Thorn Cycles Forum

Technical => Transmission => Topic started by: rafiki on July 30, 2018, 08:42:31 PM

Title: Eccentric BB setting when fitting a new chain?
Post by: rafiki on July 30, 2018, 08:42:31 PM
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?
Title: Re: Eccentric BB setting when fitting a new chain?
Post by: Dave Whittle Thorn Workshop on July 31, 2018, 08:58:25 AM
All the way back so the distance between the axle and bracket center is as short as possible
Title: Re: Eccentric BB setting when fitting a new chain?
Post by: rafiki on August 09, 2018, 12:42:36 PM
Just looking at it now Dave. Not sure how I know it is all the way back. I don't see a mark.
Title: Re: Eccentric BB setting when fitting a new chain?
Post by: geocycle on August 09, 2018, 04:41:43 PM
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.
Title: Re: Eccentric BB setting when fitting a new chain?
Post by: Donerol on August 10, 2018, 11:29:13 AM
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.
Title: Re: Eccentric BB setting when fitting a new chain?
Post by: rafiki on August 10, 2018, 04:17:32 PM
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.
Title: Re: Eccentric BB setting when fitting a new chain?
Post by: geocycle on August 10, 2018, 05:13:13 PM
Yes I think youíve got it! Itís not easy to explain, Iím afraid, good luck!
Title: Re: Eccentric BB setting when fitting a new chain?
Post by: rafiki on August 10, 2018, 05:18:22 PM
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.
Title: Re: Eccentric BB setting when fitting a new chain?
Post by: macspud on August 11, 2018, 02:39:24 AM

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.
Title: Re: Eccentric BB setting when fitting a new chain?
Post by: rafiki on August 11, 2018, 05:38:09 AM

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.
Title: Re: Eccentric BB setting when fitting a new chain?
Post by: rafiki on August 17, 2018, 03:45:39 PM
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?
Title: Re: Eccentric BB setting when fitting a new chain?
Post by: Matt2matt2002 on August 17, 2018, 04:30:29 PM
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.
Title: Re: Eccentric BB setting when fitting a new chain?
Post by: Danneaux on August 17, 2018, 05:10:22 PM
Quote
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.
Title: Re: Eccentric BB setting when fitting a new chain?
Post by: mickeg on August 17, 2018, 06:15:52 PM
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.
Title: Re: Eccentric BB setting when fitting a new chain?
Post by: rafiki on August 17, 2018, 07:36:05 PM
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?
Title: Re: Eccentric BB setting when fitting a new chain?
Post by: Donerol on August 17, 2018, 09:24:20 PM
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.
Title: Re: Eccentric BB setting when fitting a new chain?
Post by: rafiki on August 17, 2018, 09:41:24 PM
Thank you. I'll try that and compare to the current chain link count.