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.

Comments

[kelseymh]

UPDATE: Modified some of the steps today (6 September 2013) to reflect further attempts at making the project successful. They didn't work, but I've documented them here, replacing some of the previous pictures and parts items.

[Databanks]

Could I just suggest lifting the front wheel of the small bike? When my son was little I bought a product called a TrailGator - It coupled to the front of his bike but only his rear wheel was on the ground. It meant the stability was primarily through my bike and his steering had no impact on the ride. Maybe doing the same would help, it's worth a try. One feature that might help was a telescoping arm to allow for different size bikes being towed. The project is well worth the effort, though - much prefer having the kids on something that can disconnect when you get to your destination rather than something that's only good when the parent is riding.

[Lorddrake]

I think i see the problem in your design.

If you look at the store bought version there is no front wheel on it. it basically makes the back wheel of the parent's bike into the front wheel for the kids bike. In that way the kids bike will lean into the turns correctly and follow the progression of the parent's bike.

in your design you have the front wheel on the ground and the tow bar attached to the fork.
I think that if you were to elevate the front of the kids bike just enough to get the front wheel off of the ground and attach the towbar to the neck instead of the fork it would work better.
although you may need to redesign your articulation at the seatpost for better range of motion.

[kelseymh]

Yup. I tried two different modifications: Raising the front wheel (by supporting the towbar on my end), and by using training wheels. In both cases the underlying issue raised its head: the friction of the "clamp" holding the towbar onto the neck of the trailing bike was not enough to overcome the torque from the rear wheel. The trailing bike would start to fishtail, and combined with the angle of the fork, would tip to the side and flop over.

I recently stumbled across a commercial product (the Trail-Gator) which does exactly what you describe: it lifts the front wheel up off the ground, with rigid clamps at both the lead and trailing bike. The few bad reviews it has on Amazon describe the same problem: fishtailing, with slippage at the clamp, and the trailing bike falling over.

[Databanks]

Ah, never mind my previous comment - you've already seen the TrailGator. Sorry. But I didn't notice any fishtailing or tipping of my son's bike when installed correctly, if that helps

[Lithium Rain]

I wonder if placing an additional "extension" of the tow bar higher up on the child's bicycle would help with stabilization? And/or shorting the length closest to the adult bike, placing the "bend" much closer (as in the commercial products).

_{Probably dumb suggestions since you're the physicist. :P}

[Maxus_Lu]

The idea is great and cost-effective.
I think to remove instability it is highly necessary to take the front wheel of the kid's bike OFF the road. It should be hanging in the air. Otherwise it will aim to move in it's own direction wile towed, being then thrusted downwards if the direction of the Kid's bike differs from the towing one.
Or you have to tow the front wheel itself, giving/dictating it the direction of movement. Something like in openproducts's picture.
And there was an instructable here somewhere with the sort of hanging bike-stand for the front wheel, that was attached to the adult's bike rear frame triangle and axis.

[gav4god]

Have you tried attaching your trailing arm to the axels of the towed bike?
That should stabilize it and steer it as well.

[wb8nbs]

I once made a two wheel trailer for my bike This required a flexible joint just under the seat. I used a steel plate welded to the seat, and a double strip of rubber cut from a tire tread attached to the trailer tongue. The rubber attached to the steel plate with a single bolt loose enough so the trailer could swing to either side in a turn. The rubber flexed enough to allow the bike to lean. Woked well. Put hundreds of miles on it with my 4year old son in the trailer.

[kelseymh]

Yes, indeed! For a two-wheel (side-by-side) trailer, the flexible joint is critical. The trailer can't tilt to accommodate turns, ramps, bumps, etc., so you need some kind of swivel joint at the lead end.

The problem I've got is that for pulling a regular bicycle, that flex joint introduces too many degrees of freedom: the tow bar swings, the trailing bike fishtails, and my daughter gets a skinned knee or worse :-(

[openproducts]

Nice Instructable, thanks kelseymh for sharing your experiences. And a great discussion in the comments. It inspired me to test an idea I had on the shelf. Towing a child's bicycle should be possible by using its front wheel as a connector piece. The experiment turned out worse than I expected: the child would be thrown off its bike in the first curve. Read more on this unsuccessful proof of concept and the suggestions for further work in my Instructable 'Failed Project: Tow Child's Bicycle'.

[HelmutHound]

If she knows how to ride, I don't see the reason for the tow bar, tbh. But if she is set on being towed:
Why not install training wheels for more stability, and make a handle bar lock, so she can't steer, but can still hold on? (Im thinking a mix between an old steering wheel lock, and a bolt and eyelet lock).

[kelseymh]

I tried the training wheels idea this morning; it didn't work. The trailing bike stayed upright for much longer, but still ended up flopping over. It appeared to me that it got "pulled" by the tow bar, so the next thing I want to do is replace the galvanized pipe with thin-wall conduit (EMT), which is much lighter in weight than pipe.

[kelseymh]

Don't get me started :-) She can ride very well. However, crossing the "really big streets" (two or three lanes each way, traffic lights, multiple cars making left and right turns, etc.) is very intimidating. My expectation is that once we've done the tow-bar thing for a few months, she'll be "desensitized" to the traffic and better able to handle it on her on.

...Training wheels. Why didn't I think of that?!? That's probably just the right answer. They keep the back of the bike from flopping over; that's what they're designed to do. We still have her old ones, sitting in my "box 'o' stuff".

[Kiteman]

I'd be tempted to extend the angled bar (currently 18" long), so that it can be clamped along the angled cross bars of the smaller bike's frame.

I might also be tempted to remove the front wheel.

The challenge would be to come up with a way to clamp bar rigidly to bike, yet also allowing quick removal once you're off the Big Road With Scary Cars.

[kelseymh]

You've hit exactly the challenge, Kiteman! It's coming up with a way to hold the main frame to the towbar (or somewhere), in such a way that it can't fishtail. Everything else seems to work fine.

[Kiteman]

Lightbulb!

*Insert* the tow bar!

Drill through the front of the frame, where the badge is, and insert a long rod. Further down the frame, drill through the lower cross-bar, and through the inserted rod.

A pin would then fasten the rod in place, which would, in turn, stop the forks turning. The new challenge is finding a rod stiff enough to work, yet thin enough that drilling the holes through would not weaken the bike too much.

[kelseymh]

I had actually thought of an even simpler version of that -- drill through the neck and stick a clevis pin through the hole. That ties the two telescoped pipes together (the fork and frame) rigidly.

When I started drilling a pilot hole (1/8") into the neck, first I got metal shavings, then I started getting this weird black powder out. So I stopped and investigated.

It turns out that the neck of the bicycle is more complex than that. Handlebars have a height adjustment. This consists of a long bolt which runs from the base of the handlebars all the way down to the bottom of the fork. The bolt is threaded through a heavy "rubber" bung inside the fork, and when you tighten it, the bung in pressed up against the inside of the fork tube. Loosen the bolt, and you can raise or lower the handlebars.

If I had continued drilling, I would have ended up putting a hole through the height-adjustment bolt :-(

[Kiteman]

:-(

[Kiteman]

Sketch: