A bricycle can be adjusted to be a bicycle, a tricycle, or anything in between.
The spring mechanism in the rear provides a restoring force proportional to lean. Using a so called ‘zero rest-length spring’, the effective spring constant can be adjusted by moving the attachment point of one end of the spring. Clamping the spring, infinite stiffness, makes a tricycle. Detaching the spring, zero stiffnes, makes a bicycle. The interesting part is where the bicycle is no longer rideable when set to a certain spring setting.