Can anyone suggest an explanation? Having replaced both chainrings and the timing chain on our Thorn tandem , the eccentric bottom bracket won’t allow correct adjustment. It appears that to get the full adjustment I need to add a half link to the chain, which can’t be correct?
With the eccentric adjusted to its shortest the chain would be much to tight, and fully the other way it’s loose. All help would be gratefully received
Tandem timing chain
-
alexnharvey
- Posts: 1946
- Joined: 10 Jan 2014, 8:39am
Re: Tandem timing chain
If it's loose at one end of the range and tight at the other can't you set it at an intermediate point which is approximately right? As the chain and rings wear the chain effectively becomes longer and looser and the distance between the axles is increased to compensate using the EBB. You do not need to access the full range of motion of the EBB.
Last edited by alexnharvey on 4 Nov 2022, 7:50am, edited 1 time in total.
Re: Tandem timing chain
Sounds to me like you simply need to have the eccentric adjustment 'in the middle' . Assuming you have same size rings as before, don't forget there would have been wear in the old system so the new stuff would be at correct tension with the eccentric further back than it would have been on the old kit.
Like alexnharvey has said whilst I was posting this.
Like alexnharvey has said whilst I was posting this.
Re: Tandem timing chain
Agree with both posts above.
But If that intermediate position can't be found...
• Is the tension changing when the chain is fitted and you turn the cranks?
• Is it the right type of chain and is the length correct (and I've never needed a half-link)?
• Are the chain rings true, not bent and installed correctly?
Jonathan
But If that intermediate position can't be found...
• Is the tension changing when the chain is fitted and you turn the cranks?
• Is it the right type of chain and is the length correct (and I've never needed a half-link)?
• Are the chain rings true, not bent and installed correctly?
Jonathan
Re: Tandem timing chain
I've had this same issue in the past. The eccentric allows an adjustment of 1/2" IE + or - 1/4". So manufacturing tolerances can sometimes cause a slightly tight chain which is impossible to fit without recourse to a half link.
Once everything has bedded in you may be able to remove the half link.
Once everything has bedded in you may be able to remove the half link.
Last edited by rjb on 4 Nov 2022, 9:08am, edited 1 time in total.
Peugeot 531 pro, Dawes Discovery Tandem, Dawes Kingpin X2, Raleigh 20 stowaway X2, 1965 Moulton deluxe, Falcon K2 MTB dropped bar tourer, Rudge Bi frame folder, Longstaff trike conversion on a Giant XTC 840, Giant Bowery, Apollo transition. 
Re: Tandem timing chain
Agree with the above, just set it in the middle.
Not sure if this will help; I've got a bike whereby the throw on the EBB is slightly less than a full chain link. If I leave the 'extra link' then at it's longest it's very slightly too loose, and I can't tighten it further (I rode it like this, but then lost the chain on a jump and although no harm was done, it made be very nervous). Then, If i take out the link, I can't get it to reach at the shortest throw at all.
I found out when I tried to change the gearing. I bit of time on a spreadsheet, showed I was just really unlucky and just about any other cog/chainring combination should work. So I picked another one...
I don't know much about tandems, but did you change the chainring/cogs when refreshed the transmission?
Not sure if this will help; I've got a bike whereby the throw on the EBB is slightly less than a full chain link. If I leave the 'extra link' then at it's longest it's very slightly too loose, and I can't tighten it further (I rode it like this, but then lost the chain on a jump and although no harm was done, it made be very nervous). Then, If i take out the link, I can't get it to reach at the shortest throw at all.
I found out when I tried to change the gearing. I bit of time on a spreadsheet, showed I was just really unlucky and just about any other cog/chainring combination should work. So I picked another one...
I don't know much about tandems, but did you change the chainring/cogs when refreshed the transmission?
-
roubaixtuesday
- Posts: 6749
- Joined: 18 Aug 2015, 7:05pm
Re: Tandem timing chain
Recommendations above on adjusting the eccentric half way make sense to me, but I think perhaps the OP description of the issue might be confusing?
Sheldon Brown has a method for ensuring the timing chain gives consistent tension all the way around the pedal turn which may be relevant.
https://www.sheldonbrown.com/synchain.html
Sheldon Brown has a method for ensuring the timing chain gives consistent tension all the way around the pedal turn which may be relevant.
https://www.sheldonbrown.com/synchain.html
Re: Tandem timing chain
Excellent, and addresses the thoughts that I posted above if a sweet spot can't be found easily.roubaixtuesday wrote: ↑4 Nov 2022, 10:41am ...
Sheldon Brown has a method for ensuring the timing chain gives consistent tension all the way around the pedal turn which may be relevant.
https://www.sheldonbrown.com/synchain.html
Jonathan
Re: Tandem timing chain
Thanks all for your help however perhaps I need to further explain our problem .with the new chain the length of the old one , and the ebb at its longest .the new chain is too loose. If I remove a link then even on the shortest ebb position the chain is too short.
