. In my case this was one hurdle, another was track dropouts not accepting any derailleur hanger. – Mike Sep 22 '18 at 19:07
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Converting a single speed to a derailleur geared bike is generally considered unfeasible, or at least difficult and with some risks.

Single speed frames have a narrow rear hub width: 110 or 126mm. Modern road derailleur hubs with 7 or more sprockets are 130 or 135mm wide. Note that the width of the cassette does not change as the number of sprockets goes up, instead, the sprockets are spaced more closely together (although 11 speed systems do have slightly wider cassettes). Old 5 speed rear hubs did use the 126mm spacing.

The rear stays on a steel frame can be spread apart, and then the dropouts realigned to get a wider hub spacing. See Sheldon Brown's page on frame/hub spacing. https://www.sheldonbrown.com/frame-spacing.html.

A new crankset will be required (or at least a bottom bracket with a longer spindle) to get correct chain-line. Check out Sheldon Brown's page on chain-line: https://www.sheldonbrown.com/chainline.html

Also, you'll need clamp on cable housing bosses and an adapter plate that fits over the rear axle and drop-out to mount the derailleur.

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  • Thanks for your reply. The frame I use has all the mounts for the derailleurs, I was just wondering thinking to use a cable stop on the down tube shifter mounts; do I need to add an additional adapter to place the rear derailleur lower than the fitting allows? – takumar Sep 22 '18 at 13:58
  • @takumar If it has derailleur mounts and 126mm rear spacing it's an older frame designed for a 5 speed derailleur system. Does not make much difference to how you would go about fitting a modern 130mm hub. – Argenti Apparatus Sep 22 '18 at 14:13
  • @takumar - you will need the hanger extender only if you plan to use extremely large cogwheel - the one that the derailleur won't accomodate. In most cases any modern derailleur should fit. – Mike Sep 22 '18 at 19:10
  • Quick update after more than a year : I have successfully spread the dropouts, and the bike works like a charm with 11 speed Shimano 105 rear deraileur. I kept the crankset, fitted a 1x chainring (from Spécialités TA), and everything works as expected. I have now ridden more than 3000 miles with this setup. Thanks a lot for the feedback. – takumar Jan 9 '20 at 9:09

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