The torque works the other way - a longer handle bar means your arms have more leverage on the bumps, not the other way around. It’s more stable with a longer bar.
It works the same way that longer cranks give you more leverage on the wheel, and shorter cranks give the wheel more leverage on you.
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And I don’t understand how moving the seatpost connection affects how long a bar extension you use. You can choose whatever handlebar length you want, no matter where you decide to attach your seat.
I mean, it’s a cool project, and it’s great that you ended up with something you like, but it’s not working out for the reasons you think it is.
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You should check out Turtle’s V unicycle for comparison - he ends up with a similar extended riding position using almost the opposite concept, by moving the seatpost backwards instead of forwards.