This instructable shows a secure method of carrying a bike with another bike. It was created in response to a particular problem I encountered - at 10 years old, our daughter is too big to fit on the back of my bike and big enough that we can cycle to school together. But she takes the bus home - so only needs the bicycle for one way. She could use a dockless public bike, but these are heavy and not really designed for a 10 year old. And it is a 5k ride. After much thought, we bought her a Dahon dove lightweight folding bike. This was modded with 16" wheels rather than the standard 14" for stability and many of the components replaced with lightweight versions. The bike weighs just over 7kg and packs up very small.
Initially I thought the bike might fit into the seat already mounted on the back of my bike. This sort of worked (see picture) but was not very stable and I thought I could do better.
I decided to use the seat post as the anchor point. This is very stable and close to the centre of gravity of the folding bike. It can also be moved up and down very easily to accommodate imperfections in the design.
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Corrugated cardboard to make tests/template;
Aluminium or steel tube that is a good fit with the folding bike seat post (in this case around 34mm)
Plywood for mounting rack;
Heavy duty cable ties, or alternative means of securing plywood mount to rear rack;
A tubular rest/mount for the wheels (I used part of the seat that I had just taken off the rack, but could be improvised with many other parts, or just built into the plywood holder.
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Step 1: Cardboard Prototype
After working out that the folded bike could sit on the rear carrier, I set about designing a suitable support. The main support comes from a socket into which the bottom of the seat post can fit. This is affixed to a plywood board that is then attached to the rear carrier. Prototyping with corrugated cardboard, I was able to get a reasonable level of accuracy and was able to test that the bike would sit inside the holder.
I had originally imagined that the seat tube socket was something I would have to fabricate, but in the end I found something suitable off-the-shelf in aluminium. I think it was a leg for a bed or other piece of furniture. Another way to make the socket would be to build up a few layers of plywood and drill them out to the correct diameter.
Step 2: Cut Out Plywood and Affix Mounting Tube
After testing the fit with the cardboard prototype, I transferred the dimensions to a piece of 18mm plywood and cut this out with a jigsaw. The supports for the wheels were made with a piece of leftover bent tube. If you don't have anything like this, you could just make the plywood mount slightly longer and cut recesses in which the wheels can sit. The seat post socket is held in place with recessed bolts (see pic). Wood screws not recommended because of the forces on this component.
Step 3: Go for a Ride!
It is a good idea to secure the folding bike with a bungee or two to stop it moving about too much, but I found the seat-post anchor to be stable enough not to need it.
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