Design decisions

posted 18 Dec 2011, 05:23 by Jocke Selin
When venturing onto something as mad as trying to build a practical cargo bike out of an old bicycle you have to scratch your head quite a bit. There's many more design decisions than you can initially think of. Here's a few noggin' boilers that I've encountered regarding the steering on the Carrygo.

Longevity means common parts

Because the Carrygo is personal and customised to your requirements, it can't be made as a throw-away-bicycle. It has to have a long life, this in turn means that all parts and all design decisions have to be selected and made with the objective of either being indistructable or repairable with common parts. If a part is specially made and it breaks, it means that the owner will either have to source a new custom part from me, or have someone make a one-off part, or the worst; have a broken bicycle. Neither of these options are viable. Should something break or wear out, I want the owner to be able to take the bike to the local friendly bike shop and purchase a part off the shelf.
To me as the builder and designer it means I must stay away from anything that's exotic or hard to come by. In fact, I should buy all the parts from the same ecosystem where my customers would buy from. In practical terms this means that the local DIY shops, the local bike shop, etc, become my good friends. The downside of this is that some parts of the design will have some of that DIY look and inevitably will not look as "factory" made as if I would have opted for custom made parts. The image is of the steering rod and the "Rose joint" ("Heim joint" in the U.S). This is a standard part available from shops that sells ball bearings etc.

Dream steering as an example

Let me tell you about a design decision that I've had to refrain from. My friend, Mike Tryphonos, builds hub-steered motorcycles, see his website. I've been fascinated with those bikes for a very long time. Better handling and lighter weight. Then I saw Elian Cycles cargo bike with hub steering and it blew me away. The design is simple and very robust, just beautiful.
I started sketching on a hub-steered version of the Carrygo, and the benefits were obvious. I could remove about 200 - 250mm off the height of the front cargo rack. That would mean that the heavy load would be lower and thus the centre of gravity would be better and the handling would improve. It would also mean that you could carry higher loads without obscuring the view for the rider. This is all thanks to not requiring to have the fork's head tube.
The drawback is clearly that it would require a custom made steering front hub. Not only would that have required a lot of effort to design, prototype, and test, but once that stage would be complete, they would be expensive and/or time consuming to manufacture. Finally, if it would break, it would leave the customer in the aforementioned pickle; most likely with a broken bike. I really didn't want that to happen, so I've had to shelf that idea. Maybe I can revisit it as a design exercise one day.

More steering issues

Steering version 1. Not a very good option. Fixed now.
With my "organic engineering" approach, I will encounter issues that need to be improved, after they have been created. The first version of the steering was such an issue. I built the frame and the steering as I was progressing. I quickly noticed the reason why most (well, all!) front loading cargo bikes have a long steerer tube that goes all the way to the bottom, such as the Bilenky Cargo bike. However, that requires a custom frame or quite extensive modifications to an existing frame, so I didn't do it. I can laugh now, but I must admit I felt quite stupid when I couldn't get the steerer tube into the head tube. After a bit of "modifications" I had fixed the problem, but it was ugly. Ugly I could have lived with, but the way the steering worked, didn't feel right. The "problem" was that the steering rods crossed the bottom tube, and it required the "rear" steerer tube to be created in two parts. More parts, more failures. Not good. An improvement was required.

Steering version 2

This is a much better way to handle the steering. Version 2 works.
The solution was to create a "hole" in the bottom tube so that the steerer tube could fit into the headtube without any problems. This also allowed me to put the steering rods below, and parallell to the bottom tube. This made the steering a lot more pleasing to the eye and because of the improved parallellness (warning; not a word!) the steering was also improved.
By using two steering rods, I've not only improved the robustness of the steering by doubling the fail-safe, but also because the rods now push-pull, there's no requirement for a centre fixed point (such as a bearing) at the bottom of the rear steering arrangement.
The steering is now working perfectly and it's much kinder to the eye.

That's how it goes

I hope this little article has given you a bit more insight into the development and manufacture of the Carrygo cargo bike. What seems simple on the surface might have a complex story behind it. Solving these issues and making the decisions, are part of the fun. If you have any questions feel free to give me a shout on Twitter; @CarrygoBikes.

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