Failed Attempt at a Tow Bar for Child's Bicycle

My daughter has been riding her two wheeler for close to a year, now.  She has a healthy trepidation about riding on, or crossing, "big streets;" and several of her friends have those overpriced "towing wheel" cycles (see the second picture).  The kind that can't be used for anything at all if they aren't attached to a grown-up's bike.  Well, she wants a towing bike, too. 

I'm not about to drop $300 when she has a perfectly usable bike already.  Instead, I tried to build a towing rig that can attach to both of our bikes, and can also detach, leaving both bikes fully operational.

DISCLAIMER This project did not work as intended. The trailing bike is not stable when towed, and falls to the side very easily, even without a rider. I have documented my work for completeness, but the device described here is not safe for use.

I do intend to keep working on this, and try to find a way to reduce the swivel and sway to make it safe and rideable. If I succeed, I'll update this Instructable accordingly.

Update: It turns out that there's an instructable which discusses modifying a commercial product that does exactly what I'm trying to do here. I need to study that I'ble more, to see if it can help me with my issues here.

Step 1: Tow Bar Concept

The basic idea is simple: connect the two bikes with a rigid bar, with a swivel connection at the front end and a stiff connection (and a locked front fork) at the back.  It turns out that the connectors at each end are a b**ch to find.

I spent the better part of a day chasing around shops, trying to find a proper universal joint for the front end.  No luck.  A few days later, Instructables user moaibob published his Bike Trailer Hitch; exactly what I was looking for!

For the trailing end, mating a pipe to the bike at an arbitrary angle isn't easy. I finally found an "adjustable flange" (McMaster-Carr part #4698T77) used for installing stair and other railings, where you can set the pipe receiver to whatever angle you need.

So now I have a buildable project!

Note to readers: All of the drawings were made using the Unix xfig program on my MacBook. If you would like the original .fig files, please let me know in the comments.

Step 2: Tow Bar

Updated 6 Sep 2013: I had originally called out galvanized pipe for the towbar. That was way too heavy for the purpose, and it kept pulling the trailing bike over onto its side. I have replaced it with schedule 40 (white) PVC, which is much lighter weight.

The tow bar itself is the simplest bit. Two straight sections of 3/4"-trade size PVC pipe (1.05" true OD) connected with a 45° elbow.

3/4" schedule 40 PVC, 23" long

3/4" schedule 40 PVC, 18" long

3/4" PVC elbow, 45 degrees

The dimensions shown are for my and my daughter's specific bikes, with 26" and 15-1/2" wheels. For your project, you can estimate the lengths as follows, to put six inches of separation between the wheels.

• The horizontal pipe should be about three inches shorter than the diameter of the lead bicyle's wheels.
• For the angled pipe, take the difference in wheel diameters, add three inches, then multiply by 1.4, a.k.a. sqrt(2).

Each pipe needs a 5/16" hole drilled at one end:

1. Drill the horizontal (23") pipe 1/2" from the end, and exactly parallel to the downward bend of the elbow. This provides a vertical axis for swivel.  Attach and thread-lock the elbow first, and use a level on the angled pipe to get the alignment right before drilling.
2. Attach and thread-lock the angled pipe to the elbow, and use a level on the horizontal pipe to get the alignment right before drilling.
3. Remove the set screw from the adjustable flange.  Slide the fitting onto the end of the angled pipe, so that the pivot is aligned with the elbow.  Mark the position of the hole onto the pipe, and remove the fitting.
4. Drill a 1/8" pilot hole at the mark, and through to the opposite side of the pipe.  Make sure that the pipe is mounted horizontally, and that the drill goes directly across the diameter of the pipe.
5. Using the pilot holes, drill a 5/16" hole on each side of the pipe.  If you have precision bits, you may want to use a 21/64" bit instead.

I used PVC purple primer and cement to connect the two pipes to the 45° elbow. After the cement cured, I drilled 3/16" holes through each side, and double-secured the pipes with #10 pan head screws and hex nuts.

Step 3: Swivel Mount ("trailer Hitch")

Please see Bike Trailer Hitch for the original version of this component. I've just done some pretty drawings to help me fit it together.

I could find a source for the "antenna pole cross bracket" (second image) in the U.S. In response to my Question, user gadget-man recommended a split-ring pipe support, which has a 3/8" threaded hole built in. To fit a typical seat post, choose the 3/4" pipe size (1.05 in ID); to fit the frame, choose 1" pipe size (1.25 in ID).

Split-ring pipe hanger, 3/4" or 1" size

Swivel caster, 3", with 3/8" threaded stem

1" or 1-1/4" square tube, 2-1/2" long

1/4"-20 x 1-1/2" shoulder bolt

1/4"-20 self-locking nut

1/4" x 1-1/2" clevis pin with retainer clip

3/8"-16 jam nut

The next two steps show how to modify the caster and square tube. Once done, assemble the hitch:

• Insert the closed end of the square tube into the caster.
• Secure the tube to the caster with the 1/4"-20 shoulder bolt and self-locking nut.
• The clevis pin will be used to connect the towbar to the hitch, at the "wings" of the square tube.
• Attach the pipe support to the seat post on the lead bike, with the threaded hole facing directly backward.
• Screw the caster into the pipe support; tighten the jam nut against the support.

As you can see in the picture below, I wrapped my seat post with some rubber sheet inside the pipe support. This reduces vibration, and also ensures a tight grip without damaging the metal.

