Thorn Cycles Forum
Technical => Transmission => Topic started by: rafiki on July 30, 2018, 08:42:31 pm
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I shall be fitting a new chain to my Sterling soon. When I do is there a specific position the eccentric BB should be moved to before fitting?
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All the way back so the distance between the axle and bracket center is as short as possible
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Just looking at it now Dave. Not sure how I know it is all the way back. I don't see a mark.
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I always start with the thinnest part of the BB pointing along the chain stay so the distance from the BB to the hub is as short as possible. The chain should be slack. Then when the chain is on I rotate the BB so the thick side of the BB moves toward the chain stay and the chain will tighten. Moving the BB effectively increases the distance from BB to hub and tightens the chain.
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It depends on the chainstay length and the number of teeth on the chainwheel and sprocket, though. On my particular set-up I can't exploit the full adjustment potential of the eccentric.
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I always start with the thinnest part of the BB pointing along the chain stay so the distance from the BB to the hub is as short as possible. The chain should be slack. Then when the chain is on I rotate the BB so the thick side of the BB moves toward the chain stay and the chain will tighten. Moving the BB effectively increases the distance from BB to hub and tightens the chain.
I think I am beginning to understand this. The thick part is the bit with the adjustment holes. So they should be positioned furthest from the chainstay when sizing the chain, meaning the bracket axle is as close as possible to the rear sprocket. Rotating the thick bit towards the chainstay moves the axle away from the rear sprocket thus tightening the chain.
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Yes I think you’ve got it! It’s not easy to explain, I’m afraid, good luck!
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Thanks for your input. it was also helpful to me to look at the photo of the replacement BB shown in the SJS store. It put things in greater perspective.
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Rotating the thick bit towards the chainstay moves the axle away from the rear sprocket thus tightening the chain.
Rotate the thick bit down and towards the chainstay, the thin part should never go towards the bottom as the fixing bolts could damage the bottom bracket.
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Rotating the thick bit towards the chainstay moves the axle away from the rear sprocket thus tightening the chain.
Rotate the thick bit down and towards the chainstay, the thin part should never go towards the bottom as the fixing bolts could damage the bottom bracket.
Thanks, I'll remember that.
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I always start with the thinnest part of the BB pointing along the chain stay so the distance from the BB to the hub is as short as possible. The chain should be slack. Then when the chain is on I rotate the BB so the thick side of the BB moves toward the chain stay and the chain will tighten. Moving the BB effectively increases the distance from BB to hub and tightens the chain.
I will do this job at the weekend, hopefully. When I measure the exact length of chain required to go round both sprockets how much more should then be allowed in addition to give the correct amount of slack?
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I always start with the thinnest part of the BB pointing along the chain stay so the distance from the BB to the hub is as short as possible. The chain should be slack. Then when the chain is on I rotate the BB so the thick side of the BB moves toward the chain stay and the chain will tighten. Moving the BB effectively increases the distance from BB to hub and tightens the chain.
I will do this job at the weekend, hopefully. When I measure the exact length of chain required to go round both sprockets how much more should then be allowed in addition to give the correct amount of slack?
I don't fit chains very often but the last time I did ( KX1 as it happens ), I carefully measured the old chain and carefully cut the new one accordingly.
Only to find it was too short!
Dan kindly offered his advice that I should have counted the links.
Perhaps Dan will chip in here and confirm my memory?
Good advice above re BB setting.
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I don't fit chains very often but the last time I did ( KX1 as it happens ), I carefully measured the old chain and carefully cut the new one accordingly.
Only to find it was too short!
Dan kindly offered his advice that I should have counted the links.
Perhaps Dan will chip in here and confirm my memory?
Yep, as you related, Matt. It is an easy mistake in the moment. Old chains "stretch" (they elongate through wear of the pins and ovalization of the bushings and rollers) so they become effectively longer through increased pitch (spacing) between links. If one uses the stretched old chain as a guide in sizing the new, the replacement will be too short so yes, best to count the links and match the number of links when making the replacement.
A good reminder for us all.
All the best,
Dan.
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And there is the chance that someone pulled a link or two out of a chain after it stretched because they ran out of adjustment in the eccentric. If you owned the bike for the full history of that chain, you would likely know if you took out any links.
But if you bought a used bike with a very stretched chain, counting links could result in a too short chain. It is unlikely but it could happen.
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That could be the case in point. I can't remember if I or my LBS removed any links. If that were the case if I pull the new chain around the sprockets and get that length, how many links extra should I allow on top of that?
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Here is a formula by Chris Juden to calculate the chain length:
chainstay length in inches (i.e. BB spindle to rear axle) x 4
plus
1/2 (total teeth on chainring + total teeth on sprocket).
Round the answer up to the next even number to give the minimum number of links. You may want to add two more for luck to start with as you can always them remove them once you've tried it out on the bike.
For measuring the chainstay I would rotate the eccentric BB to give the shortest length to the axle. Depending on your particular set-up though you may find, like me, that you can never use that position - unless perhaps with a half link.
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Thank you. I'll try that and compare to the current chain link count.