I have been advised that I should force the chain on and live with the tightness until it loosens up. I’ve tried that but it’s so tight that the pedals don’t turn freely.
Oh the fun we have on bikes
I have been advised that I should force the chain on and live with the tightness until it loosens up. I’ve tried that but it’s so tight that the pedals don’t turn freely.
Oh the fun we have on bikes
Re: Tandem timing chain
I suggest you provide more information about your tandem, including photographs if possible. It seems that there is something wrong from your description, but without more information all people can do is guess, such as guessing that you have a Rohloff hub.
Re: Tandem timing chain
Have you changed the size of the chainrings?
Failing that, have you changed the chainline slightly? possibly put the chainring on the otherside of the spider?
Failing that, have you changed the chainline slightly? possibly put the chainring on the otherside of the spider?
Re: Tandem timing chain
This also relies on the frame builder locating the bottom bracket shells correctly so that the eccentric when it's it's shortest postion is an exact imperial length measured in 1/2 inches which will then accommodate the chain stretching, such that replacement time occurs at the maximum adjustment. 
This works with even toothed chainring pairs ie 34, ,36, 38 etc If you used odd toothed ring pairs 35, 37, 39 etc then you may well be 1/4" out.
This works with even toothed chainring pairs ie 34, ,36, 38 etc If you used odd toothed ring pairs 35, 37, 39 etc then you may well be 1/4" out.
Peugeot 531 pro, Dawes Discovery Tandem, Dawes Kingpin X2, Raleigh 20 stowaway X2, 1965 Moulton deluxe, Falcon K2 MTB dropped bar tourer, Rudge Bi frame folder, Longstaff trike conversion on a Giant XTC 840, Giant Bowery, Apollo transition.
Re: Tandem timing chain
Good points above.
A thought has just occurred, have you had the eccentric out of the frame or had it particularly loose in the frame? Is the eccentric further across than it used to be ? Back to front? Look at the tandem from the front, do the timing chainrings line up? If not that could be where you are losing adjustment potential. We had a tandemist who used to come out with the CTC group who was no mechanic . He had taken his eccentric out (cranks off) and re installed it back to front causing both timing chain and gear change issues. And he denied having done so until me and my mate pretty much called him out on it because it was the only reason the machine was behaving as it was.
A thought has just occurred, have you had the eccentric out of the frame or had it particularly loose in the frame? Is the eccentric further across than it used to be ? Back to front? Look at the tandem from the front, do the timing chainrings line up? If not that could be where you are losing adjustment potential. We had a tandemist who used to come out with the CTC group who was no mechanic . He had taken his eccentric out (cranks off) and re installed it back to front causing both timing chain and gear change issues. And he denied having done so until me and my mate pretty much called him out on it because it was the only reason the machine was behaving as it was.
Last edited by Barrowman on 4 Nov 2022, 5:57pm, edited 1 time in total.
Re: Tandem timing chain
A Thorn tandem is likely to have a Rohloff, and consequently to have two eccentric bottom brackets.
On a solo with an eccentric, for a given chain length there will usually be two possible eccentric positions (an upper and a lower position), e.g. 1 o'clock and 5 o'clock, 8 o'clock and 10 o'clock, 6 o'clock and 12 o'clock etc.
On a tandem with two eccentrics I suspect that in some cases one of the two possible upper and lower positions on the drive chain eccentric might not be usable, because with the drive chain eccentric in that position the range of movement of the timing eccentric will not include a position giving a satisfactorily tensioned chain.
The procedure for setting up eccentrics on a Thorn Rohloff tandem is probably something like this:
1. Adjust the drive chain eccentric.
2. Adjust the timing chain eccentric.
3. If there is no suitable timing chain length, i.e. as in the OP's situation the timing chain is either too tight or too loose, change the drive chain eccentric to the alternate position for its chain length, e.g. if the drive chain eccentric is at 1 o'clock, change it to 5 o'clock, and then adjust the timing chain eccentric.
On a solo with an eccentric, for a given chain length there will usually be two possible eccentric positions (an upper and a lower position), e.g. 1 o'clock and 5 o'clock, 8 o'clock and 10 o'clock, 6 o'clock and 12 o'clock etc.
On a tandem with two eccentrics I suspect that in some cases one of the two possible upper and lower positions on the drive chain eccentric might not be usable, because with the drive chain eccentric in that position the range of movement of the timing eccentric will not include a position giving a satisfactorily tensioned chain.
The procedure for setting up eccentrics on a Thorn Rohloff tandem is probably something like this:
1. Adjust the drive chain eccentric.
2. Adjust the timing chain eccentric.
3. If there is no suitable timing chain length, i.e. as in the OP's situation the timing chain is either too tight or too loose, change the drive chain eccentric to the alternate position for its chain length, e.g. if the drive chain eccentric is at 1 o'clock, change it to 5 o'clock, and then adjust the timing chain eccentric.