Here’s my theory why some tandem teams don't perform as well as they should
Many stokers do OK with 170s on their single bikes. But often pedaling at a slower cadence than their captain's. Perhaps because they ride less and/or have shorter legs. When you put them on the same bike, with similar crank lengths, the only cadence where both riders are effective is a narrow range that includes the high end of the stokers useful RPM range and the low end of the captains. (Assuming they overlap at all.) Lots of shifting is the usual result.
Example; Stoker 60-80 rpm, Captain 75-95 rpm = Team 75-80 rpm.
Shorties can raise a rider's comfortable RPM range. And in most cases the range is wider, losing little, if any, off the bottom and gaining quite a bit on the top end. (Great for single speeds. You can gear lower and still have a decent top speed.)
In a perfect world, lengths would be such that the stoker's and captain's efficient RPM ranges would be matched. But lacking perfection, getting the overlap wider will get the bike down the road better.
When one rider’s comfortable cadence is 70 and the other’s is 90, going from 170 to 165mm, about 3%, isn’t going to make a noticeable difference when trying to increase the stoker’s RPM by 28%.
When a person is uncomfortable at higher RPM it isn't due to the muscles switching from extension to contraction more often. It is because their muscles are extending and contracting at a speed that is too fast for them. This recruits more fast twitch muscles, which produce more heat and lactic acid. Shorties reduce this speed by moving the muscles a shorter distance per revolution. Allowing more use of slow twitch fibers for a higher comfortable cadence.
Most of my tandem stoker sets have been 150mm or 145mm with a few even shorter. Tandem stoker sets are made from the right arms of two cranksets. Short captain's sets are made from a single set and I just use the LH tap in the arm with the chainring and vice-versa.
For MTB tandem cranks I use SRAM S600 arms. (shown)
If you want 110/74mm BCD
(Bolt Circle Diameter) I start with Andel crankarms
BTW Before Terry Osell retired from framebuilding, he would usually put shorties on the back without even telling the buyers. He says no one ever noticed.
For left side timing rings I use "Single Speed" rings as they have taller teeth.
I can also provide 170 or 175mm "Captains" sets.
A couple of comments from satisfied customers;
“I got the cranks two days ago, and about ten minutes later, they were on our tandem. The next day, we went for a ride, and boy, was she happy to be able to pedal those little circles!”
"Mark, we are totally on the same page! I share the same theory. Especially the part about the captain usually being the more experienced rider. So the captain's natural cadence is likely higher, as in your example. That is the case here, and was also the case with my previous partner. In hindsight, I greatly erred in not getting her shorter cranks. Our ranges had no overlap, and my wishing that her range would come up to mine didn't happen. I've grown up a bit since then.
Back to the present, now that I have the short cranks on the back of my present tandem (thanks to you), I no longer over-rev my partner. In fact, I've really let the RPMs come up, and have asked her if she is still OK, and to my surprise (and delight), she is.”