How to build a bike trailer¶

I’ve been meaning to build a bike trailer for, well, eh, about twenty years actually. This page documents my various attempts, successes, failures, inspirations and notes from my research in the process.

Early attempts and failures¶

I’ve now built several trailers that work and had a couple that did not. My first failure was my first attempt about twenty years ago. It was a single wheel trailer inspired by the Bob Yak. It was built using a childs scooter which back then where large and styled like BMX. It failed repeatedly under load as my welding was crap. Twenty years on, my mark II attempt works but that long awaited success came only after I built my first successful two wheel trailer. In between, I started another two wheel trailer which was destined to fail. Additionally, before succeeding with the single wheel trailer, I did a version of it with a seat post hitch which was a disaster.

So what did I learn from these early attempts and failures? Well, I learned more about welding and the limits of what I could expect from an ARC welder. I learned more about balance, stability and why certain designs are more popular with others.

My Twin Wheel Trailer¶

This was surprisingly quick and easy to make. It cost nothing to build beyond the cost of grinding disks and welding rods. I was given two 20" BMX wheels with tubes and tires – a front and a back wheel. I used Dexion (or Dexion style) angle iron to form the axle mounts with an angle grinder to form slots for the axles to sit in. I then welded mounts on rusty old 3/4" electric conduit which I used to form the trailers frame. I hand bent the pipes to shape by inserting it down inside the bumper of my van as some kind of improvised holder. The frame was welded together to form one piece including the hitch arm which was bent up a little as it moved away from the trailer so that it would meet the hub on the rear wheel of the bike as the arm swept inwards.

I had planned to make a flat bed with a wooded base but I couldn’t find a piece of suitably sized wood and couldn’t be arsed to cut anything. Instead I looked around for another option and came up with a plastic stacking tray (like a bread tray but with equal height sides all round). It fitted perfectly into the frame creating a recessed box with a lovely low centre of gravity. Ideally I’d have more than one of these as they stack either into or on top of each other – very handy. The box just lifts out, which is great for transferring the contents indoors.

The hardest bit was the hitch. I tried several methods for the mount but have currently settled on two links of chain, a bit of inner tube and part of an old fashioned side pull brake caliper. Basically, one end of the chain is inserted into the end of the pipe which forms the hitch arm. A hole is drilled and a bolt put through to hold the chainlink in place. The other link is cut, opened a little and hooked into bolt hold on the bit of the caliper then hammered closed. The inner tube is slide over and cable tied in place. It serves two purposes: making the chain links less noisy when they move, and protecting the paint on the chain stay where the hitch point touches when fitted. It is fitted by removing the quick release axle off the rear wheel of the bike and replacing it after threading it through the hitch mechanism.

The only problems I can currently see or predict with the trailer are as follows. The pipe bending was poorly done and introduced kinks into which will have weakened the metal. With inconsistent bends and no accurate measurements anywhere, the frame isn’t entirely symmetrical. This means that the box isn’t quite a perfect fit, it has about 1cm play side to side and one corner tends to drop a little. The shape of the hitch arm is incorrect and reduces the available turning circle in one direction as the rear wheel sometimes hits the arm. The arm probably isn’t super strong where it meets the rest of the frame – I expect it will snap there if anywhere. Finally, the wheels I used were different widths (always the case with front and back wheels) and so the axle mounts only fit with the wheels on specific sides. In future I’d either try to use two front wheels (or two back, although in theory they are not generally as strong), or I’d add spacers to the axle on the front wheel so the spacing was the same as the back.

Update. As I expected, the hitch arm broke off which gave me a chance to reshape it a little before welding it back on. I slipped a smaller diameter tube of metal inside before re-welding to make it stronger and for good measure I also welded a 4" strip of metal on the underside.

Update 2. After another few months of use the arm broke off again. I’ve not yet fixed it and since I was finding the hitch annoying anyway, I think I’ll change the design and switch to a seat post mounted system.

My Single Wheel Trailer¶

The single wheel trailer was much harder to build successfully. I didn’t build a frame or flat bed, instead the front and back parts of the trailer were both welded to short lengths of Dexion angle iron which in turn were bolted to the front and back of a plastic stacking box previously used by Halfords (very similar to Pickfords removal boxes).

