Introduction: Single Wheel Bicycle Trailer With Suspension.

About: Architect

I was looking at getting a BOB Ibex trailer for hauling all of my trail building and maintenance tools out to the trails because I'm sick of walking them in. I figured I would give a go at building my own, so here is my attempt.
I believe the BOB trailers are very well designed, particularly the patented attachment method, so I didn't stray to far from their design.

I've included  photo of the BOB Ibex trailer for comparison.

Step 1: Components and Materials

I started by collecting the primary components so that I could do my layout around those.

Wheel: I was thinking of going with a 20" bmx wheel, but ended up using a 16" wheel like the BOB trailers. I found a few used kids bikes with 16" wheels, but went out and bought a new one for this build.

Shock: I found a coil rear bike shock on ebay from a moped dealer for about $13 including shipping. Would have liked to use an air shock, but they were more costly. I found some 1/2" axle bolts at my local Ace hardware store along with some nylon spacers.

Swingarm and rear triangle: I used an old BMX fork I had lying around as the swingarm for the rear wheel. The pivot is a steel bike hub the local bike shop gave me. Another fork was used for the vertical part of the rear triangle.

Frame: The frame is fabricated from 1/2" EMT (electrical metal tubing). 10' lengths from Home Depot cost $2 each. I used 4 with some waste and leftovers. CAUTIONARY NOTE: I use a flap wheel to remove galvanizing from the EMT before welding.

Miscellaneous: I used various pieces of steel plate (1/4" and 1/8") and steel angle and channel I had lying around from previous projects. Old spokes are used as retaining pins. Also used some scrap 1" EMT. Bottom was made from expanded steel.

BOB quick-release axle: This is the key to the Bob patented bike attachment. I probably could have made something similar, but I knew this was tried and tested. I got mine from Amazon for $26. There is a different version for use on bikes with solid rear axles.

Step 2: Tools

The major tools I used are as follows:
  • 4" Angle grinder fitted with cut-off blades, grinding wheels and flap wheel
  • Metal files - round and flat
  • Pipe cutter for cutting the EMT. Also used the cut-of wheel.
  • EMT bender. I only had a 3/4" one but used it with the 1/2" EMT.
  • Drill press and hand drill
  • Bench grinder and wire wheel
  • Wire feed/mig welder
  • Jigsaw with metal cutting blade
  • Marking tools, scribes, punches

Step 3: Swingarm and Rear Triangle Construction

My first step was construction of the swingarm.
Since the bmx fork was for a 20" wheel and the wheel I was using was a 16", there was a couple of inches to play with and this turned out to be the perfect place to attach the shock.
I added a piece of shallow channel (angle would have worked as well) across the top of the fork to carry the shock attachment angles.
I cut two shock mounts out of a piece of 1.5" x 1.5" ( or maybe 1.25" x 1.25") steel angle.

I cut two more shock mounts for the top and mocked up the rear triangle. I used another bike fork for the vertical legs of the rear triangle and placed the steel hub in the fork as the pivot for the swingarm. The steel angle clamped to the top shock mount in the photo was used to mark the length of the vertical fork for cutting the fork down and was used to attach the shock mounts to the fork vertical legs.

The fork tube was cut down and a rounded groove was ground into the end to receive the steel hub/pivot. Since the fork tube diameter was greater than the hub, I also hammered the the fork tube for a better sized groove for the hub to fit into.

The channel was welded to the swingarm fork and the bottom shock mounts were welded to the piece of channel.
The steel hub was welded to the swingarm fork tube. (The axle and bearings were removed first.)
The steel angle was welded to the tops of the cut fork legs and the top shock mounts welded to the angle and then the edges were ground down.
Please excuse my crappy welds, It was windy outside and then my gas ran out. Plus, I'm not that great a welder.

Lastly, the vertical fork legs were bolted onto the steel hub axle and the shock bolted to the top and bottom mounts using the 1/2" axle bolts and nylon spacers.

Step 4: Trailer Fork Dropouts

The part of the trailer that connects to the rear axle of the bike is called the fork in this application (I'm following the nomenclature used by BOB for their trailers). I had to wait until I received the quick-release skewer mount which I bought on Amazon in order to start this part of the project. The dropout connection to the special BOB quick-release axle skewer allows the trailer to pivot in the up/down direction around the bicycle's rear axle. I needed to mount the quick-release on my bike and confirm clearances as well as measure the diameter and width of the dropout connection points.

I started with a sketch of the dropout made from an online photo and using the measurements noted above and then made a paper template. I used some scrap 1/4" thick steel plate to cut out two dropouts. The round scrap steel with a big hole in the middle wasn't exactly large enough to match my template, but it was close enough. I cut most of this with a jigsaw with metal blade after drilling for the 9/16" diameter dropout hole. The steel plate was also thicker than the slot on the quick-release mount, so I ground the plate a little thinner at the connection point only.

