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Yeah, I always assumed it was to do with friction in the steering, but I guess with the right geo it should be possible. I never had shimmy on my bakfiets, but no-hands was a no-no.
On the Bakfiets (and other Long John style bikes), the reason you can't ride no-handed is because they have a very slack head tube angle relative to the amount of rake they have. That means the tyre patch is trailing some way behind the steering axis and that means the weight of the bike is always trying to tip the steering over centre, one way or other.
I still don't know why they're set up like this. It would make more sense to have a fork with more rake if the head tube angle is that slack. That way the contact patch is more where it would be on a normal bike (in relation to the steering axis) and it would mean the bike would try and self-centre itself on the steering rather than fall over, whichever way you lean it.
We already know that Long John bikes with steep head tube angles feel twitchy and either suffer from shimmy or death wobble. Plenty of us on here have ridden a cargo bike like that. That's why they have slacker head tubes. Opposite end of the scale, however, is a slack head tube with not enough rake which means it won't ride no-handed.
See image for head tube angle, rake and trail.
It's definitely more difficult than on a standard bike but I think that's to do with length rather than the steering linkage.
Well, I guess it could be the weight of the linkage and second steerer tube plus the added friction from the whole system.
It's not that I want to be doing it all the time, more just that I'm glad this one will do it, the last one (in both cycletruck and linkage steering mode) would just start shimmying all over the place soon as I took my hands off the bars.