In an effort to combine my two greatest passions, some years ago I bought a bicycle trailer for my daughter. Back then I was looking for a trailer that would be rigid, have a roll cage and properly designed seatbelts to protect the kid from (God forbid) impact, can be quickly folded to fit in the trunk, and is convertible into a stroller. So I went for a sturdy German-made Croozer.
I did not consider suspension important, imagining that a toddler hangs in a dedicated hammock anyway, and an older kid will do with a pillow under the bottom.
How wrong I was! The younger the child, the more important it is to protect the spine from vibrations and the head from sudden jolts. For the trailer to have suspension is a must, so I had to add swing arms.
Croozer does not sell a factory solution, or rather, their kit is as expensive as a new trailer, so I had to go after an aftermarket solution.
You'll see that the design is fairly simple and could easily be adopted to fit other carts or trailers.
• 4 bearings: 6001-2RS-C3
• 1 threaded rod: 12mm
• 4 bolts: 6mm*30mm
• 6 nuts: 12mm
• 4 nuts: 12mm, locking
• 4 nuts: 6mm, locking
• 1 stainless steel 14mm tube (wall thickness 1mm)
• 2 bungees
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Step 1: Selecting a Solution
Welded solutions above were out of question – without a workshop with a welder, a drill press, or a belt sander, I was unable to proceed. The last solution was the most elegant, but expensive and impossible to find on the market.
Thingiverse to the rescue! I’ve found a post by Guilliaume from Switzerland, who has designed a set of swing arms for his bicycle trailer, a solution that would work for me. After some modifications to the original idea, I am using the original openings in the trailer’s frame, and the add-on is 100% reversible.
So, in step one, I downloaded the design from Thingieverse:
Step 2: Making the Measurements
Then I made some scientific-grade measurements and created professional blueprints. Having analyzed the design in the light of measurements, I bought the parts mentioned in the intro.
Step 3: Print the Swing Arms
I asked a friend who operates the DIY printer we have in our office to print the swing arms for me.
Since I wanted to have them as sturdy and solid as possible, he tweaked the design a little to increase the infill.
Step 4: Strip the Existing Parts
I disassembled the trailer to expose the mounting holes. The two small ones held the original brake, the large one accomodated the axle. That is where the pivot axis will end up.
Step 5: Fit the Sleeves and Rods
I used Dremel to cut the rods and tubes into pieces. Then I fitted two pieces of the 14mm steel tube in the printed parts. The tube goes all the way through and acts as a sleeve for the wheel axle. Due to inaccuracy of the printing process, the opening for the steel tube was a tad too tight, and the tube would not fit.
The solution was to soak the swing arms in boiling hot water to soften the material and then press the pipe through again.
Step 6: Press the Bearings In
I pressed the bearings into the sockets on each side of the swing arm. To do this, I aligned the bearings with openings on each side of the arm and pushed them in slightly. Then I fed a threaded rod through and used two nuts to drive the bearings into their sockets. You can use a hammer if you like.
Step 7: Assemble Spacer Packages and Install Axles
The 14mm tube is cut into several pieces. In the original design, two of them go over the threaded rod and act as spacers that control the wheel base, as seen in the picture. I later replaced the washer/tube set with two threaded nuts for two reasons: stiffness and to make the wheelbase narrower.
The other end of the rod that you see goes through the opening in the trailer frame, where the original wheel axle was.
On the inside of the frame, I added yet another washer and secured the rod with another locking nut.
Step 8: Think of a Place to Attach the Bungees
The original Thingiverse design used plastic plugs that were bolted or riveted to the trailer's bumper. I wanted my contraption to be 100% reversible, so no riveting was possible, and I something else. In my friend's magic sched we found a piece of steel bar, and we devised a pair of attachements for the bungee cords, as can be seen on the drawing.
I attached two bars, one on each side of the trailer, by feeding 6mm bolts through the holes where the original parking brake was attached, and securing them with locking nuts.
Step 9: Attach the Bungees and Ride!
Then I threaded the bungees through top holes in the swing arms, and hooked them to the rods. The second image shows the setup I ended up with.
I could have gone out riding but as you can see, two passes between the arm and the rod were too long. Three were just about right in length but too stiff. The bungees needed a couple of tweaks, but I finally got the lengths right.
I wish I could record a video, but I don’t think I have the right camera for the job. Here’s one from my phone, but it does not give any justice.
Step 10: Further Changes...
At the end of summer the bungees had shown a lot of wear, one of them actually tore through (surprisingly).
Images show how I replaced the bungees with springs that I salvaged from an old sofa bed. This setup has just survived a week of abuse on rocky Tatra valley beds, but it also made the ride way too stiff and it really takes a big load or a big hump to makem give.
So I will either return to the original concept with the bungee cords, or install a different set of springs (in the meantime I collected quite a number of them just sniffing around).
Step 11: ...and Plans
As for the future plans, the original trailer has a parking brake, and the aftermarket design does not. For the time being I lock one of the wheels with yet another bungee cord, but my plan is to add a brake by attaching a little door latch. I initially wanted it to be spring-loaded, but since there is very little space on the swing arm, I will use a piece old tube to hold the latch in place when not engaged.