For the back end I used the rear triangle and wheel from a childs bike with 16" wheels. I cut the triangle up a bit and re-welded as I wanted to save the bottom bracket for another project. I didnt actually need to modify the triangle and I could have used the front forks instead but decided I’d have another use for them too.

The front part of the trailer, the part which hitches to the bike, was finally made using the front forks from mountain bike plus the headset from the kids bike. The mechanism forms a pivot point which allows the fork and the trailer to swing up/down in response to the bike dipping or climbing up or down bumps or kerbs etc. The headset welded to the trailer end of the fork acts as a second pivot point allowing the trailer to swivel left or right as the bike turns.

The hitch itself attaches to bolts fitted to the braze-ons either side of the rear axle drop-outs on the bike. I created a clever quick release mechanism which is hard to describe but seems to work well. Basically the fork is maneuvered so that the braze-on bolts are positioned in horizontal forks drop outs. About an inch further up the forks are two hook shaped bits of metal which are loosely bolted so that they can be hooked over the braze-on bolts. That’s it, the trailer is now attached – not tools needed to attach or remove.

Finishing touches to the trailer included a bright yellow paint job and a glassfibre tent poles supporting a yellow plastic bag flag at drivers eye height. Still to come will be a mud guard and drain holes in the box as I am sure it will not be rain proof. I might even consider a water proof cover with elasticated edges.

The trailer is great, I love it. It tracks perfectly behind the bike allowing you to weave in and out of lamp posts, trees, pedestrians, parked cars etc without ever clipping them like you would with a two wheel trailer. Riding in traffic is pretty amazing as you can still nip down the gaps you would if you didn’t have a trailer. When heavily loaded the bike does feel a little strange, different to how the two wheel trailer makes it feel. Stopping, loading or unloading is a bit of a pain as you have to find something to lean the bike and trailer against. Slow speed close quarters maneuvering, especially reversing, can be a real pain, especially when you are off the bike and wheeling it. The turning circle in this scenario is awful. Nether-the-less, it’s great and I think I’ll use it a lot.

Update: I’ve now given the trailer away and will soon be building another very similar one. What was really nice about giving it away was being able to unbolt the front and back parts then put them into the box itself. Yup, the trailer fits into its own box for easy and convenient transport or storage! Mounting the trailer on my friends bike proved a bit of a pain due to a pannier rack using the braze-on holes. I didn’t have bolts long enough to hold the rack, the mud guard supports and the trailer arms along with nuts to keep it all tight. I ended up mounting to holes on the pannier rack itself.

I started building another one (actually two now), basically the same. I considered switching things round a little, perhaps using front forks for the back part and back forks for the front, or both fronts, or both backs, but I think I’ve concluded I may have chosen the best configuration the first time round. I also toyed with the idea of using rear suspension, like the most expensive of the commercial Bob trailers. I also wanted to trim a couple of inches off the ground clearance and refine the quick release with some kind of spring/elastic and locking mechanism. Sofar progress is good and I think this one is a little lighter than the first. I used 16" front forks for the back end but cut them off the stem (which I then used in the front swivel). I cut the two prongs of the fork apart and then swapped them over before welding them individually onto the dexion piece that bolts to the box. The construction is ultimately very similar to the modified rear triangle I used on the first but without the seat post/upright part. This means it is a little lighter but meant I had nowhere to stick the flag pole. I wasn’t entirely sure this would be strong enough and I needed somewhere to attach a mudguard and flag pole so I welded on a couple of diagonal braces. These start half way along the forks on the V brake mounting points and then angle in and forward to slot into the handle area of the box. A horizontal piece is welded between the two providing the mount for the mud guard and flag. I haven’t yet build the quick release and there are subtle differences in the dropout of the forks I am using which make me concerned.

