Although extremely simple and effective, the design involves certain compromises: Firstly, the Flevo trike has a rear suspension, but only the seat is suspended. The luggage boot is mounted to the rear axle and is thus not suspended. Secondly, the fixed rear axle and luggage boot, combined with a heavy load may cause the rear part to tip over during hard cornering even when the rider is leaning far into the turn. Incidents of this kind have been reported, sometimes resulting in a catastrophic rollover. Moreover, the fixed axle design puts a lot of lateral stress on the rear wheels if cornering under heavy load.
The Fleovo tilt mechanism is shown below.
One obvious improvement would be to keep the basic Flevo steering arrangement, but to make the rear wheels and luggage compartment tilt in conjuction with the front wheel. There are many ways to do this, but in keeping with the elegant simplicity of the original Flevo design, and with the added requirement of an improved suspension system, how is this to be achieved?
Such a "circular leaf spring" might be constructed out of carbon fibre and made strong enough in the right places to hold the stub axles and at the same time be flexible enough to act as a spring and to allow tilting. As the diagram shows, tilting is possible through deformation of the circular spring. The load is attached at two points shown as black circles in the sketch. Tilt is induced by shifting the load, e.g. leaning the body to one side.
The basic principle can be illustrated if you roll a sheet of paper into a cylinder and flatten it slightly by pressing it between your hands. Compressing the roll is the "spring" action. If you then roll the paper between your hands while holding them parallel, there is no resistance. This is the tilting action. The "leaf" would have to be quite broad so as to keep the wheels parallel in a fore-and-aft direction and to bear up under loads due to braking torque. The wheels are kept parallel in the vertical plane by virtue of being attached at opposite points on a circle: Tangents at opposite points on the circumference of a circle are parallel. Deformation of the circle, such as flattening it, will not upset this basic relationship. However, assymmetrical deformation does lead to loss of alignment. On a human-powered vehicle this might be tolerable. Thus, we have a tilting suspension design in which there are no ball joints or other pivots, and no tie rods or suspension arms.
Initially, I thought this "tin can spring" could in fact serve a dual function as suspension and luggage container. Later I abandoned that idea in favour of a second container suspended inside the "spring". In the model this was realized as a second tin can that is about 8cm in diameter, closed at one end. The seat and front end of the trike are attached to this inner can, which thus has a structural role: It both carries the weight of the luggage and bears the load of the rider in the seat above. How the model tilts is shown in the composite photograph below. The picture also illustrates how the inner container is attached to the spring.
The picture clearly shows the deformation of the spring. To check the parallelism of the "wheels" I joined the top and bottom of each with a string. When a string goes slack this indicates the distance between the top or bottom of the wheels has changed. In fact, this does happen, even though the strings themselves actually help to keep the "wheels" parallel. It seems this fact is the main disadvantage of the "tin can spring" - which could lead to a high degree of tyre scrub in practice.
Note that the attachment points of the inner can to the spring are modelled as pivots, but that these could just as well be fixed in the same as the stub axles are fixed. There is very little actual movement in these pivots, and deformation of the spring would be sufficient to allow tilting.