Biga, the Bike Trailer




About: I'm an Italian freelance structural engineer, graphic designer and photographer, and I'm teaching physics in Waldorf high-schools. I always investigate electronics, robotics and science in general, I'm a pas...

In support of a bicycle workshop I have decided with my friends to build a trailer to carry the tools around the city.

We were not skilled in welding iron frames, but that didn't stop us, and with some advice we succeeded in building a nice, strong, and effective trailer, which remembers me an ancient Roman "biga".

[first two pictures are created with pixlr express]

Step 1: The Pieces

We started to cut an old and cheap mountain bike frame, so that we had different measures tubes. As wheels I found a pair of children bike forks, so to have an easy wheel connection and to use 20" wheels, not big as 26" but not as tiny as a cart. The green tube comes from another old bike frame, it has an elliptical section, and it matches very well with forks' heads.

Step 2: The Project

As you see the project is quite simple. The forks are horizontal and connected by two little pipes. All this acts as a surface plane, where we will link the toolbox. The main problem in a frame like this is to avoid big leverages on the weld joints, and I planned to stiffen the structure with diagonal pipes for both longitudinal and cross direction. In this way the weak part is the middle section of the vertical pipe, but it proved to be strong enough.

Step 3: The Hitch

The joint has to link generally the bike to the trailer leaving the three axis free to rotate. This is not true if the trailer has only one wheel, indeed in that case the longitudinal axis rotation has to be held by the bike.

We used a kick scooter front part, which appeared to be so perfect for that purpose. Indeed the ring attached in place of the wheel will be coaxial to the seat tube, and both vertical and transverse rotation axis don't add any torsion on the seat post. The scooter steering wheel mechanism acts to freeing the last longitudinal axis rotation, although it's not in the exact horizontal direction. Indeed the three rotation axis need to be not aligned, but not exactly perpendicular one to each other. As you will see in the next animation this tilted axis will help to steer.

Step 4: Final Design

The 3D model came out pretty nice, and I was able to make a 3D animation, from which is clear how the trailer behaves during a turn. The bike and the seat post will tilt around the longitudinal axis, the trailer tries to follow it, but since the longitudinal axis rotation is linked to the vertical axis one, it's imposed to also move toward the inner side. That results practically in a much easy turn and better stability, at both high and low speed.

[click on the image to see a better quality animation]

3d animation

Step 5: Shaping the Tubes

With the angle grinder refine the tubes' heads so that they match with the adjacent pipe section. This will help to make a better weld.

Step 6: The Welding

This is the fun part. With the shielded metal arc welding machine link the tubes together, trying not to pierce them. That will be difficult since the bike frames have usually tubes with thin sections. We used 2.5mm electrodes, but you can probably try with smaller ones. As I said we are not welding masters, actually I'm the exact opposite ;-)

Step 7: Remove Slag

With the right tool remove the slag from the weld, beat it hardy and check that all the edge is well welded. If not put your mask on again and weld the weak sections with more material.

Step 8: Welding Completed

The welding part is completed and at full night we can finally carry the trailer home (we made it at the bike workshop). A real satisfaction, especially after the discover that the feeling is to have nothing to pull. Indeed the trailer is also light and very fluid in movement, with a very low center of gravity, especially if unloaded.

Step 9: Some Finishing

The ugly welding joints need some finishing. So I added a thick layer of two components plaster, I left it dry and then I smoothed it with file and sand paper. Not the best work, but actually the full trailer has many imperfections, and we didn't want to put too much time in this.

Step 10: The Tail Light

To build the tail light I searched a plastic cap which exactly fits on the extremity of the trailer horizontal pipe. I removed a bike LED light from its silicon case. Then I glued the edge if the light inside the cap. Pushing the transparent shell you can easily turn on and off the red light.

Step 11: Painting

With the painting process the trailer arises in all its beauty and (im) perfection ;-)

We painted the frame with a white spray paint, then we added light (which was not so cool) and dark blue layers. We opted for an ochre tint for the kicker scooter joint. Paint on the rotation joint doesn't affect its behavior, but you ought to cover the seat tube ring, so to handle it without ruin the paint.

Obviously two components paints are much more resistant to scratches and abrasions, I really suggest to use them instead of the classic spray cans.

Step 12: Assembling

Now you can assemble the joint using a fast saddle clasp or bolt and nut. Then put the wheels on and insert the tail light in the tube. Don't glue the light, since you need to remove it to change batteries.