Step 4: Modify Caster for Swivel Mount

1. Remove the wheel from the caster, either by undoing the bolt, or by removing the axle pin (use a thin hacksaw blade or a cut-off wheel)
2. Cut off all but 1/2" to 3/4" of the caster stem, and put on the jam nut.

Step 5: Prepare the Box-beam Swivel

1. Drill 5/16" holes centered at each end of the square tube, 1/2" from each end on perpendicular axes.
2. Cut off the non-drilled sides of the square tube, back to 1-3/8" from the open end.
3. If you are using 1" tubing, bend out the two wings about 3/32" on each side, so that 3/4" pipe (1.05" OD) fits loosely between them.

Step 6: Reduced Hitch

The full three-axis hitch was too flexible to support the trailing bike. I attempted to make things more stable with a simpler swivel hitch.

	Split-ring pipe hanger, 3/4" trade size
2	#10 x 1" pan head screws
2	#10 flat washers
	1/4" OD (#10) x 3/4" spacer
	1" square steel tube, 3" long
2	3/4" x 3/4" corner brace
	1/4"-20 x 1-1/2" pan head screw
	1/4"-20 self-locking nut

I still used the split-ring pipe hanger, but connected it directly to the box beam using a pair of corner braces.

1. Attach the two corner braces on either side of the box beam, with the 1-1/2" screw through all of them. Secure the screw with a self-locking nut.
2. Cut the 3/4" spacer in half to make two 3/8" lengths.
3. With a washer at the head, insert a #10 x 1" screw through each corner brace, and slip a spacer onto each side.
4. Put half of the split-ring pipe hanger onto the two #10 screws, and hold the assembly up against the seat post.
5. Thread the screws into the other half of the pipe hanger, to secure the assembly to the seat post.

I also notched one side of the box beam so that it could be angled horizontally against the post and pipe hanger, as shown in the picture.

Step 7: Fixed Mount

The trailing bicycle must be locked in a straight path. If the front wheel is turned, it can destabilize and cause both bikes to crash.

The assembly shown attaches to the front fork, with a 1/4"-20 bolt through a hole at the base of the front fork. I originally tried using a pipe strap (as shown in the pictures), but that allows the frame neck to swing, making the bike unstable. The key component is the adjustable angle flange (PDF below), into which a standard 3/4" trade size pipe (1.05" OD) can be fitted.

	Adjustable angle wall flange, 3/4" pipe
	5/16"-18 x 1-1/2" shoulder bolt
	1/4"-20 x 2" pan head machine screw
	Felt pad
	Antenna pole U clamp (e.g. RadioShack #15-826)
2 ea	3" x 5/8" flat mending brace
2 ea	1/4"-20 x 1/2" pan head machine screw
2 ea	1/4"-20 self-locking nuts

Modify the flange as shown in the next step. Install the flange onto the child's (trailing) bicycle as shown in the first figure:

1. Slip the U-clamp between the frame members with the opening forward, and secure the toothed brace against the front of the neck with the included nuts.
2. Slide the end holes of the mending braces onto the U-clamp, and secure them with 1/4"-20 self-locking nuts.
3. Put the 1/4"-20 x 2" screw through the single hole in the flange below the fitting, and through the hole at the base of the fork. Secure the screw with a self-locking nut.

Step 8: Adjustable Flange Modification

The set screw provided with the flange is replaced with a 5/16"-18 x 1-1/2" bolt:

1. Remove the set screw from the flange, and use the threaded hole as a guide to drill a 1/8" pilot hole through the opposite side of the fitting.
2. Turn the fitting over, and enlarge the 1/8" pilot hole to 5/16".
3. Do not drill into the original threaded hole! Set the stop on your drill press for a depth of 1/2".

Cut a circle from the felt pad the same size as the flange, and attach it to the bottom surface. With a utility knife cut an 'X' at each of the three bolt holes.

Attach one mending brace in front of each of the "upper" holes using a 1/4"-20 x 3/4" machine screw and lock nut.  Orient the braces so that they point straight up, as shown.  The 1/4"-20 x 2" machine screw will go through the "bottom" hole, and through the hole in the fork.

Step 9: Putting It All Together

1. Insert the end of the horizontal towbar member between the wings of the swivel mount. Put the clevis pin through all four holes, and secure it with the included clip.
2. I wrapped a bungee cord around the towbar and secured it to the cargo plate behind my seat.  This reduced the "flopping" of the towbar as I attached the trailing bike.
3. Insert the end of the angled member into the slip-on fitting. Put the 5/16"-18 x 1-1/2" bolt through the three clearance holes, and tighten through the threaded hole provided (for the set screw).

The towed, tandem bikes are now ready for use.

Step 10: Failure

In fact, this lovely, detailed project doesn't work.  There are too many degrees of freedom, so the towbar tends to flop to one side, taking the trailing bike with it.  When I replaced the swivel mount with a more fixed version, that reduced some of the flexibility, but not enough. 

I replaced the original galvanized pipe towbar with PVC, and the trailing bike would stay balanced at rest, but as soon as I started moving (even just pushing the bike), it fishtailed over to the side.

Even with the handlebars secured, there is enough sway from the trailing bike that it swivels the towbar, which throws the trailing bike off balance, and collapses.

Possibly, a completely fixed anchor on the lead bike would work, but I have my doubts about doing this with off the shelf parts.