The concept described in this Instructible has been inspired on the discussion in the Comments Section of the recently published openproducts' 'Parent-Child Tandem' (July 22nd, 2013 - based on a commercially available product) and kelseymh's Instructable 'Failed attempt at a tow bar for child's bicycle' (August 21st, 2013).
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 principle connecting a child's bicycle in tow to an adult bike is that the degrees of freedom of the following bicycle are being limited. Of the three possible rotations two are allowed: tilting forward and backward (important for crossing a speed bump but also in curves) and turning left and right (important in curves, comparable to the principle of an articulated bus). The only rotation that is to be suppressed is tilting side to side: the child's bicycle should always keep the same position as the towing bike (i.e. upright, or tilted towards the inside bend in a curve).
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.
Step 2: Suggestions for Further Work
Unless you intend to automatically eject children from their bikes (or you're only driving in perfectly straight lines) this design is useless for towing a second bicycle. However, there may be a few hypothetical applications that are worth mentioning:
1. The trouble is, as already mentioned, the inclination of the following bike's fork. This inclination has a purpose for the bike's stability so there will be no sensible bicycles without it. However, especially for small bikes such layouts may exist. For those bikes the concept described here might work.
2. If the child's bike is being connected backwards (i.e. the rear wheel is connected to the wooden piece) the couple might be stable. Most important problem however is that the adult's bike now has a long tail, which will cause the towed bike to graze along the sidewalk or get stuck in the roadside. This is also not safe! Moreover, the pedals cannot spin anymore because the wheel has been fixed (with a back-pedal brake it might not work at all). The fact that the little rider experiences an opposite direction of traffic is also worth mentioning. A possible meaningful application of this configuration is to convey a bike (without its rider).
3. If you're working in a circus connecting two adult bicycles this way might be fun to experiment with. It should be possible to zigzag in a straight line: both riders are keeping each other in balance while continuously tilting over.
The next step spends some words on the CC-BY license of this Instructable.
Step 3: License
This Instructable is being made available through a Creative Commons Attribution (CC-BY) license. Special design features of the concept have been described in Step 1. Note however that the design does not work as intended.
Republishing this Instructable is allowed, provided it is being attributed properly (cite the name openproducts, link to www.openproducts.org, www.instructables.com/member/openproducts, or the original Instructable. For other arrangements send a Private Message through the instructables member page (www.instructables.com/member/openproducts).
If this design infringes any rights then refer to Article 28 in the Terms of Service (www.instructables.com/tos.html).