Timing chain and eccentric BB adjustment

[Frank9755]

I could probably work this out, or experiment, but I expect someone will know the answer.

I've used up most of the adjustment in the eccentric bottom bracket.  When it is right round to the front, and the timing chain gets slack, could I shorten the chain by a link and flip the eccentric back to the back? 

Or is there an adjustment gap - ie what I get from the eccentric is less than a full link?

Or should I really replace the timing chain at that point?  Happy to replace it but happy to run it on if it is normal practice to do so.

[Paul H]

My experience with EBB's is with IGH's rather than tandems, however the principal is the same.  An eccentric ought to cover a little more than that needed for a full link.  Whether it's worth doing is another matter, have you already used the full throw of the EBB?  If so, I'd consider it worn enough to replace.  If the EBB started in a mid position, I'd be more inclined to remove a link. Either way, you could always measure the chain and decide the risk. 

[Russell]

Stupidly and annoyingly ecentrics often offer less than a whole link. The newer more expensive ecentircs, Bushnell and so on, do offer 1" of adjustment.  Try a link removed but it may not work or do as I need to do and use a half link.

Our drain pipe, custom sized, is not an even number of pitches so I need to use a half link.  It does have an adverse effect on oil/grease/muck build up as rather than being pushed into empty gaps the half link keeps redistributing it.  Not really a problem though.

[Ian H]

Not bought a tandem for a few decades, but back in the day you would need a half-link.  Not sure they're available other than in 1/8" chain.

[Frank9755]

Thanks all.
I think there's enough adjustment to get us through PBP next week so I'll leave it for now, but will most likely require attention soon after.

[Tim Hall]

I once made a half link, using a whole link,pliers and a moderate quantity of Bad Swears, for a Dawes Super Galaxy Twin that didn't have enough adjustment in its eccentric.

[mzjo]

About 40 years ago Tony Oliver wrote an excellent article for the Tandem Club journal which pointed out that tandem eccentrics typically do not allow removal of a full link to allow for chain slack. He also included the maths necessary to design a tandem with a relay length that made the most of the length of a new chain, not wasting valuable adjustment taking up the slack from new. IIRC the choice of odd or even relay chainring sizes was part of the calculation. One way of taking up the slack when there was none available on the eccentric was by having two sets of relay chainrings, one odd and one even size, to be swapped as necessary.

Perhaps someone out there has a copy of the article? It might even have been part of a series on tandem design.

[Ian H]

The design of our touring tandem was such that we could only get full adjustment using timing rings with odd numbers of teeth.  Something to do with 1/16ths of an inch, but I can't work it out in my head.  Obviously Steve, the builder, didn't either.

[iandusud]

We use a half link. Available in 1/8" and 3/32"

[Frank9755]

That's what I've done - put a half link in!

[Blodwyn Pig]

being new to this tandem lark, I had a thought about this.  As the BB is eccentric and rotational,  presumably as the chain wears, and the bb is rotated, it would change the position of the BB in relation to the saddle. Is this something that is detected? it would be like constantly moving the seat back.?

[Paul H]

being new to this tandem lark, I had a thought about this.  As the BB is eccentric and rotational,  presumably as the chain wears, and the bb is rotated, it would change the position of the BB in relation to the saddle. Is this something that is detected? it would be like constantly moving the seat back.?

Some are more sensitive to that than others.  A full link's throw is 12.5mm, I set my saddle at the mid point, so I'm never more than 6mm off that, occasionally I think I notice that, but never enough to do anything about it.