Nomad Mk3 eccentric slipping

[karola]

Hi everyone,
Quick question about the eccentric BB on a Thorn Nomad Mk3 (the newer design with the two clamping bolts under the BB shell, not the older grub screw type). Mine slowly rotates after about 50–70 km and the chain becomes loose again.
I tried using Dynamix assembly paste with friction particles but it didn’t solve the issue. What do you usually run there in practice: grease, assembly paste, or something else?
Best !
Karol

[PH]

It shouldn't require anything, I have two Thorns with this style of BB and had a previous bike with something similar, they've never needed anything other than the bolts pinched tight.  Not even super tight, Thorn's first version of this style of EBB had four bolts, I assume they dropped that because it wasn't necessary. I put a bit of copperslip on mine, to avoid galvanic corrosion, but it wouldn't stop it slipping, if anything it would increase the likelihood.
Sorry that's not very helpful. It's such a simple mechanical design, if the parts are within tolerance I can't think why it wouldn't hold.  These are used on tandems, so it isn't likely you're just to powerful for it! Have you had the bike long?  Has it always done this? Do the lugs on the split meet when tightened?  I had that on a seatpost clamp, which is basically the same design. It's maybe not impossible that the insert is slightly undersized, if you have some accurate callipers, give it a measure. If you don't find a simple solution, maybe give Thorn a ring, it's possible they've come across it before. 
Good luck.

[karola]

Thanks for the reply, I appreciate it.
The bike is quite new to me and it has been doing this more or less from the beginning. I tried Dynamix assembly paste, but I also ran it without any paste and it behaved the same.
Good point about the split in the shell I’ll check if the lugs meet when the bolts are tightened and I’ll try measuring the eccentric with callipers as well.
Thanks again for the suggestions.  Karol

[Andyb1]

When I got my secondhand Raven I checked the eccentric shell was not seized in the frame and was surprised how loose a fit it was.  I expected a fairly tight fit but it was a  loose rattle fit!   It is secured by the old style pointy headed screws so is different to yours, but I do wonder if there are undersized eccentric shells around? 

[Matt2matt2002]

When I got my secondhand Raven I checked the eccentric shell was not seized in the frame and was surprised how loose a fit it was.  I expected a fairly tight fit but it was a  loose rattle fit!   It is secured by the old style pointy headed screws so is different to yours, but I do wonder if there are undersized eccentric shells around?

Thats an interesting point.
Like your self, my second hand Raven has the pointy head screws. Never had an issue but I always questioned in my mind if this was the best method of securing the EBB.

[Andyb1]

I might shim mine a bit tighter….. I lockwired the pointy bolts but after 1000 miles the lockwire needed to be undone so I could tighten them a bit more.  If the eccentric shell is undersized all the downwards load is taken by the pointy bolts as the shell is not touching the bike frame at the 6 o’clock position.
Not loosened since so maybe the bolts are deep enough in the shell now?
There must be a reason why Thorn changed their EBB design.

[mickeg]

I might shim mine a bit tighter….. I lockwired the pointy bolts but after 1000 miles the lockwire needed to be undone so I could tighten them a bit more.  If the eccentric shell is undersized all the downwards load is taken by the pointy bolts as the shell is not touching the bike frame at the 6 o’clock position.
Not loosened since so maybe the bolts are deep enough in the shell now?
There must be a reason why Thorn changed their EBB design.

Years ago one of my pointy end bolts was loose.  I was afraid it might fall out.  I now wrap a rubber band around the two bolts, thus they can't turn on their own from vibration.  I think I have shown this photo on this forum at least a dozen times.

Someone on this forum years ago suggested I cut rubber bands out of inner tube rubber, that is more weather proof than the rubber bands sold for office and household use.  Thus, the ones in the photo are black.  Two bands for redundancy.

Simpler than wiring them.

[mickeg]

Hi everyone,
Quick question about the eccentric BB on a Thorn Nomad Mk3 (the newer design with the two clamping bolts under the BB shell, not the older grub screw type). Mine slowly rotates after about 50–70 km and the chain becomes loose again.
... ...

Several years ago I had a seatpost (painted black) that slipped.  I tried it in several bike frames, it was the post that was a bit undersize, not the frame being oversize.  Standard 27.2mm post.  I do not have a caliper that has great precision to measure a thickness greater than an inch, so I could not measure it to assess actual diameter. 

I spray painted it with more black spray paint.  Three or four heavy coats.  Then let the paint harden for at least a week outside where the sun could shine on it on sunny days.  I knew how much of the post I wanted exposed above the frame, so I masked that off so I only painted the part that went inside the frame.

After that it stopped slipping.  Three heavy coats of paint, well dried, was sufficient.

I have no idea if that would be a solution for your eccentric or not.  If it is just a tiny bit undersize, it might work?

[UKTony]

Hi everyone,
Quick question about the eccentric BB on a Thorn Nomad Mk3 (the newer design with the two clamping bolts under the BB shell, not the older grub screw type). Mine slowly rotates after about 50–70 km and the chain becomes loose again.
... ...

Several years ago I had a seatpost (painted black) that slipped.  I tried it in several bike frames, it was the post that was a bit undersize, not the frame being oversize.  Standard 27.2 mm post.  I do not have a caliper that has great precision to measure a thickness greater than an inch, so I could not measure it to assess actual diameter. 

I spray painted it with more black spray paint.  Three or four heavy coats.  Then let the paint harden for at least a week outside where the sun could shine on it on sunny days.  I knew how much of the post I wanted exposed above the frame, so I masked that off so I only painted the part that went inside the frame.

After that it stopped slipping.  Three heavy coats of paint, well dried, was sufficient.

I have no idea if that would be a solution for your eccentric or not.  If it is just a tiny bit undersize, it might work?

 I was reading recently about press fit BBs. There was mention that a product called Retaining Compound “can be especially useful to account for poor or marginal fitment on press fit cups and bearings”. Maybe this would be worth considering? (I’m sure there’ll  other sources apart from Park Tools).

https://www.parktool.com/en-int/blog/repair-help/the-park-tool-guide-to-bicycle-lubricants-and-compounds?srsltid=AfmBOorVxlDdxMMqpg4VmQcmQ9xbh4taxRaiAQ9h4QiN58S-M9LUIF9I#:~:text=Retaining%20compounds%2C%20such%20as%20Park,press%20fit%20cups%20and%20bearings.

[JohnR]

Make sure the two faces of the opening in the frame are free from dirt or anything else which would prevent the gap from closing enough to clamp the EBB when the bolts are tightened. Would a threadlocker be suitable as a gap filler? I recall one of the Mapdec Youtube videos discusses fixes for loose bottom brackets. It might be this https://www.youtube.com/watch?v=b9uV_8zLSVY.