This weekend project is meant to contribute to the discussion on towing child's bicycles. It was meant as a proof of concept but the attempt failed miserably.
This device does not work as intended, it is very dangerous since the child might be thrown off the bicycle in a curve.
See the next step for a more detailed description of the design failure.
If you like failures you might also enjoy an earlier openproducts' adventure: 'Failed Project: Keeping Snails Away from a Vegetable Garden' (June 3rd, 2013).
Step 1 in this Instructable documents the 'design features' of this towing construction, while Step 2 provides some suggestions for possible further work. Finally, Step 3 spends some words on the CC-BY license of this Instructable.
Step 1: Design Features
The basic idea of the connector documented in this Instructable is that the two required degrees of freedom (tilting forward-backward and turning left-right) can be provided perfectly by the front wheel and the steering wheel of the towed bike. A rigid connection of the front wheel of the child's bike to the frame of the towing bicycle will then ensure (but it doesn't!) that the rear bicycle remains upright. Note that the front wheel is hovering above the street.
The problem however is that the inclination of the child's bike fork causes a twist to the back bike: in a curve the rear bicycle will tilt to the outside curve, which is extremely dangerous. The child might fall off its bike.
There is also a constructional issue to be mentioned. This design has been based on a new function of the fork, which has not been designed for this purpose. Keeping a bicycle upright including its rider brings about much higher forces and moments in the wheel and the fork than expected for normal operation, likely to cause breaking of the rim, the spindle or the fork.
Since this Instructable was meant as a proof of concept the rear bike has been fixed using clamps. In a more advanced testing stage these would have been replaced by a different fix. Moreover, wooden beams are not the most suitable for carrying these heavy loads.
The next step provides some suggestions for further work, if any.