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

[Moronic]

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.

[Moronic]

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:

https://youtu.be/ewb8Miaksmw

[Moronic]

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.

[steve216c]

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

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!

[mickeg]

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.
https://www.ebay.com/itm/223058360956

[Moronic]

Thanks for the responses, gents. The bottom bracket is a VC Components BC in 73mm, as in the video I posted a link to. One of these:

https://www.vitalmtb.com/product/guide/Bottom-Brackets,25/VP-Components/VP-BC73,14278

I wandered into a local bike shop today for a crank puller and BB tool, and was kindly given extensive and relevant advice. It appears the chain side BB carrier is indeed meant to be tightened against the BB - or in this case the eccentric - in which case mine has or is unwound significantly.

Hard to know how that would have happened. I seem to recall being surprised when the bike arrived that the carrier wasn't flush, but as I was ignorant of modern BB design thought it must have been set that way. OTOH it is hard to attribute such an oversight to the Thorn assemblers, given how neatly everthing else was prepared.

Having ascertained that likelihood, your replies make lots of sense to me and are helpful.

So it sounds like I pull the cranks, unwind the chain side carrier a few turns, give the threads a clean and regrease, unwind the off side carrier a few turns, wind in the chain side carrier until it is good and tight, then wind in the off side carrier until it snugs up against the internal housing. Replace the cranks and I should be good to go with no knock.

I'm surprised how little a replacement sealed bearing unit would cost - from memory under 20 pounds on the Thorn invoice and about $35 over here. The local shop pulled out a Shimano unit to demonstrate the componemts and even it was only $55. The Shimano unit came with Loctite or similar preapplied, so unwinding by implication is a common problem.

[JohnR]

Thorn used to use a Shimano UN55 bottom bracket but Shimano stopped making them so Thorn had to find other compatible parts and have got a range of compatible products (73mm length 107mm) listed at https://www.sjscycles.co.uk/square-tapered-other/ . Eventually they'll find out which is the best of the alternatives. Did you report your problem as it's clearly a black mark against the VP components product? When you do the maintenance check for threadlocker.

[Andre Jute]

Moronic says:

I'm surprised how little a replacement sealed bearing unit would cost

I think it is very likely that most bottom brackets owe all but a fraction of their retail price to branding and promotion rather than to development and materials and manufacturing cost.

Check out, for instance, the Stronglight JP400 sold by SJS for STG15:

https://www.sjscycles.co.uk/bottom-brackets/107-stronglight-jp400-plastic-cups-sealed-bearing-square-taper-bottom-bracket/?geoc=IE

They don't have it in your size but this mechanically superb bottom bracket is made by Kinex, bearing specialists in the Czech Republic, and branded for anyone who buys a big unit load, and sold around the world at surprising prices, low and high. It comes as a standard fitting on Eurobikes from the very top drawer, manufacturers who test every component compulsively. It's on permanent sale elsewhere in Britain for a tenner, and under other brand names than Stronglight has been under ten Euro on the Continent many times. It is made with a choice of plastic cups, as in the one SJS offers above, aluminium cups, or steel cups. The plastic cups aren't just for weight weenies: one manufacturer told me they fitted the plastic cups after their outside test engineer advised them that the small amount of movement the plastic cups permit is desirable for the longevity of the threads in the BB and for the exclusion of moisture because the fit is permanently tighter (presumably for the lifetime of the BB -- I didn't think to ask because these people don't sell anything you can't pass on to your grandchildren unless it is deliberately intended to be replaced at some specified interval). I was surprised to hear the high-buck, high-reputation names of some of the bottom brackets this single-euro (in wholesale bulk) BB beat out in a tough lab. I've had it on several bikes and it just soldiers on; I eventually went to the Shimano UN55 to get an axle length which suited a desired tread width, because Shimano stopped making the UN26, the previous Shimano value champ. A Kinex BB I took off and gave to someone else and fitted for him to ensure the job was done right is still working well after years of abuse like being parked outside in the rain for months.

It's probably fair to say Kinex occupies the place Europe that Tange occupied for decades in the Far East, as the makers of everyone else's branded bottom brackets. The Tange BBs under their own brand that SJS stock in a fair size range are near the bottom of the page JohnR referenced.

[mickeg]

...
So it sounds like I pull the cranks, unwind the chain side carrier a few turns, give the threads a clean and regrease, unwind the off side carrier a few turns, wind in the chain side carrier until it is good and tight, then wind in the off side carrier until it snugs up against the internal housing. Replace the cranks and I should be good to go with no knock.
...

Yup.  But, keep in mind that one side is left hand thread.  Unfortunately I do not recall which side, I always manage to figure it out so quick that I don't remember which.

I assume you have the tool that fits in the bottom bracket.  Those tools are cheap.  Mine fits on a half inch drive ratchet.

I think it was about five years ago, I had a clicking noise in the bottom bracket area.  Changed pedals, tried lots of other things, nothing fixed it.  So, decided to change bottom brackets, and that was when I discovered that my non-drive side bushing was tight, but not tight enough.  When I went to unthread it, it came loose much easier than it should have.  So, instead of replacing bottom brackets, I tried tightening up and reassembled.  And that fixed the clicking noise.

[Moronic]

Thanks again guys. Interesting background, Andre. John yes I emailed Thorn Monday. Haven't heard back so I'll likely phone them today. Don't really want to work on the bike till I've had a conversation with them.

