Firstly a disclaimer. As this bike was a personal experiment then I didn’t use the regular amount of planning and modelling. Many methods would not be used to make a proper product and currently I’m constructing a jig to hold everything in place when making this kind of bikes in the future.
With that cleared away, let’s dive into this DIY Cargo Bike project. It was actually started 2,5 years ago and I finished the bike roughly 8 months ago. I wanted a long john type cargo bike when we decided to move to Estonia, as there will not be a luxury of so many cycle paths. So already when still in Copenhagen I started the project.
- The geometry.
Something that irritates me a little when people see my bike is they say – oh did you make a Bullitt replica. In part that is true. But also true is that long john type cargo bikes were used a hundred years ago, so you shouldn’t be afraid to make something similar. Of course most of the geomtery should be similar to an existing bike that has been tested and sold with success. So you can use a lot of the geometry here: http://larryvsharry.com/wp-content/uploads/2014/08/technical_wireframe_bg1.png
Improvements for next time: a) Plan as much ahead as possible. If you have access to 3D CAD program then use it. Model everything, check that nothing collides when steering etc.
- Chopping an old bike into pieces. Welding a new down tube.
I knew I wanted to do as little work as possible, so I found an old bike, dissembled everything and started cutting away. After cutting away the down tube I took a long 2 meter tube I wanted to use for the new down tube. Then I aligned it to the middle of the fork and welded to the bottom bracket. It is important to keep the fork on the bike at this point as I was not using a jig and needed some reference points. Keeping the new down tube long would also be giving me reference points for later alignment of the new fork. Before welding, I made sure the seat tube and new down tube were parallel by bolting both the welding table. PS! If tubing sizes are different then you need to use something for distance under the frame tubes if frame is of smaller tubes. Improvements for next time: a) I would consider building a jig even for 1 piece. b) If not then I would run another tube under the down tube to align the new down tube with the rear axle center. c) I would make the down tube from round tubing as it will have some rotational stress.
- Welding on a new top tube.
Now I cut off the steering tube and cut the old top tube shorter, using it as a guide for the new top-tube alignment. This way I could make sure the new steerer would end up in the middle of the bike. Had I used a men’s frame I would have probably kept the original top tube. Note: keep the guiding tubes on the frame as long as you can. They help against heat distortion from welding. Improvements for next time: a) Make this from round tubing for more resistance to rotational forces / torque.
- Welding on the new head tube.
Now this one was the biggest mistake and can also be for you, so pay attention and read improvements. Steerer postition will affect the whole handling. So what I did correct was to make sure the head tube is in position before welding the cargo bed. All the tubing in between the bottom braket and head tube can be misaligned without a problem – if the head tube, rear triangle and the bottom bracket are located correctly. So using the 2 meter long new down tube, I could be quite confident that the middle of the front hub would be in the correct position. But getting the fork exactly straight in relation to the frame is more tricky. I used an extra piece of tubing and welded it to the new down tube. The other end was bolted to the fork.
Now I didn’t double check the alignment properly here. I would strongly suggest using an inclinometer here. Or at least a spirit level to make sure the seat tube and head tube are aligned. Next comes the head tube angle. You need to make sure you have proper amount of trail in order to keep the steering stable. I didn’t pay enough attention to this and ended up with almose half the trail of a Bullitt (more about trail and steering here). Improvements for next time: a) After setting up the fork take a photo of the frame as best as you can from the side. On your computer open the photo with image editing programme and calculate the head tube angle or just compare it to this blue print of a Bullitt. b) I would buy an inclinometer from the web to check all angles. They are quite inexpeniseve , starting from 10€ on the web.
- The Problem.
I mentioned a the steering will be where you can make the biggest mistake. What I ended up with is speed wobble when running over bumps at speeds over 30 km/h. Science is not exactly sure why it happens. In my case I think it is small trail combined with slightly misaligned front and back wheel and square tubing for top tube and down tube which make the frame flexible sideways. What I did to compensate is that I added a motorcycle steering damper. This worked, but I want to experiment further. This year I will cut the fork again and try to increase the trail. I will post an updated later this year whether this worked. I believe Bullitt actually also has trouble with high speed steering stability as they also now have a steering damper for as an extra option.
To be continued…