One test we did at R+E (Rodriguez/Erickson) while I was there about '79-'80 was ride the same frame with two different internal bracing schemes, and then with none (open frame). We actually brazed in tubes, and then hacksawed them out before the next test. Between the ādirect internalā and the double-diamond, no one could feel the difference. Not that there wasnāt one, just that experienced riders couldnāt detect it. With the open frame, there wasnāt any more āwindupā in the frame like from sprinting or climbing, but there was a perceptible amount of bounce when your cadence matched the natural resonance frequency of the frame. Iād describe it as just perceptible, not at all unpleasant/distracting/inefficient. And that was with old-school small tubes, like 1-1/8" toptubes. I think with modern tube sizes, youād be hard pressed to notice.
Thatās why I think for modern tandems, any mid-tube is just a place for a second H2O battle for the stoker. Not worth the extra weight, but no real harm done if you want one.
I think the open frame has been proven plenty stiff enough considering itās been in use since at least the 1940s (Singer, Herse et al.) and quite a bit more popular in recent decades.
One interesting factoid about open frames: If the stoker TT is horizontal, parallel to the keel, and the two STAs are the same, you have a parallelogram. If you imagine the joints as pinned, free to pivot up down like a hinge, then I think you can see that as the capān seat goes down, the front of the frame is rotating CCW around the front hub (as viewed from the right), but the rear seat is forced to rotate CCW also around the rear hub, which means the stoker saddle is forced UPWARD. The captain being so often heavier than the stoker means cappy might be jamming stokieās saddle up in her nethers.
No one uses pinned joints in this way, itās just a visualization aid. And tandem rear frames arenāt usually parallelograms, not exactly. But still I think this might be part of the reason the stoker saddle is usually reported to be less comfy than that personās single.
Stokie not being able to see bumps coming is probably part of it too. I donāt know what other factors there may be. Could there be a doctoral thesis in this? itās an important question. Future generations might thank you if you solve this one!!
Or just use some suspension and/or fat tires, problem solved.