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On the inside of the bottom bracket shell, the thread does not go all the way around. At the places where the tubes are welded (or attached) into the bottom bracket shell, there are holes in the thread.

This frame is from a custom made touring bicycle that has a Rohloff.

The gaps are on both the drive and non-drive side.

The frame manufacturer, says that it is also possible to mount external bottom brackets.

Below some pictures.

My questions / concerns are

  1. Are these gaps in the thread normal?
  2. Is this a badly designed frame?
  3. Is this a defect?
  4. The bicycle came with a Shimano UN300 internal bottom bracket. Would it be possible (given the gaps in the thread) to mount external bottom brackets?

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Picture of the internal bottom bracket that came with the bicycle enter image description here

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  • You can clearly see in the pictures the threads do go all the way around Mar 10 at 0:50

3 Answers 3

23

Are you referring to the absence of material at the chainstay openings in the bottom-bracket shell? This is absolutely normal. This is what a BB shell looks like before being built into a bike:

bottom bracket shell

Lugged frames might not be the norm anymore, but they were for about a century. If this were a problem, it would have been solved by now.

8

There's no defect or issue here. Most shells with the chainstay ports in the spot you'd want them for wider tire applications have this going on. Framebuilders choose shells to meet their design needs (namely what angles they need everything to fall at, give or take some ability to modify those angles by reworking the shell) from a relatively small number of available options. What is true is that a lot of classic road and bike boom type bikes have the chainstay ports further in, so there's more uninterrupted thread, and this is going to form the basis of what a lot of people view as normal since there are many more of those bikes around than anything handmade and contemporary. If you look around at all the various frame part suppliers you'll see plenty of shells just like yours. It's normal. It also doesn't have any restrictions on whether you run an internal vs external BB.

5

An internal vs external bottom bracket won't make a lot of difference - both styles still thread into the same threads in the bottom bracket shell.

That the threads are "interrupted" won't matter as long as they line up again once the threads resume.

It is an interesting design choice by the frame manufacturer, possibly to make manufacturing easier. A better design would be to weld the frame, then insert a sleeve inside the BB, and tap threads into that sleeve.

However this frame should not have a shorter life just because of these threads being interrupted.

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  • 2
    There could /potentially/ be a problem here if there was so much material removed on one side that the cylinder could "open up" slightly when stressed e.g. by a bump in the road. However since it is reinforced by the lugs (coming out at right-angles) and the strength of the tubes welded (etc.) into them, this is most unlikely to be the case: that's a solid assembly, with the geometry of the threaded portions dictated by the overall shape. Mar 5 at 14:27
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    Threading a sleeve seems worse. Now you’ve introduced a yet another interface between two shells that are loaded pretty heavily.
    – Paul H
    Mar 5 at 18:07
  • @MarkMorganLloyd its also reinforced by the BB threadded into the shell.
    – Criggie
    Mar 5 at 18:26
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    @PaulH although it doesn't seem to cause problems for the interrupted threads used on artillery piece breeches (I just wish I knew how they were machined...). Mar 5 at 18:32
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    @MarkMorganLloyd artillery machines are considerably less weight sensitive than bikes and are far more tolerant of loud operation Mar 10 at 0:52

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