Two wheeled trailer two¶

This trailer was built using parts from a failed, non welded trailer built using two 16" wheels and two 26" front forks. The original attempt was designed to have a very low center of gravity with a flat bed. The wheel were mounted right at the back which meant about 50% of the weight was placed onto the bike which is not normal for two wheel trailers. at first I tried a seat post hitch but as soon a weight was put on the trailer the front part would hit the ground. I then tried a chainstay hitch which worked much better but if weight was place at the front on the derailer side of the trailer then that corner would dip down and hit the ground.

I scraped the project when a friend wanted to build a trailer. We took it to pieces and started again, this time mounting the wheels more centrally. The construction used angle iron (from road signs) which was welded to the forks. The frame flexed and twisted a lot until we gave up with the flat bed idea and built sides and a back to create bracing and protect the wheels from the carried load. The design criteria called for a trailer narrow enough to go through doors and with a seat post hitch so that it could be used as a hand cart when needed. We used a bent piece of 3/4" conduit for the hitch arm then rubber hose and a piece of rubber cut from the side wall of a car tyre for the actual hitch.

Apart from the problem with the twist which was solved with additional bracing, the other main problem was the fact that the forks where positioned in such a way that the dropout slots face up and towards the rear. This increases the likelihood of a wheel popping out which is exactly what happened when we tested it will a person on the back.

Construction Considerations¶

Two wheel vs One wheel (mono wheel / single wheel)¶

Single wheel trailers track accurately behind the the bike, but apparently they may end up steering the bike and putting excess stresses on its frame. Carrying more than 30kg on a monowheel trailer is not recommended. They are apparently best for off-road use or smallish loads too large for panniers, but the load is still attached to the bike like bicycle panniers. About half the trailer-weight is on the rear axle of the bike. With bicycle panniers, monowheel trailers, and courier bags the load is rotationally attached to the bike, whereas with a two wheel bicycle trailer it is not.

This means that two wheel trailers are theoretically more efficient at carrying a given load, and have the least effect on bicycle handling. Two wheel trailers have their centre of gravity just in front of the wheels, so very little weight is on the bike. If you can do it right, zero weight is on the bike. This cannot be done with a single wheel trailer. For mostly on-road use and/or carrying moderate to large loads, a two-wheel bike trailer is considered the best option.

In terms of construction, two wheel designs are a lot more forgiving of bad design and shoddy construction. There is a lot more flexibility in terms of hitching types on two wheel designs than there are with monowheel designs.

Two wheels¶

much greater potential cargo capacity
stable when loading and unloading
usually wider (so harder to weave through traffic)
can roll over when rounding a turn at speed
More stable at low speeds
Climbs better (since it does not need momentum to remain upright)
Easier to make

One wheel¶

narrower (great for trail riding or congested streets)
leans with bike (= less likely to roll over when rounding corners at high speed)
comparatively low cargo capacity (both in weight and volume)
must be loaded and unloaded symmetrically to keep trailer from tipping over
some have reported they can wobble when loaded heavily
Easier to add suspension
Not as forgiving of poor design and construction

Choice of Wheels¶

Smaller wheels theoretically bounce more and are ‘slower’ due to higher rolling resistance but in practice it is probably irrelevant. Smaller wheels make it easier to build something with a low centre of gravity and I think they are probably stronger. Wheels are either steel or alloy. Alloy is lighter but may not be as strong as steel. Really it’s a case of using what you can get your hands on, but ideally I’d say the best bet are 20" BMX wheels with loads of spokes. BMX wheels are very strong but not as easy to obtain as 26" mountain bike wheels. Kids bikes can provide good wheels too but check there are actually bearings as some don’t. Wheel chair wheels with half axles are also very handy but harder to come by than bike wheels.

Some possibilities¶

27"/28"(700mm) road/racing bike (weakest option)
26" mountain bike (very east to obtain)
24" kids mountain bike (good compromise)
20" bmx and kids bikes (best option for two wheel cargo trailers)
16" from kids bikes (good, best for single wheel design)

Guide to Kid’s Bike Sizes
Age Child’s Height Bike Wheel Size
2 – 5 26 – 34 inches 12 inches
4 – 8 34 – 42 inches 16 inches
6 – 9 42 – 48 inches 18 inches
8 – 12 48 – 56 inches 20 inches
teen 56 – 62 inches 24 inches

Construction material choices¶

Metal¶

Angle Iron¶

Proper angle iron is about 4mm thick and almost 2" in either dimension. It is generally too heavy for bike trailers. You can get smaller angle iron which is used to make signs used at road works so it’s easy to obtain. It measures about 3mm thick and 1" section. Another type of angle iron which is lighter is the stuff with holes in (Dexion) which is often used for industrial shelving. It is generally about 2mm thick and either 2" square or with one face wider than the other.

