Follow thru and experimentation is the only way to design technical projects
Frame attached bags for city bikes need to solve for technical and practical problems.
Carried loads should remain undisturbed, even when cornering, hitting bumps, tram tracks, or emergency braking. Cyclists need to be able to quickly detach the bag and its contents off the bike to leave nothing tempting for thieves. Bags should resist weather and abrasion while staying reasonably light.
As a commuter with errands, I had a few more rerquirements: THe bag should be trim enough to carry into a musuem with a laptop and sketchbook, but with the expansion capactiy to bring groceries or takeout home when Priss texted asking forr bread and milk.
Why create something new?
Why not panniers or a basket? Well, neither fit easily to my bike. Panniers have good technical attributes such as center fo gravity and quick release, but poor practicality when carrying bulk items such as our ‘friday sushi’ upright. Panniers also tend to carry quick release harware that can snag on things. Its always better to balance pannier loads by having 2 but this is less convieineint for carrying into buildings. Retrofit baskets tend to work well for carrying but can affect handling if loaded. They still need bags. And why not pbackpacks? Its not a great idea for my back.
Early Idea, First try
This was my first bag design, and a with first of everything, somethings just always go wrong.
Sizing was done by fitting a cardboard maquette, then doing tech drawings. For attachement, I thought the bag could quick secure by hooking under the back platform then anchor to a stap attched to the seat tube. The bag would slip between bars on either side of the platform to provide three points of connection and lateral stabilty. A laptop pocket on the bottom provides additional stability as the laptop and padding provide dimensional support. two standard buckles on either side allowed me to attach a shoulder strap. An expanadble compartment coverted the bag from a laptop bag to a duffel large enough for a full grocery load, bulky takeout, or a weekend trip.
Tough Traveler in my hometown of schectady did the cutting and sewing of the prototype.
Fast forawrd a few weeks and a prototype shipped to me.
First fail:
Just before fabrication started, I looked to see how close the template came to my cycling shoes when pedalling, by pedalling stationary and photographing the overlap. Turns out that more than 1/2 inch of overhang with the plaform back results in a heel strike. A quick adjustment fixed this and drove the next issue.
The follow on fail
IN adjusting the bag shape, there wasn’t enough lateral support from the foam reinforced cordura. The bag would swing wildly by the seatpost with no provocation.
A solution from unexpected places:
In experimenting with by wrapping the shoulder strap around the platform back, I could stabilize the bag.
The fix idea came when we had a cushion fixed for a smooth surface wooden bench in our place: upholsterers commonly glue or staple high friction fabric to slick surfaces to keep cushions from moving. I created a paper template for a matching sleeve on the platform back also made of cordura, with matching buckles to snap the bag into– the friction of the fabric and buckles kept things in place. The fabricated sleeve returned a couple weeks later
Its been a couple of years and the bag proved itself- with groceries, commuting, short trips, bike-packing.
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