[Moronic]

Had a an amiable chat with Robin Thorn. He apologised for what was likely not enough tightening by the assemblers. The fix is as above, although he suggested removing the drive side cup, cleaning it and the mating threads and applying some thread locker before reassembly. He observed that even if threads had been damaged through being ridden loose, on a Mercury the threads were in the eccentric rather than in the frame, and so could be wholly refreshed with just a new eccentric. Obvious but I hadn't thought of that.

Not sure what I think of the Loctite idea. That would mean tightening a big fine steel thread dry into aluminium, on a part of the bike that's exposed to splashes. Otoh lubing the threads invites a repeat loosening. I suppose if Shimano supplies its BB with threadlocker preapplied, thats the direction to take.

[JohnR]

Not sure what I think of the Loctite idea. That would mean tightening a big fine steel thread dry into aluminium, on a part of the bike that's exposed to splashes. Otoh lubing the threads invites a repeat loosening. I suppose if Shimano supplies its BB with threadlocker preapplied, thats the direction to take.

I use threadlocker as a matter of course on everything except the pedal spindles (which get grease) to reduce the risk of vibration causing things to work loose. It's possible that whoever assembled the bike was so used to using the Shimano UN55 with some pre-applied threadlocker that they omitted to apply some when using a different bottom bracket. There's a description of the different grades at https://www.henkel-adhesives.com/uk/en/insights/all-insights/blog/difference-between-threadlockers.html. Keep away from the red stuff and you'll have no problem with disassembly in the future.

[mickeg]

I am not sure which country you are in, if you are in USA, this is what I bought last time I bought some thread locker.
https://www.truevalue.com/6-ml-removable-blue-thread-locker-1

There are lots of brands, I wanted a small bottle that was cheap and that was what I saw at my local hardware store.

Note that it says removable.  I think blue is a common color code for removable, it says blue on the package even though the bottle is red.

Shake well before using.  The tip on the bottle can clog, I keep a paper clip attached to the bottle with a rubber band to use to unclog the tip.

After several occasions on tours where I met people that lost rack bolts or some other important fastener, I have started to carry a small bottle of that thread locker on tours.  But lately most of my tours were on my S&S coupled bike, so I was installing and removing racks at the start and end of the tour.  Thus, I primarily carried it to use on my own bike.

I use that thread locker on all rack bolts, bolts on cantilever brake posts, kickstand bolts, recently added shoe cleat bolts to that list too.  But use grease on water bottle cage bolts, seatpost bolts, stem bolts and just about everything else.  I usually have used grease if it was a steel and aluminum interface, such as an aluminum bottom bracket thread on a steel frame. 

Dave W, formerly of SJS had commented that dissimilar metal corrosion can be an issue with Rohloff drain screws, so I started adding thread locker on those.  Apparently I use the wrong thread locker as the one I use he says is soluble in oil, but I continue to use that thread locker on my Rohloff drain screws, which i continuously re-use.

Some bike shops do not even have a bottle of thread locker in the shop, they just tighten things way too tight, some mechanics use a lot of grease and some use too little.

Grease is a very viscous fluid, so it can also function to reduce the chance of a loose bolt from vibrating loose.  So, even though grease is a lubricant, it can help prevent loss of some fasteners.

Some times specifications say to use grease or threadlocker, but sometimes they say not too.  But I think most people working on a bike never bother to read any such specifications.

I am not saying that what I do is best, just describing what I have settled on doing after decades of wrenching on bikes, mostly my own but I also worked in a bike shop before I went to college.

If you have an interest in learning more than you ever wanted to know about greasing threads, an interesting discussion in part of this post on that topic.
https://www.velonews.com/gear/tech-wearables/technical-faq-velon-transponders-greasing-fastener-threads-and-warm-shoes/

[PH]

Glad you got a response from Thorn,shame if it was an assembly error, but on one's perfect.
Shimano have pretty much relegated ST to the cheap end, they don't make any higher end ST chainsets so it's unsurprising they downgraded the BB as well, add to that the less common width and there isn't a lot of choice. I've been swapping to high end IRD when mine need replacing, 10 year guarantee and the bearings are replaceable, I'm hoping they'll be the last BB's I need.
Park Tools recommend  threadlock, or if you haven't got that, anti-seize paste, or if you haven't got that grease - Mainly they're telling you not put it in dry! it's a bit of an odd one, because they also tell you to tighten to the recommended torque, but that will vary depending on the thread treatment.
I use Copperslip and have never had a BB loosen or seize, but I also remove and re-apply on an annual strip down service.  I save threadlock for those things I don't intend to disturb, racks, bottle cages, light fittings.  I see no reason not to use threadlock on the BB, just for me it's easier to clean and re-apply Copperslip.
A BB in a EBB shell is better protected than one in a frame, it only has to deal with what's coming at it from the crank side not internally, I once removed one from another touring bike and a cup full of water poured out!

[lewis noble]

Well done for getting it sorted, and Robin for giving clear advice.

Threadlock vs copperslick . . . . . I used to put threadlock on pretty well everything, cycling and non-cycling bolts - but found that some became too hard to undo if needed - too many rounded nuts and bolts.  Probably I used the wrong grade of threadlock.

So now, on all of my bikes,family bikes etc, I use copper slick / copper grease on pretty well everything, rack, guards, stem bolts etc., and nothing has ever worked loose, I suspect because the threads 'seat' better and over the whole of their surface.  No problem in undoing them however wet they get / exposed they are.

Not expensive, small tube in toolbox lasts for ages, usually starts leaking or gets squashed before it runs out.  And if repairs need doing on tour, no problem and no need to take the tube, as enough grease stays on the threads for 2 - 3 dismantlings at least. 

Lewis