Step 13: Adding Toolbox

The toolbox should stay between wheels, over the two small tubes. Keep that in mind when you design the trailer dimensions. You can lock the box with a belt, or trying a smart method as illustrated in next step.

Step 14: Fasten the Box

To fasten the box to the frame you can use four broom holders like ones in the pictures. Attach them under the box so that they hang the forks in the positions shown in the first photo, the toolbox should now stay firm in every direction. I still have to attach mines.

As you can see the toolbox can be opened still on the trailer. Anyway, with an easy and fast method to secure it, as the broom holders are, you can lift the box and remove it from the frame. The holders will act also as rubber feet.

Step 15: Ride Your Bike and Biga

Your trailer is ready, secure it to your bike removing the seat post and inserting it on the pivot ring. If the ring moves up and down along the seat post you can add a nylon band on the seat tube.

The biga carried a person (about 65 Kg) for 20 Km with no difficulty so it's sturdy enough to carry heavy loads too.

It's time to enjoy it, but remember to avoid vehicular roads, it's certainly not the safer vehicle to drive on them.

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    38 Discussions


    4 months ago on Step 2

    I really appreciate this design. The main element that I like is the re-use of bike components.


    Question 6 months ago on Step 3

    For the hitch, did you reverse the steering tube end with the wheel-holding end? That's the only way I can see to get the trailer-holding part to angle down in that direction. As it comes on the scooter, the part that attaches to the scooter platform that becomes the part that attaches to the trailer tongue comes off in the opposite direction.

    Isaac newman

    2 years ago

    nice trailer. Love the hitch idea so much making one now.


    4 years ago

    I can now officially say the bike trailer I made is an absolute disgrace!
    Sweet design, looks really cool. I love that hitch too..


    4 years ago

    Hi Andrea, really like the project, I have built several bike trailers in the past and this one is great! I'm currently building a petrol trike and I'm also learning to weld while doing so, can you please explain in greater detail the size and thickness of the electrodes, I'm debating whether to use larger or smaller diameter ones. Anyway, thanks and love the build!

    1 reply

    Hi Boogie, actually I'm not very skilled in welding, but I can suggest to use a thin electrode to weld light frames, that will reduce the chance to melt the pipes and create holes.


    5 years ago on Introduction

    Ave, Roma! :) A great Instructable, and this might serve also to tow a second Bicycle, to pick up someone...

    Voted, and went to my Blog:

    2 replies

    I rather suspect that when loaded this trailer will be too nose-heavy, with likely severe consequences for handling of the bicycle-trailer combination, particularly when cornering or cornering and braking. When combined with the high-hitch, a bicycle with rear suspension could have its suspension overloaded.

    It is notable that commercially designed and made bicycle trailers (and probably all commercially designed trailers for cars too) have a load platform with the axles, positioned slightly behind the centre of the load platform.

    3 replies

    I don't understand what you mean... if the wheels axis is positioned "slightly behind the centre of the load platform" the nose will be very heavy, but in my trailer the load center of gravity is very near to the center of wheels, so the nose is not heavy at all. The trailer has been already used for many kilometers, it behaves marvelously in cornering and breaking.

    Anyway we are planning to extend the loading platform toward the rear area between the wheels, in that way the load will be even more centered.

    I'm sorry but your reply makes no sense and is self contradictory. If what you say is true "but in my trailer the load center of gravity is very near to the center of wheels, so the nose is not heavy at all.", why would you state: "we are planning to extend the loading platform toward the rear area between the wheels"?

    That only makes sense if what I said were true. If the trailer worked as claimed (when loaded) why would you modify it, except to increase its capacity? *

    Extending the load platform rearwards makes it much more like tried and tested commercial designs, which was exactly the point I was trying and clearly failing to convey.

    *A rearward extension on an already well configured trailer is a recipe for tail-heaviness, which is even more dangerous than nose heaviness because it can unweight the rear wheel of the towing bicycle (especially during braking, turning and conditions encouraging low adhesion), causing loss of grip and jack-knifing.

    We are planning to extend the loading platform toward the rear area between the wheels exactly to increase the trailer capacity, not much in load weight but especially to use that small area to keep a low storage box, which will still leave space for the toolbox to open.

    You are right about the tail heaviness, but here we are speaking about only a few kilos.

    I think that your trailer and mine are quite similar in terms of load and weight positions... in both trailers the centre of gravity is slightly in front of the axle, which I think as you that is the best configuration.