Howdy folks. I'm doing the 2,000 mile check up on our tandem this weekend. I've never jacked around with the timing chain/eccentric until now and have a question.
There is a bit of droop/slack in the timing chain that I plan on tending to this weekend. I assume this is natural chain stretch and should be corrected by adjusting the eccentric. I'm wondering how often you folks replace the timing chain?
1/2 - 3/4" of play in the chain is OK. As for chain replacement, you have a couple choices....
Treat it like the drive-side chain and replace it once it “stretches” into the 1/16 – 1/8” range (12” ruler method) or gets to the “replace” marker on your Park Chain Checker.
Never replace it until both it and the timing rings are completely worn-out, using ½ links as required to control the position of the eccentric, i.e., your saddle set-back position can start to get screwed up as you rotate the eccentric to take up slack and/or when you remove links once it stretches too far for the eccentric’s rotation to work.
Note: If you find you’ve unintentionally left a chain on too long and damaged the profile of the chain ring teeth you can move the stoker’s timing ring to the front and yours to the back to get a fresh start without buying new chain rings. Don’t try to figure out why your stoker’s chain ring teeth are wearing down on the opposite side as yours – it’s just one of those tandem things that you can't do anything about.
measuring the pins in the chain. You can get a tape measure and
check your chains by setting the ruler for 12 inches and placing it next to
the chain. The 1 inch and 12 inch marks should line up exactly on two
rivets center to center.
The standard way to measure chain wear is with a ruler or steel tape measure. This can be done without removing the chain from the bicycle. The normal technique is to measure a one-foot length, placing an inch mark of the ruler exactly in the middle of one rivet, then looking at the corresponding rivet 12 complete links away. On a new, unworn chain, this rivet will also line up exactly with an inch mark. With a worn chain, the rivet will be past the inch mark.
This gives a direct measurement of the wear to the chain, and an indirect measurement of the wear to the sprockets:
If the rivet is less than 1/16" past the mark, all is well.
If the rivet is 1/16" past the mark, you should replace the chain, but the sprockets are probably undamaged.
If the rivet is 1/8" past the mark, you have left it too long, and the sprockets (at least the favorite ones) will be too badly worn. If you replace a chain at the 1/8" point, without replacing the sprockets, it may run OK and not skip, but the worn sprockets will cause the new chain to wear much faster than it should, until it catches up with the wear state of the sprockets.
If the rivet is past the 1/8" mark, a new chain will almost certainly skip on the worn sprockets, especially the smaller ones.