Technical > Transmission

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

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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:

--- Quote ---I'm surprised how little a replacement sealed bearing unit would cost
--- End quote ---

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:

--- Quote from: Moronic on November 09, 2021, 09:07:06 AM ---...
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.
...

--- End quote ---

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.

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