Technical > Transmission

Thorn Mercury Mk3 bottom bracket click or knock - any solutions?

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Note that this problem has been solved, as reported in reply 22. Over a further few hundred kilometres since then it has not rearisen. Reply 10 details the response from Thorn.

Hi to the owner custodians of Thorn mechanical knowledge.

Unfortunately my near New Mercury has developed the dreaded bottom bracket knock, otherwise known as click, which from a search of this forum seems to have no known cure.

Before I chase up Thorn mechanical support from the shop, is anyone aware of a cause and repair that has eluded my research or hasn't been posted?

My reading tells me that most replies on this topic suggest that the noise originates elsewhere. Believe me, this noise originates at the bottom bracket.

I suspected the pedals, one of which had a loose spindle. So I've replaced them with new Shimano XT.

With that cause ruled out, investigation led to successful static reproduction.

With the bike upside down, and the cranks vertical. I can generate the click by pressing down hard by hand on the lower pedal. That is, by generating torque that would rotate the crank spindle end to end.

Having done so, I can rotate the pedals 180 degrees and generate the same click by pressing down on the opposite pedal. It feels as though this returns a moving part to its former place.

Clearly there is movement available in the BB area. However, the movement is fine enough that it is not easy to observe.

Certainly: 1. It is not the eccentric moving in the frame. The securing bolts are well nipped up and if that were moving I believe I would see that movement.

2. It is not a loose spindle on worn or maladjusted bearings, as far as I can determine. I've not removed the cranks but the axle spins nicely with no discernible play. Plus BB bearing is all but new and hasn't been badly treated.

Hence, it appears movement is taking place between the bearing housing and the eccentric that holds it.

I reckon I can just about see this, but it's subtle enough that I wouldn't swear to it.

Does anyone understand the mechanicals well enough to see how this might be possible? And if so, what possinilities are there for a fix.

(Optional extra question: will damage occur or worsen if I continue to ride the bike? My intuition is that it will.)

Thanks in advance for helpful thoughts.

Okay a bit more research tells me my bottom bracket is one of these, which threads into a non-eccentric BB shell as shown in this slow but fairly clear video:

And since a view from the bottom of the BB shows this:

I am going to hazard a guess that the plate between the BB eccentric and the chainwheel should be flush against the eccentric, and therefore that the BB housing on the drive side has been unscrewing itself from the eccentric.

In which case the appropriate response will be to purchase a crank puller and BB socket, pull the drive side crank, give the exposed threads a clean, and then tighten the BB housing.

Any chance I am on the right track here?

I'll talk to Thorn as it looks like a warranty issue. Not sure what they can do from the other side of the world.

I think the technical term for this noise is bearing creak- but whatever you call it, it is annoying nonetheless  ;D

I don't have a Thorn, but the bottom bracket bearing creak can happen on any bike once those bearing stop working quite as they should do. I've replaced bottom bracket bearings on bikes with free sitting bearings sitting in a cup, with ball races and with sealed cartridge. The noise is the same when the bearing gremlins get to work. It is just a question of time before it happens. But it is NOT the end of the world and it is unlikely to be an issue with your eccentric. And you ruled out pedal bearing creak too, suggesting it can only be the BB.

If you can cope with the noise, then you may continue riding. If it is the bearings, the worst you can do is make the bearings and the associated noise worse. It will only damage that component more, which you are likely to need to replace anyhow.

Saying that- can you fix it?

I'm going to assume you have a Shimano UN-BB 55 bottom bracket or similar. Technically speaking, these have no user servicable parts being a 'sealed unit'.
When I purchased my 2nd hand Rohloff powered bike, there was some BB bearing creak. Not much but enough to irritate. As I did not know the size nor model of BB to order, I removed it. If memory serves, I have a Shimano UN-BB 61 which I could find nowhere as identical replacement, but I ordered 3 UN-BB 55s of same size to replace with. And I carefully cleaned the old BB to allow me to use the old BB till the new ones arrived.

Using a toothbrush, I carefully removed all the debris/dirt collected on the chainwheel side. Holding the BB at an angle where no liquids could get in, I then dipped the toothbrush in terps to thin and cleaned the area as best as I could so that no residual dirt could be seen. I then tried packing as much lithium grease (marine grease or alternative grease would probably work too) around the chainwheel side and  non chainwheel side where the axel meets the housing of the BB. I used the Shimano removal tool to try and push against the grease evenly in the hope that some would get into the sealed unit gaps or, at the very least, stop further ingress of additional debris.

After refitting the old BB, I carefully filled out additional grease where the tool at removed it when refitting.

And that BB creaking stopped for about another 3000km after 20-30km of riding must have drawn enough new grease into the sealed unit.

Very slowly, it did gradually develop a bit of play, not discernable at the time the noise first became apparent but eventually easy to spot. But when that noise did return, I just switched it for one of the UN-BB 55 brackets which had arrived soon after my experiment but which had been sitting for around 4 months while I continued to ride on the 'fixed' bracket.

There are youtube videos on alternative ways to open up a Shimano BB, but my method was pretty non invasive and worked for me. You may even be able to clean and repack crease without removal of the BB from bike just from the chain wheel side.

You've got nothing to lose by giving a clean and grease if you are going to have to replace it eventually anyway. I know the BB-UN 55 is hard to source in 'Thorn' size. Be wary of the entry level alternative replacements. I did that once on another bike, and it was worn out within 3-4000 km. There is the fabled Phil Wood BB, but a decent quality alternative that is not entry level might be enough to keep you creak free for a few more years.

Good luck!

Do not try to just tighten the bottom bracket from the drive side.

Before I remove a crank arm, I like to mark the crank arm and bottom bracket spindle so that I put the arm back on the spindle the same way as it was before.  This probably is not necessary these days with computer controlled manufacturing, but decades ago I had a spindle that the square taper was really rectangular, not square, and I had a problem re-fitting a crank arm that had deformed to fit the crank only one way.  If you do not want to mark them, skip this step.

Remove both crank arms. 

Loosen the non-drive side bottom bracket bushing.  One side is left hand thread, I do not recall which but it might be this side.  Probably does not need to be fully removed, but you might want to.  The point here is that if the non-drive side bushing is in too far, the drive side can't be fully tightened.

Then you can tighten the drive side, but if I was doing this I would first remove the drive side completely and try to figure out if it came unthreaded on its own, or was there some reason that it was not threaded all the way in the first place.  In other words, inspect the threads.  I usually grease my threads before I assemble.   I usually do not use a torque wrench on this, but if it really did loosed by itself, you might want to use a torque wrench.

Then install and tighten the non-drive side.

Then put on the crank arms.  When I do this at home where I have a torque wrench, I ususally use that.

And if the pedals were off, re-attach.

I am assuming you can re-use your bottom bracket.  If you need a different one, I am not sure if the Mercury is 68 or 73mm width.  My Thorn Nomad Mk II uses a 73mm width bottom bracket.  Before you order one, measure your old one to make sure you order the correct one.

I do not know if you have a crank puller or not.  And I do not know where you are located.  If you are in USA, I like the XLC crank puller for touring because it is small and most important it uses a 15mm open end wrench, thus a pedal wrench works with the XLC.


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