fooferdoggie 

With a mid drive in front the front can freewheel so the cranks are not locked together. The chainring in front is 15t but it is a 2.5 to 1 gearing so its 37.5t and the back is 40t. But I have a wireless cadence sensor on the back and front and the back is about 8 to 9 rpms faster then the front. I guess its s the crank arms? Since the back has approx. 175 mm cranks and the front about 172 (just using a tape measure.
Would like to get my cadence unto 90 but my wife cant handle more then about 90 so right now we cant do it. What’s the best way to get them closer to the same?

WheelsNT

No experience with mid drive e-tandems, but... crank arms don’t change cadence. They change the perception of cadence because longer cranks make a larger circle, requiring more foot speed to go around the circle at the same cadence. Whereas shorter cranks make a smaller circle, requiring less foot speed to go around that circle at the same cadence. BUT, the cadence itself is the same regardless of crank length.

The difference in front vs rear cadence is due to your gearing. If you want to rev higher than her, you need a smaller gear. If you’re on 15t, can you get a 14t? Or, put a larger ring on the back, take the rear to a 42, 44, 46? One tooth on the front is a bigger percentage change than one tooth on the back, so to make an equivalent change you need to get several teeth larger at the rear.

Edit:
Thinking further — at what cadence is the 8 to 9 rpm difference? If it’s at 80 to 90 rpm, then it’s around 10% different. If you make the rear ring 10% bigger, which would be a 44t, then all you would do is get the cadence equal between you. If you want to make your cadence higher than hers, then you need to go MORE than that. What is her ideal cadence, and how far below your ideal cadence is it? If she wants to be 10% below you, and right now she’s 10% above you, then we want a 20% move overall. So in that case get a 48t ring for the back. You will also need a longer timing chain.

fooferdoggie 

I had wondered about the crank arm length. 15t is the smallest they make and it gets noisy going small chain rings. the new bosh motors use regular chain rings. I was thinking a 42 or so. How will that effect overall peddling?. I have a 36 chain ring and a 11-42 cassette so we can climb 16% grade hills. we have about 22 or so max peddling speed before my wife spins out but changing her cadence may help.

WheelsNT

Sorry, I was editing while you were typing. Read the second half above.

fooferdoggie 

she seems Fien at 80 to 80 but anything above 90 gets a bit sloppy. or I found if I turn the assist level down its better. so yes when she is at 90 I am at 80 or 82 it jumps around bit but that's pretty close. I would like to at least get to 86 90 would be better. but going to 44 will take off some of the top end though right?

WheelsNT

Changing the rear timing ring changes your cadence (and your e-motor’s cadence) relative to hers. You and the motor will have to spin faster for the same mph speed of the bike. If your top end is coming mostly from the motor, then yes it will reduce that. But you could change the 36t to something bigger to get that back.

fooferdoggie 

ok that makes sense. when my wife gets stronger I can add a larger chain ring anyway. the difference in climbing on a tandem and a regular bike is so different. my regular bike is a bosch powered bike too the same torque but it goes 28mph. but even on full assist (that I don't usually need on my regular e bike) climbing is a lot more work. a 16% grade with the 36-42 and on turbo its still a lot of work to get up.