Originally Posted by PaMTBRider
The timing chain on our Co-motion needs replaced and I was wondering what to get. Is there anything special about a timing chain other than length? Should it be replace with an 8 or 9 speed chain? If it matters we have Ultegra cranks. Thanks.
8 or 9 speed will work; whatever you can find on sale noting that you'll need 1.5 standard chains to create the new timing chain. In other words, consider buying 3 of what ever chain you intend to use, make your new one with two and then make up another "spare" timing chain with the left-over 1/2 chain and the 3rd chain for use as needed.
A couple tech notes:
So long as a timing chain can be adjusted to the proper tension and isn't making noise, you can run the darn things until they have turned the timing rings into a saw blade (12,000 miles?) and then replace the timing rings and chains at the same time. It's not my preference, but it underscores that the timing chain is not a precision part of the driveline like the drive train since it isn't required to move between different sprockets or drive rings.
With that latter in mind, how worn out was the original timing chain? Note that if the timing chain was allowed to wear too long it may have deformed the pitch of the timing ring teeth. If so, when you a new chain on it will not mesh cleanly, usually evidenced by a lot of driveline noise or a sloppy feel from the pedals. If so, fear not, you can usually "flip" or "rotate" the timing rings and essentially end up with "new" timing rings. I usually just rotate, i.e., move the front chain ring to the rear and the rear chain ring to the front.
When installing or re-installing timing chain rings, make sure that you get the rings properly aligned that that the teeth are all in phase: keeping the manufacturers ring marks (brand names, tooth numbers, etc..) aligned usually accomplishes this. You'll also want to try and get the chain rings centered on the spiders to remove any biopacing / cam action from your timing chain tension. Bolt-on timing rings don't always perfectly align with the spiders and if they're both off at the wrong place you can end up with the timing chain being too tight and alternatively too loose as the cranks go through a full rotation. This can make setting the timing chain tension a bit of a trick in that you need to catch it when it's at the tightest setting instead of the least tight to make sure it doesn't bind.
Ideally, you should have about 1/2 of slack along the top of the timing chain run between the cranks and, keep in mind, when you tighten the eccentric the front axle will move forward 1mm - 2mm. Therefore, you'll want to get your chain's tension adjusted where you want it, and then you'll want to back-off of tension (rotate the eccentric) a little so that when you tighted up the eccentric the chain tension will be where you wanted it.
Finally, in regard to road tandems and timing rings, while it can greatly simplify things to have the same width of chain as the driveline it's not essential. Timing chains tend to be trouble free so long as they remain properly lubricated: after all, they only transmit power from the captain to the stoker whereas the drive chain carries the power from both the captain and the stoker. Breaking one usually implies that the chain rivets used to mate the chain weren't properly installed and the latter can be eliminated by using re-useable chain links, e.g., Forester SuperLinks, SRAM PowerLinks, or similar products from KMC and Wipperman. The Shimano one-time use rivets and a bad run of SRAM chains in the late 90's seemed to bring about a surge in broken timing chains which tarnished the trouble-free reputation of timing chains in general. However, and again, I'm talking about ROAD tandems, breaking or damaging a timing chain is a rare event that can usually be traced to root cause problems like a bent timing chainring, running a timing chain too loose, having different length front and rear bottom bracket axles, or an otherwise out of alignment chain line.