Instructables
Picture of Failed Project: Tow Child's Bicycle
Openproducts_-_connector_test.jpg
Openproducts_-_bike_connected_-_fine_for_straight_line.jpg
Openproducts_-_bike_connected_provisionally.jpg
Openproducts_-_bike_connected_-_big_problem.jpg
Openproducts_-_bike_connected_-_very_dangerous.jpg
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.
 
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Step 1: Design Features

Picture of Design Features
Openproducts_-_connector_test.jpg
Openproducts_-_connector_mount_to_luggage_carrier.jpg
Openproducts_-_connector_ready_for_tow.jpg
Openproducts_-_bike_connected_for_proof-of_concept.jpg
Openproducts_-_bike_connected_provisionally.jpg
Openproducts_-_bike_connected_-_fine_for_straight_line.jpg
Openproducts_-_bike_connected_-_big_problem.jpg
Openproducts_-_bike_connected_-_tilting.jpg
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.
KimberlyM3 months ago

is the child supposed to stay on the bike while being towed? why don't you instead of clamping the front wheel clamp the rear wheel and when you turn the bike will turn with you.

josh1 year ago
this is great!! no success comes without failure! I'm looking forward to what you come up with next!!
ToddKrejci1 year ago
Keep at it
I love the concept
Great idea for riding kids to school so u can take the bike home with you aswell
movemaine1 year ago
I've been thinking about my own DIY recumbent trailer for my daughter. Something along the lines of: http://rideweehoo.com


openproducts (author)  movemaine1 year ago
Nice product movemaine, thanks for sharing the link. The purpose of my attempt was to connect an entire child bicycle to the adult bike in a simple way. The advantage of such a combination is that after uncoupling the kid can also ride completely on its own, which provides an additional challenge and a welcome change on longer journeys. But keep us posted on your progress!
Beowolff1 year ago
Hi, I´m new and it´s my first post here :)
Excuse my modest English and drawings..

PROBLEM
In my opinion, you have got 2 Problems that lead to the failure..
1: The small problem: the front wheel is “locked” by your construction, which allows the transmission of torques (a torque is a turning force..) when your child tries for example to turn the handle bar. This torque can destabilizes the bike, but it could also stabilize it..
2: The big problem: the connection occures at the bottom of the bike, which means that the weight of the bike+child acts as a destabilizing action. That´s because the center of gravity of Bike+child is higher than the bearing (connection point on your childs bike). The result of this, is that when the bike tilts to the side, the weigth of bike+child creates a destabilizing torque which amplifies the tilt, which amplifies the destabilizing torque, which amplifies the tilt, which ........... which throws your child on the ground. If your child is strong enough it could try to stabilize the bike by turning the handle bar to create a stabilizing torque as said in point 1..

SOLUTION
You have to move the connection point with your childs bike upward, so that the center of gravity of bike+child is deeper than the connection point. In this case the weight of bike+child will have a stabilizing action on the bike. I would recommend to connect it on the frame, where the handle bar joins the frame. If this isn´t high enough, you can increase the stability by tilting the bike so that the front wheel goes more up (see draft on picture).

TIPS FOR CONSTRUCTION
Use triangulation for the construction, the triangle is the only geometrical form that can not deform even with an articulation at each edge, so it is very stiff compared to other geometrical forms like squares for example.
I would try it like this (see draft on picture) the triangular framework is composed of the blue, green and red bars. I think that the blue bars are included in the carrier (I don´t really see it on your picture..) So you only have to make the green and red bars.
With this draft the front wheel will be able to turn, so there will be no destabilizing/stabilizing torque. That also means that if your child becomes unbalanced it may have some difficulties to equilibrate..
You should take into account that if you connect the green bars to the wheel axle, the axles will be exposed to heavy additional shear forces. It might be better if you connect them directly to the blue bars.

I hope I could help you.

Best regards,

Beowolff
Draft1.jpg
openproducts (author)  Beowolff1 year ago
Thanks Beowolff for sharing your ideas. The picture you add is very illustrative, well done. I'm not entirely sure though whether your suggestions will work. For example, where in your construction do you allow turning the rear bike relative to the towing bike? They shouldn't be connected in a rigid way, see my text in the first paragraph under Step 1. In addition, it is not clear whether you suggest to have the front wheel of the child bike touching the ground. You write 'with this draft the front wheel will be able to turn' but the towed bike should not be able to go a different direction, you'll have two captains on a ship... Compare it to towing a bicycle by simply using a cord: this requires skills that cannot be expected from young kids. See also the discussions in kelseymh's Instructable 'Failed attempt at a tow bar for child's bicycle'.
You can try to rig a device that will keep the front handlebars from rotating. I made a similar rig awhile back and it worked great. If you stop the pivot, it should not tip. Also. The front tire needs to be set in the holster a little more
openproducts (author)  tincupchalice1 year ago
It would be great to see your solution tincupchalice. Did you take a picture that you can share? You are right about the front tire. In this proof of concept the spokes hit the holster, which could have been carved a bit to allow the wheel further in. My solution would have been to create a third support point at around 8 o’clock.
Big kudos for documenting a failure. Let's do more of this so we can all learn something. I wish the scientific community would get a bit better at that too. Thanks for the hard work. Carl
Thanks for posting this many people who don't invent never consider the many attempts it takes sometimes to get something right.
I think every idea works perfectly until it hits the material world
wilgubeast1 year ago
This is how to document a failure. Nobody loses when you learn this much from a mistake and share it with all of us. Especially the toddler testdrivers. I think they come out on top in all of this.

Well deserving of a feature to the homepage.
kylemoe1 year ago
Have a look at how single wheeled trailers are set up for towing behind a motorcycle. There is a lengthy thread on advrider.com
kelseymh1 year ago
Yes, indeed! Thank you for posting this, and for confirming my own experience. Your picture of the trailing bike tipped on its side is exactly what happened in my tests.

With the front wheel "fixed," what ends up happening is that the back end of the bike swings freely around the fork/neck axle, and falls to the side. I also found that using, for example, U-clamps to try to lock the handlebars and frame together at the front fork was insufficient. The handlebars wouldn't turn if you pushed them, but the bike would still sway/topple in use, because of the large torque from the frame length and mass.