I try to break down the "Tumbler Toy" effect as experienced by me on my specific tilting tadpole trike, which I was not riding for several years but plan to do so in 2026 again. My summary will help me to get into it again.
Tilting. Sure, but what actually tilts? For my trike, I would argue at first, tilting and steering are statically not entangled. When at rest I can use the handlebars to steer the front wheels while the frame of the trike stays in an upright position. And the other way around, at rest I can tilt the frame to the left and to the right by also using the handlebars while the front wheels always show straight ahead. But you already migh feel how I'm cheating here: I'm not sitting on the trike, but stand next to it with my feet on the ground, which provides the required counterforce when moving the handlebar in the different directions. The other cheat is to look only at the tilting angle of the frame.
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A second glance reveals that even when keeping the frame in an upright position the front wheels (26") themselves already tilt at steering. In the most extreme steering position the inner curve wheel has a tilting angle of about 25° and the outer wheel has about 5°. The specific numbers don't mean much without precisely describing the construction, which I avoid for the moment. These angles have been measured at rest with an upright frame (0° tilt angle of the frame) and with dismounted seat which allows even more extreme steering (otherwise the seat would physically stop the handlebars earlier, leading to smaller front wheel tilt angles). The different front wheel tilt angles are by construction (I avoid technical names like caster angle and ackerman steering). Leaving the frame alone, for the front wheels steering and tilting always come together.
Sitting on the upright but resting trike means to sit on the top of a potential curve describing an unstable equilibrium. In this position you can fell over to the left or to the right side. With an upright frame and when steering to the left, then falling to the left side feels expected. But falling to the right side is possible as well (when steering left), and I was very surprised finding me in this position for the first time, an impossibe position which never would happen while driving at speed, and which could damage the contruction if I remember correctly.
Static considerations are easier to follow, because you can check them out while the trike stands in front of you. Dynamical concepts are harder to get into. But riding such a trike feels easy and natural once you learned it (similar to a normal bike). Dynamic stability of bicycles, uff. Maybe there are differences between classical upright bikes and tilting trikes, maybe these are only small. I noticed, while cornering is a real nice experience, riding a straigh line is more difficult with that trike than with an upright bike. I wonder why. I see two differences: the height of the CoM is much less and the force vector is not perpendicular to the contact surface between the wheels and the ground (hmm, at the next regional fair I should test riding such a modern tadpole tilting cargo bicycle). For the moment I believe that the low CoM makes balancing more difficult (shorter lever), a straight line with my trike is a sequence of steering corrections to just keep on top of that potential curve. A tilt break could have two functions then: keeping the direction and not falling over and off.
In the picture above you see all the pivots and connections. Bending the back introduces tilting the frame, which tilts the wheels und introdcues cornering. The handlebars steer the wheels in a direction, they tilt and take the frame with them. Until now all was about leaning into the curve or falling off the trike or rolling from that potential curve. How to get back from there?
Too much for me, I would say. So I stay with what I experienced. From the two ways to lean into a curve only the handlebars would bring the bike in an upright position again. Hmm, I should probably test out two things during this year: can I ride straight ahead in the tilted position of the frame (probably only with counter steering) and can I get in an upright position again by just leaning over extremely to the other side? Gloves and helmet mandatory.
Eventually the already mentioned acceleration at low speeds to bring back the trike into the upright position. This accelerates the whole trike via its rear wheel. I would image that this superimposes an acceleration to the front wheels which helps them leaving their curve on a tangential trajectory for that short moment. This kicks the trike out of the curve and helps getting the front wheels a littler more upright again into a more stable parameter regime from where the rider can pick up the trike again instead of falling over.
This is a small version of that "Tumbler Toy" effect with my trike. The experience with your contructions is different, as you outlined before.