Steel Pipe¶

Another easy to obtain metal for construction is conduit or steel water pipes. These seem to typically be 3/4" but apparently you can also get 1" and even 1/2". It is often galvanised so you there is a major health risk associated with welding or using a grinding disk. If you are going to use it then you should sand off the zinc and work in a well ventilated area, ie outdoors.

Box Section¶

Box section / square section metal could be good to use. It should be more resistant to buckling than angle iron. You find 1" square quite common – often found forming the frames of office desks/table etc. Smaller sections may be found on some types of chair.

Shopping trolley www.instructables.com/id/Shopping-Cart-...

Wood etc.¶

Wood is light, strong and easy to work with. You don’t see many wooden trailer but don’t let that put you off. If you have tools and skills to make good joints then go for it. Use angle iron to make the axle mounts then go to work with whatever wood you have available (pallet?).

2×2 Timber (often found holding up estate agents signs)
2×1 Timber
Plywood sheet
Bamboo

www.instructables.com/id/Blistering-Bar...
www.instructables.com/id/Simple-Wood-an...
www.instructables.com/id/Bike-trailer_2

Plastic¶

PVC pipe www.instructables.com/id/pvc-bike-trailer http://www.instructables.com/id/PVC-Bike-trailer_1/
Plastic stacking boxes

Bike Bits¶

Wheels (obviously)
Front Forks (good for holding wheels)
Rear triangles (also an option for holding wheels)
Side pull, steel, brake calipers (make hitch points)
Head sets (swivel joint for single wheel design)
Bottom brackets (could provide bearing races for axle)

Construction methods¶

Constructions methods depend on your material choices but also on the tools you have available. If you have metal and an welder then welding is an obvious choice. If you have a drill, bolts and timber then you use bolts.

If you have no tools then using lashings. U’clamps are a very versatile device and you could even use hose clips to hold your trailer together.

Hitches¶

Possible hitch points¶

Seat-post: Temporary or permanent clamp assembly attaching trailer hitch to the seat post.
(see this interesting post www.tonystrailers.com/singlewheelers/)
Rear axle – one side
Rear axle – both sides
custom pannier rack mounting
Chainstay: Two-piece sandwiching clamp screws tight over left rear triangle, with protruding socket-and-pin receiver.

Hitches at the seat post are handy as they provide a handle for the trailer allowing it to be operated by hand separately from the bike. However, under braking (especially with heavier loads) there may be a tendency for the trailer to try to lift the back wheel. Arguably a hitch point near the rear hub is much more stable, although there are contradictory schools of thought on this. When a bike turns the trailer is at an angle to it and it brakes are applied while turning then the trailer will push against the seat post and may cause the bike to jack-knife. The further the trailers hitch point is from where the bike tyre touches the ground the greater the force generated is.

Hitch types¶

Ball joint / Ball Rods www.instructables.com/id/How-to-make-a-...
Universal joint
Hose pipe www.instructables.com/id/Easy-Bike-Trai...
Spring
Castor wheel www.instructables.com/id/Bicycle-traile... http://www.instructables.com/id/Building-a-strong-flexible-bicycle-trailer-coupler/
Airhose quick release www.instructables.com/id/Quick-Hitch-fo... http://www.instructables.com/id/Bicycle-Trailer-Hitch—-Quick-Release-hose-couplin/
Others www.instructables.com/id/Trailer-Hitch http://www.instructables.com/id/A-bike-trailer—Thats-unheard-of/step4/Rear-front-fork/

Commercial www.halfords.com/webapp/wcs/stores/serv...

Build a bike trailer