Two small holes were drilled so I could screw bolt the plates together with machine screws and grind them at the same time to get them as close to equal as possible. One of the holes will be used for a stud to hold the retainer pin in place. I will use a large steel nail cut-off as the stud and weld it in place.

The retainer pin prevents the dropout from coming off of the  quick-release mount. It is made from a stainless steel spoke. Small holes are drilled through the dropout legs (see sketch and photo). The spoke is inserted into the holes, then bent at a hard angle and then sprung into place behind a projecting bolt or stop that is in the hole location noted above.

Step 5: Trailer Fork

The trailer fork also allows the trailer to pivot side-to-side behind the bicycle.

This pivot tube was made using an approx. 10" length of 1" EMT with 5/16" x 1 1/4" fender washers welded to the top and bottom. 
The fender washers were held in place using a length of 5/16" threaded rod and nuts, which will act as the actual pivot in this assembly. The washers were ground down after being welded to the EMT tube. A cap nut was used at one end of the threaded rod.
CAUTIONARY NOTE: I use a flap wheel to remove galvanizing from the EMT before welding. 

The frame for the trailer fork was made with lengths of 1/2" EMT bent using an EMT bender. The frame members were slotted with a cut-off wheel on the 4" grinder before being welded to the dropouts. The bottom two fork frame members were first welded to the dropouts and then welded to the pivot tube. Then the upper two fork frame members were welded to the dropouts and pivot tube.

Thin nylon washers were place at the top and bottom of the pivot tube to assist with providing a smooth pivot motion.

Step 6: Trailer Frame

The trailer frame was fabricated from 1/2" EMT.

I started with two long lengths for the top and bottom main rails. Since I was unable to achieve a uniform large radius at the front of the trailer like the BOB trailers use, I opted for a small radius bend at the front center with short straight sections leading to the flanking small radius turns to the side rails. I wasn't concerned with the overall length at this point, as long as the front bends on the top and bottom rails were pretty well coordinated.

As with many of my projects, I start with a sketch and work out many of the details as I mock up the components as I fabricate them. So is true for this project.
I cut pivot plates out of 1/8" steel and welded them to the fronts of the bottom and top rails, using angle established from the mockup. The pivot plates located on the top and bottom of the pivot tube determined the placement of the top and bottom frame rails.

Next I welded vertical struts between the top and bottom rails at each side of the front pivot point.
After another mockup to determine the desired frame length, I decided I would run a top frame member over the steel angle at the top of the rear triangle vertical fork legs. I welded the top cross member at the rear, cut the side rails to length and welded the vertical struts at the back corners.

Diagonal members on each side to stiffen the frame. Large steel nails were cut down and welded into holes at the underside of the diagonal members to act as bungee cord connection points.

Step 7: Frame Bottom

The bottom of the frame is expanded metal. They sell sheets at Home Depot, but the sheets at my local HD are too small to cover the entire bottom in a single sheet. Someone gave me an old garden wagon that had seen better days so I cut the expanded metal off the wagon and ground down the rusty spots.

Before cutting the expanded metal to the final size, I welded the vertical part of the rear triangle into the rear of the frame, cut the expanded metal around the fork legs and tack welded the expanded metal to the bottom of the frame. I then ground down all of the sharp/rough edges of the expanded metal.

Step 8: Ready for Paint

Now I am pretty much done and ready for paint.
I'll need to finish that when the weather is better.

Below is a short video of the test ride at the cemetery across the street from my house. Not tested on the trails yet, but I think it will work great from the test ride. Video was shot using homemade underseat mount seen here:

Step 9: After Paint

Finally got the trailer painted and it looks pretty good.
I used Rustoleum Hammered Bronze figuring the hammered effect would soften some of the welds that don't look too great, but I thin the fact that I sprayed it in 34 degree temperatures didn't help the hammered effect.
However, it has a nice bronze metallic finish that I am very happy with.

Finished weight is 22 lbs.

The last thing I need to do is make some kind of lanyard attachment for the retaining pins so that they don't get lost if they come lose while riding. Maybe later today.

Homemade  DIY Bob Ibex trailer

Step 10: Additional Touches

I later added the following two items:

Retaining pin securing strap:
I made straps out of narrow strips of inner tube, pierced one end with the retainer pin and the other with a screw which was then secured to a hole near the fork dropout.
This makes sure the retaining pin doesn't get lost if it pops out and keeps it at hand when hooking up the trailer.

Top tube padding:
I used foam pipe insulation (two 6' lengths for $0.97 each from Home Depot) as padding around the perimeter of the top tube and used zip ties to secure it in place. This protects the paint from oversized items sticking out of the cart and provide a little give and slip resistance when strapping these items down to the top tube.

Video of trailer in use: