A tilting three wheeler is a three wheeled vehicle whose body and or wheels tilt in the direction of the turn. Such vehicles can corner safely even with a narrow track.
Several configurations are practical. Two front wheels and one rear, where all three wheels tilt, use the acronym 2F3T (i.e. two front three tilt). Other variations include 1F1T, where only the front wheel tilts, an example being the Vandenbrink Carver, made in the Netherlands.
There is no general agreement on how much, or even what use to make of the third wheel on a tilter. Sometimes it does little more than a kickstand with a caster, employed at low speed. These are called free-leaners and they must be balanced through countersteering (however subconsciously) before turning, just as on a motorcycle or other single track vehicle. For comfort, the free leaners are the best for cyclists, as the legs don't get even thrown to the side when you hit a bump on one side, and no muscles are used for side support of the torso.
Other tilters use the third wheel to resist wind loads and force a faster lean to set up for a fast corner or emergency lane change. Opinion is divided on how best to control this benefit, however. Some sort of pendulum and servo is the obvious solution, but how to get it to act fast when you need it without chasing every bump? Some think that the driver's anticipation will always be essential in some situations, requiring at least a manual override for the tilting function. Various electric and hydraulic systems have been employed to modify TTW behaviour, with notable success on the Carver. One intriguing possibility is a system that has the driver controlling the tilt only, and the chassis steering accordingly, so the third wheel is well used, and it may be easier to learn for drivers.
Due to the tilting, there is not necessarily any load transfer between the wheels in cornering, so the rule about tadpoles (2F or "reverse trike") understeering and Deltas (1F or standard trike) oversteering does not necessarily apply. Thus deltas are less dangerous, providing that the tilting is done very accurately during hard braking. Tadpoles can still get through more kinds of trouble with grace.
Another system that seems obvious just links tilting to steering angle. This sometimes gets refined to vary the ratio according to speed, but it is still a recipe for disaster in the event of oversteer. It might be safe on a tadpole with a limited range of tilt. That simple arrangement also leaves you sitting awkwardly sideways on cambered road edges.
Some tilting three wheelers use two buttons on the steering wheel connected to an electric motor to allow the driver to adjust the angle of leaning while some more modern designs use computer controlled tilting.