Author Topic: Chain length  (Read 299 times)

ianatstanage

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Chain length
« on: June 08, 2019, 07:47:14 PM »
Hi,

I'm in the process of fitting a new sprocket and chain to my Thorn Mercury.

I am going from a 44x19 set-up to 44x21.

The only way I can get the chain (KMC) fitted is having it very tight (98 links), which does not feel right.

I was hoping to use a chain link, but even with the eccentric bottom bracket in the shortest position, the chain is too long. Moving the eccentric bottom bracket forward does not take -up enough slack.

I am hoping that the new chain will soon stretch enough for it to feel 'just right'. I will then be able to use the eccentric to take out further slack as it wears.

To extend the chain means adding two links, taking it to 100 links, which makes it too long for the slack to be taken up by the eccentric.

Am I missing something?

I have a long ride next weekend and don't want to have a mechanical due to too tight a chain.

Many thanks,

Ian
 

PH

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Re: Chain length
« Reply #1 on: June 08, 2019, 10:29:24 PM »
I wouldn't run a tight chain, it's probably the quickest way to knacker the bearings, how quick will depend on other factors, but I can say with certainty it isn't good for them!
The smaller EBB of the Mercury does mean that only a limited number of combinations work, it's in one of the pdf brochures somewhere, I'm running 45/19 which is one of those that offers maximum adjustment. If you're wedded to the idea of a non ideal combination, or just want to use up the parts you have, there are half links available, which do just what they say, you could then remove it when the chain wears. I've used one in the past (Not on a Thorn) and had no issues, just make sure you get the width that matches the rest of the chain
https://www.sjscycles.co.uk/search/?term=half%20link

ianatstanage

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Re: Chain length
« Reply #2 on: June 08, 2019, 11:59:03 PM »
So I am missing something!

Half link ordered by express delivery.

Many thanks for the advice.

Ian

IanW

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Re: Chain length
« Reply #3 on: June 09, 2019, 04:53:00 PM »
I am essentially having the same problem.
I am dropping the gearing a very little bit.
I was previously running 44 x 16.
But I am now trying to run 42 x 16.
For me the shorter chain run is impossibly tight (i.e. I cannot even join the short chain run).
But even at full chain-slack-take-up the eccentric bb does not really take up enough slack.
So it looks like I am going down the 1/2 link route too.

PH

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Re: Chain length
« Reply #4 on: June 10, 2019, 05:43:22 AM »
I am essentially having the same problem.
I am dropping the gearing a very little bit.
I was previously running 44 x 16.
But I am now trying to run 42 x 16.
For me the shorter chain run is impossibly tight (i.e. I cannot even join the short chain run).
But even at full chain-slack-take-up the eccentric bb does not really take up enough slack.
So it looks like I am going down the 1/2 link route too.
Or you could drop it a little lower, 43/17 works

ianatstanage

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Re: Chain length
« Reply #5 on: June 11, 2019, 04:58:07 PM »
My chain length is now sorted - many thanks for the advice about about the half-link.

I now have 44x21, which is ridiculously low, but it will be easier on the road with full touring kit and my 91kg weight, as well as off-road with less kit.

Of course, I have now read a bit more and notice that Thorn don't mention a 44x21 combination for the Mercury!

It has been a bit of a faff getting this sorted, but it was the first time I needed to change the chain in nearly 5 years of reasonable use, so I shouldn't complain too much.

Why are jockey wheels and Rohloff hubs not popular? Surely it would make life a lot easier not having to mess around with the eccentric bottom bracket?

Ian




geocycle

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Re: Chain length
« Reply #6 on: June 11, 2019, 05:33:28 PM »

Why are jockey wheels and Rohloff hubs not popular? Surely it would make life a lot easier not having to mess around with the eccentric bottom bracket?

Ian

I think because jockey wheels are another thing to get damaged or gunged up whereas eccentrics are pretty robust and need virtually no maintenance.  The mercury is slightly different from some of the rohloff bikes as the range of the eccentric is not as great so chains will need to be replaced more often.
 

martinf

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Re: Chain length
« Reply #7 on: June 11, 2019, 07:52:16 PM »
Why are jockey wheels and Rohloff hubs not popular? Surely it would make life a lot easier not having to mess around with the eccentric bottom bracket?

Jockey wheels and hub gears do work, I have a Surly Singleator tensioner on the big visitor bike with Nexus Premium 8-speed hub and vertical dropouts. There is no other easy way to get proper chain tension on that bike.

Downsides are one more thing to clean regularly (the single jockey wheel on the tensioner, which is no big deal), very slight added friction (not noticeable for me), and the fact that a tensioner makes it impossible to fit a Chainglider (a real nuisance for me).

Absence of a Chainglider means that I have to clean the transmission (chain/chainwheel and rear sprocket) on the big visitor bike at least 3 times more often than that on the small visitor bike, which has older-type long semi-horizontal dropouts and a Chainglider. On the latter bike the rare adjustments needed for chain tension are achieved by sliding the wheel forwards or backwards in the dropout, not really very much hassle and IMO neither better nor worse than the eccentric bottom bracket system used on my Thorns.

PH

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Re: Chain length
« Reply #8 on: June 11, 2019, 09:54:38 PM »
I have both😁
An EBB on a Mercury and a tensioner on an Airnimal folder.  The EBB is better for the reasons others have given, but there isnít as much in it as some would have you believe.  I noticed Mark Beaumontís round the world record bike (1st time) used a Rohlof tensioner.
« Last Edit: June 11, 2019, 11:04:18 PM by PH »

IanW

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Re: Chain length
« Reply #9 on: June 14, 2019, 12:42:37 AM »
I have also sorted my 42x16 using a half-link.

I think there are basically 3 chain-length adjustment mechanisms:
1) Sliding rear-wheel drop-out
2) Eccentric bottom bracket
3) Chain tensioner
4) On a motorcycle, I have even had an eccentric rear hub
They each have their pros and cons

A chain tensioner is an implicit part of a deraillure gear system
But on hub-geared bicycles we have a free-er choice.
In my opinion the sliding rear-wheel drop-out is probably the *simplest*,
the chain tensioner is the laziest, and the eccentric bottom bracket is the most aesthetically pleasing.
But I really would like the Thorn eccentric bottom bracket to have very slightly greater adjustment range.
Say 3 x 1/2" pitch chain link lengths, i.e. minimum of 1.5 inches total chain run.