I have a Thorn Me'n'u2 triplet (three seater tandem), and was having a problem where whenever I tried to accelerate suddenly (either solo or with stoker(s)), the front chain seemed to fall off (happened at least once every time we went out).

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I tightened it up (removed a link and turned the offset bottom bracket) so it was then pretty tight (taught top and bottom, only moves a bit when squeezing the middle), and that problem stopped happening - so far so good.

Later on tour we saw someone with a similar tandem but their front chain seemed very saggy - they were stopped, and it was drooping very noticeably both on top and bottom. I didn't have a chance to talk to them, but presumably they weren't having the same troubles. Does this suggest something else is/was wrong on mine, and that it should be ok with a much looser chain?

  • I suspect it's something about the way you ride, damage to a chainring, or possibly that a replacement part is not an exact match. If you tilt the bike under power that could help a loose chain fall off, and likewise a slightly bent tooth can be enough to drop the chain. And an over-long BB can mis-align the chains to give the same effect.
    – Móż
    Aug 15, 2016 at 22:34
  • Give your chain a measure with a chain tool, or use a ruler to carefully measure 12 links. Each link is exactly 1 inch long, so 12 links is 12 inches when new. If its 12 1//8 inches, time for a new chain even if its working fine.
    – Criggie
    Aug 16, 2016 at 1:24

4 Answers 4


The sync chains should always be tight enough that they're not physically capable of falling off, and there shouldn't be any easily perceptible droop, ideally in any rotational position. If you pull the chain up and down, it should have a little bit of flex and "give" to it, the feeling of the pins having some aggregate movement to give in their inner plate bushings. Some sources like to give specific amounts of free movement that indicate a properly tensioned chain, but this is the wrong approach because it depends on the length and the internal clearances of the chain. Too tight is when you can feel any binding or extra friction. Sometimes parts with poor concentricity or damage force compromises where there may be some spots with slight binding/friction. On a nicer bike like this one would hope not, but it's not the end of the world.

Incorrect chainline will encourage dropping in a sync/singlespeed chain capable of it. But mostly the right answer is the chain shouldn't be capable of it, and if it is then problems are likely.


Sometimes whether or not you can see the droop depends on the angle. Whether it is a problem might depend on pedalling style.

As long as you still have a little slack at all points around the pedal stroke, it's not too tight. Tight spots can cause resistance turning the pedals and additional wear to the chain and rings.

As Sheldon Brown notes:

Once you have the chainrings centered and secured, adjust the eccentric to make the chain as nearly tight as possible without binding. Notice how freely the cranks turn when the chain is too loose. That is how freely it should turn when you are done, but with as little chain droop as possible.

Here's a simple explanation/adaptation of sync chain adjustment from the Thorn manual for anyone who hasn't seen it.


Opinion answer - maximum of 8 mm of slack/displacement when you push the top of the timing chain down. Ideally 4 mm would be better.

On the other extreme, if its rock hard then its too tight, and will accelerate wear on timing chainrings and the chain itself.

If you replace the timing chain, get 1/8 inch chain, not 3/32 inch chain. There are no sideways forces, and the timing chainrings are intended for wider chain.


Both chains should be evenly tight. Other wise it won't work properly

  • Welcome to Bicycles @Christopher. We recommend that new members take the tour to get to know how to make best use of the site. This is a very short answer, it could be a comment instead. When you have 15 reputation you'll be able to do that. Meanwhile, I suggest that you edit this answer to expand it, explaining why it won't work properly or what would go wrong. See also How to Answer
    – andy256
    Aug 17, 2016 at 7:28

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