Interestingly enough, I recently changed out the cranks on both road tandems going the other way. I'm 5'8" with about a 29" inseam and have always ridden with 170mm cranks on my single bikes... well back to the beginning of time (1970's).
When we when shopping for tandems back in 1997 and knew zilch the spiel from the dealers and manufacturers was, longer-crank arms (175mm/170mm stoker), wider bars, and more upright. More upright went out the window pretty fast and a 2mm bump in handlebar width was all I could stand. However, the longer cranks didn't seem to bother me. In fact, we ended up putting 175mm/170mm cranks on the next five tandems. Moving between the track/fixed gear w/165mm, my road bikes with 170mm, and the tandems with 175mm cranks didn't take but about a few miles to adjust, but I wasn't logging long miles on anything but the tandem which saw 99% of my cycling time.
After spending more time back on my single bikes and then moving to the tandem last fall I began to notice that I was really laboring to keep my cadence in the 90's on longish hard rides and by the end of the ride even keeping them in the 80's was tough. I attributed some of that to my declining fitness (something I still need to fix) and 10 years of aging (not much I can do about that) but also decided to experiment with 170mm cranks on the front of the tandem. After going back and forth a few times I really found the 170's to be much more familiar and easier to spin consistently thoughout the ride, regardless of distance.
So, at least for me, I'm now of the mind that the cranks should be the same on single road and tandem bikes unless you feel you need to ****** your cadence by going longer or to bump it up by going shorter.
Last edited by TandemGeek; 06-15-07 at 09:02 AM.