[SRidge]

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.

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[TandemGeek]

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.

[zonatandem]

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.

[woog]

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

[ElRey]

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.

[Lost Coyote]

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?

[TandemGeek]

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.php?p=1416636&postcount=9