Three years ago engineers from Campagnolo, Shimano, Sachs and KMC all advised Santana that the new 9-speed systems were not suitable for tandems (or even single mountain bikes). They were right.
The problem? To stuff 9 cogs in frames originally designed for 7, Campagnolo and Shimano specified a narrower chain with pin length reduced from 7.3mm to only 6.6mm. This change required near-flush pins. While the new narrow chains were just as strong for a straight pull, they could be pried apart twice as easily during forced downshifts. The execs at KMC recalled a demonstration where an overweight employee of a major US bike manufacturer performed a forced downshift on a mountain bike while charging up a 20-foot high pile of dirt ouside the back door of the factory. The guy pried apart 7 new chains (various brands) in 7 attempts---a 100% failure rate on a test course only 50 feet long! Because road bike shifts are performed at high RPM with closely-spaced cogs, the original 9-speed chains were only adequate for road bikes with no more than a three-tooth spread between adjacent cogs.
Within two years the 9-speed chains from Shimano, SRAM and KMC all featured a "mushroomed pin" technology to help prevent pry-apart. Today's chains combine carefully controlled pin and plate tempering with mushroomed pins, and are as pry-resistant as tandem-proven 8-speed chains.
So everything's fine, right?
Yes, and no. Although the newest chains are strong enough for tandems, customers need to take special care when installing or repairing these chains.
There are actually three broad generations of chain technology. I'll call them:
The pre-HG chain used relatively loose pins that extended beyond the edge of the outer plates. The loose-fitting pins were easy to install, and allowed users to remove and replace pins for cleaning or splicing. Relatively idiot proof.
The advent of HG (HyperGlide) ramped cogs created unanticipated chain pry-apart problems. The chain makers responded with tight-pin technology. The resulting chain was strong on the reel (chain is made in a continuous length), but had a weak spot where the pin was pushed out for sizing and then re-inserted during installation. Depending on the quality of the tool and operator, the pry-apart strength of the spliced link was cut by no less than 10% (a trained operator using a pneumatic tool) to as much as 40% (an average Joe with a hand-held chain tool). To help solve this problem Sachs, KMC and Shimano developed special installation links. Trouble is, some people (even bike shop mechanics) ignore these installation links and splice chains together "the old fashioned way." The result, especially on tandems and mountain bikes, is a high incidence of chain failure.
As if one weak spot created during installation wasn't bad enough, some enthusiasts will "split" a chain to remove and clean it, and create a new weak spot in the process. Or they'll install a larger cog or chainring and splice in additional links (and two more weak spots). Still, because HG 7/8 speed chains are strong enough for tandems and mountain bikes, on a single road bike you can usually get away with haphazard chain splicing.
The newest 9-speed chains have deformed-at-the-end "mushroomed" pins. Because up until now chain manufacturers have done a lousy job of telling people when and why to use installation links (for fear their warning will cause you think another brand of chain is superior), enthusiasts are bound to experience an epidemic of failures with the newest 9-speed chains. Why? Because when you drive out a mushroomed pin with your chain tool you will simultaneously destroy the end of the pin while reaming the hole it was in. When you push the shattered pin back into the enlarged hole, the resulting link has lost 70% of its pry-apart strength.
Advice? Chain tools should only be used to push out the pieces of an outer link, which should then be discarded. Use a KMC "Missing Link" to join undamaged inner links. If you want to make a chain longer, you should use two Missing Links. And if you want to be able to perform reliable enroute repairs, you'll need a chain tool, Missing Links, and enough chain to replace a mangled section.
Confused? Simply use a chain tool to break a chain and Missing Links to rejoin it! If you don't have Missing Links and use a chain tool to effect an emergency repair, shift gently on your way home and replace the chain before your next ride.