[Image: Commissioned from Robin Davis for engtect. theodor.livejournal.com ]

And why not? I mentioned in my Brompton review that I had plans for such a bag. Shortly after carrying the bike it becomes heavy. Also it is an awkward hunker-chunker to carry. I think it was due to carrying the bike down a narrow staircase, with it at the end of an outstretched arm and my other arm on a handrail, that I put my back out. Since then I have been careful about lifting it. However it was before that day that I had decided to make a bag in the hope of reducing the burden of carrying it.

There are Brompton bags available: a lightweight nylon job (£25) ideal for storing a wet bike at the office; a large padded bag (£100); and a bomb proof hardcase (£150). These bags are designed for storing the folded bike safely at the office, boat or garage, and the latter two also suited for transporting it. I want a bag to carry the folded bike which I can easy take with me when riding the bike.

So I have made one.

I have had fun designing and making it, as I did for my reading stand but hope that it actually gets used. Although with my dissertation kicking off I guess the reading and copy stand will become useful.

I suppose my folding bike-bag design brief was:
• Carry folded Brompton safely
• Empty bag is minimal size so it is easy to take on bike or in rucksack
• Easy to use
• Durable

Early Sketches
I did these during revision for last exams…

I started thinking of the bike as a box to which I removed the unnecessary parts of the surfaces. I started to think about putting the bike inside it and flexibility.

Cardboard box model
I wanted something three dimensional to fumble to better appreciate how my initial designs did work and didn’t work. The picture below shows how a design could carry the bike on the back or under an arm, for example if I already had a backpack with me.

After playing with my Brompton I decided not to have the dual aspect of the bag position as it really is heavy and can only go on my back.

More sketches
I kept drawing the same things really but it kept me thinking. Also I seemed to be drawing squashed frogs.

Final Sketch
So I reckon I found the design:

There are pockets in the back part that house pieces of 6mm ply to stop the Brompton from sticking into my back. And a notion of folding:

CAD Drawing
I took some slap dash measurements of the bike and me to make a computer aided drawing in AutoCAD at work, which if I did again I would use a scaled photo taken from distance as the base for my Brompton dimensions.

The left image is a rough outline of me. The middle image is a rough outline of the folded Brompton. The right image is the parts that would be against my back.

Using this info, my final sketch and loads of steering at my bike, I came up with this:

I printed off a copy to see how it folds which was not especially useful and I daydreamed that the finished product would fold to this size.

I did print out another to make a 1:6 model. This was useful for my mother, who had offered to sew the final one together, to see how it fits and works.

Mum traced around a 1:1 print I gave her, used some Hessian like material, sewed straps and Velcro bits in, and did all the edging. An old rucksack of mine was sacrificed for the buckles.

The Prototype
Here it is folded:

and on my back:

Conclusion
So it works: it is actually comfortable to wear and the weight is very easy to carry. Lifting the bike onto my back is fine for me to do, although I expect others might choose to rest it on a little wall or something handy. There are a few parts of the pattern that could be better shaped to fit the bike now I see how it works - so maybe next time. The bike is securely on and the bag is durable. The question is whether I would use it or not? For example the next time I go the Tate Modern will I walk around with it on my back? That would take quite some balls I think…