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My Rohloff touring bike has an internal bottom bracket (BB). I would like to replace this with an external BB.

Current components

  1. Front Sprocket: Gates Carbon Drive, CDX, 4 bolt 104mm BCD, 50 teeth
  2. Cranks: Truvativ SRAM K28JAN21 / 175mm
  3. Internal bottom bracket: Shimano BB UN300, 127 mm length, casing width 68 mm
  4. Belt: Gates Carbon, CDX CenterTrack, 120T CDX, Part number = 11M-120T-12CTS BLACK, Belt Length: 1320, Tooth Count: 120, Pitch: 11"

I want to replace

  1. 2x Cranks (blue)
  2. Internal bottom bracket (green)

I want to keep

  1. Front sprocket (red)

New components

  1. 2x Cranks
  2. External bottom bracket

In pictures:

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Questions

  1. Is it technically possible to replace the internal BB with an external BB?

1.1. The front sprocket and the rear sprocket need to align so that the belt is not bent.

1.2. The installation should be simple, e.g. no adding washers and other hacks to get the 2 sprockets to align.

  1. Because of costs I would like to go with Shimano parts (hollowtech2).

2.1. Would this be possible?

2.2. What other manufacturers are available?

1 Answer 1

7

You want to transplant the 104 BCD Gates ring you have on to a hypothetical external BB 104 crank and wind up with correct belt line a Gates sprocket on a Rohloff. A lot of the pertinent information here is in the Gates For Rohloff Manual.

Belt line on Gates for Rohloff is 55mm for 135mm and 142x12 OLD bikes. This number basically corresponds to what you get if you put a Gates ring in the outer position of most 104/64 triple external cranks. In other words, design attempts were made to create a fairly simple path to doing this. However, there are different versions of the Gates rings out there you might have that are intended for slightly different applications and may or may not just give you the belt line you want. The manual spells out that it's CDXF450 and CDXF450E that land at 55mm. You also won't be able to use your guard. It will just be the bare ring in the outer position. (One could imagine using the guard along with extra long chainring bolts and spacers, but it sounds sketchy to me).

Gates indicates a belt line tolerance of +/-1mm is acceptable. In practice, the system will run smoother the closer to perfect it is, and tuning the rear belt line isn't really practical (mostly impossible). When setting up this kind of drivetrain, as Gates indicates, it's the intention to use spacers to tune the front belt line. If using spacers to tune the belt line is a dealbreaker, you probably shouldn't put any crank on that's not explictly intended to give the 55mm Gates for Rohloff belt line.

The best belt line for Rohloff Gates bikes in practice is achieved when the belt line starts a little bit in on the crank end, and then is tuned to perfection with spacers. It's impossible to tune it the other direction if it starts any amount out in front. Having it be perfect without doing anything to it is obviously ideal, but you may not be able to achieve that because +/-1mm is enough that normal frame alignment tolerances can disrupt it even if you had an external crank built for the purpose.

If the goal here is to get a more robust or premium BB, another approach is just get an SKF or other top-tier square taper BB.

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