crank length
 

Common tandem crank sets are 165-170,170-170,170-175,etc.,ours is 170-175. My single is 172.5,hers is 165. I like to keep a high cadence. Our problem is when I'm spinnnig comfortably she's bouncing of the seat. Is there any reason why I shouldn't go with a 172.5 on the front as this will have us spinning about the same circumferance.

Not really...

Some will repeat the argument offered by some builders that a longer than usual 1/2 bike crank is preferrable for a tandem captain since it will provide for increased leverage. Frankly, I think it's just a way of glossing over the decision to use 175/170 cranks as the default standard, much the same as most 1/2 bikes end up with 170 as a default, regardless of how large or small the frame is.

The flip-side of the coin is, not all cyclists are sensitive to a +/- 5mm change in crank length, myself being one of them. My fixed-gear bike has 165mm cranks, my singles have 170mm, and my tandems 175mm; go figure. It takes me about a mile to adapt but, then again, I'm just an old hack rider these days.

If you are comfortable with what you are using, keep 'em. If not, change.
We have been riding 170mm cranks, front and rear, for 30+ years. Pilot is 5'7", stoker 4' 10 3/4".
Some folks will never know the difference of 2 1/2, 5 or even 10mm in length; others would be quite sensitive.

If she's an experience cyclist put her on a fixed gear so she can work on her spin. Otherwise pedal slower. :)

I've had 170s to 177s on racing bikes, but settled on 175s for both road and TT years ago. I hvae 170s on my tandem and they are fine.

Is there some reason you can't use 165s for the stoker and 175s for the captian? If you are looking for a smooth spin, why not?

None whatsoever. I believe we recently covered this in another thread:
http://www.bikeforums.net/showpost.p...36&postcount=9