Electric Full Suspension Cargo Bike
Inception
During the summer of 2022, Cal Poly Bike Builders was contacted by an individual looking to buy a custom full suspension cargo bike for personal use and potentially a startup. My friend Colin Reay and I accepted the project, and begun a very long and challenging project to create a one-of-a-kind bicycle.
Concept
Longtail cargo bikes such as the RadWagon have become extremely popular in recent years for their electric assist and cargo space. However, the small wheels make for a bumpier ride than a standard bike. Our primary goal was to make a "fun to ride" cargo bike, mainly by adding front and rear suspension.
Yuba Kombi Cargo Bike
Yuba Kombi and Radwagon overlaid for geometry comparison
Design Process
We started by comparing existing cargo bikes to look for features and geometry that they had in common. This also allowed us to see whether packaging a rear suspension swingarm would even be possible within the rear cargo cage. After determining that such a mechanism might be possible, we started looking for various frame geometries that would be strong and aesthetically pleasing.
Design Iteration
We went through multiple design iterations to refine the frame geometry and manufacturing method for each part. Cost wasn't a huge issue in our case, but the part count was extremely large, and minimizing machining time for each part was critical. We finally settled on a design which we thought had the most likelihood of success, in strength, stiffness, and rideability as well as manufacturability.
Initial sketch based on the geometry of other bikes. Mainly used to verify that the rear wheel would have enough travel within the rear subframe
The second geometry concept which allowed for the battery and used a "single pivot" suspension for the rear. Single pivot refers to a bike suspension system that doesn't involve any linkages, which creates a fairly linear force curve over the suspension actuation.
Final Bike Design
We switched to a suspension system with a linkage in order to change the leverage of the shock though the travel of the arm. This creates a suspension that activates easier on small bumps, but stiffens up on large bumps preventing the suspension from bottoming out. While definitely improving the bikes handling, it did add a lot of complexity to the design and manufacturing.Â
3D view of the full bike frame
Bike with third party Yuba accessories added. Designing around these added extra difficulty to the already complicated packaging.
Being that this is a cargo bike, the weight distribution is variable and somewhat unknown. The center of mass of the bike greatly affects the rear suspension and how it feels. We decided we had greater odds of achieving a bike that is "fun to ride" by allowing for two different suspension configurations. The top left image shows the "low pivot" configuration, and the bottom left image shows the "high pivot" configuration.
3D view of the high pivot configuration
The rear subframe enables the attachment of crates, panier bags, child seats and footrests.
No cargo bike is complete without a beefy kickstand to hold it up
Fixturing Rabbithole
I learned a lot about manufacturing through this project, specifically fixturing for welding and CNC machining. We started with creating the bicycle design manufacturing in mind. Welding the many tubes required welding fixtures, which in turn required CNC machined parts. These CNC machined parts required fixtures/workholding in order to perform machining operations. This same process happened in reverse during the actual manufacturing of the parts. The rabbit-hole of fixturing was not something we had properly accounted for during our initial proposal for the project, and as such it added a considerable amount of time to the project.
Parts I Worked On
Kickstand Fixture
I designed the Kickstand Fixture plate with locating blocks for both tubes and mounting plates. Locating blocks were made symmetric to reduce CAM and operation setup time
Kickstand Fixture with kickstand tubes and brackets
Locating blocks that I machined in 4 operations
Completed Kickstand Fixture. The blocks were located using 8mm dowel pins.
Kickstand tubes and brackets in fixture
Kickstand after some exellent welding from Colin
Kickstand Main Plate
Kick Stand Plate after Op1 (Machined From a large piece of C-Channel)
Kick Stand Plate after Op 2
Final Kick Stand Plate, designed by Colin and machined be me. This was certainly a challenging part to machine due to the unique stock shape.
Kickstand Fully Assembled
Lower and Upper Swingarm Fixtures + Welding
Lower Swingarm Fixture design. The base plate is shared between both the upper and lower fixtures, and the locating blocks are swapped out.
Lower Swingarm Fixture design with lower swingarm overlaid. The pegs used to locate the tubes ended up being abandoned as they actually over constrained the tubes.
Upper Swingarm fixture design
Upper Swingarm Fixture design with upper swingarm overlaid
Base plate after a lot of drilling and rigid tapping
Lower swingarm fixture dropout blocks. Second op was held using softjaws.
Upper and Lower Link locating blocks
Upper Clevis locating blocks
Lower swingarm fixture after tacking the tapered "chainstay" tubes to the cups that Colin machined
Cross tubes mitered and a few more beads laid down
Some closeups of my tig beads. Definitely room for improvement, but a far cry from the holes I blew and mounds of filler I put down on the e-pixie. I've found that stainless filler rod is much more controllable than mild steel.
Left and right "seatstay" tubes welded to more of Colin's cups
Adding the lock mount bracket
Completed Upper swingarm
Upper and Lower Swingarm side by side
Checking clevis fitment after welding
Battery Mount
To better support the battery I designed 2 slotted plates to provide an adjustable mounting solution and torsional stiffness.
"Bottle bosses" I fusion welded to the bottom of the frame to bolt the battery plates to.
More Stuff (There's a lot of it)
Colin bending the rear top tube with our fantastic Cobra TOOB Bender
Colin is an absolute maniac (in a good way) when it comes to designing, machining and all around bike building. He works at an inhuman pace and put an incredible amount of hours into this project. While the parts I owned were significant in bringing about the final bike, without Colin this bike would never have had a fighting chance of becoming a reality.
What follows are pictures of various parts of the fixtures and bike that I grabbed over the course of this project. There is a ton of fixturing, workholding, machining, mitering and welding that are missing from these.
Linkage arms after the first op
Completed linkage arms
Steel dropouts with aluminum inserts
Rear front triangle tube fixture. This assembly contained the two different pivot positions, whose positional accuracy was important.
Rear front triangle fully welded with both pivots and rear subframe mounts
Extrusion "pictureframe" loaded with the fixture plates for the front triangle.
Down tube, bottom tube, rear tube, and head tube in the fixture
Seat Tube and top tube added
Fully welded front triangle. First tacked in the fixture, and then fully welded off of the fixture for better access.
Rear subframe in the repurposed front triangle jig. The sheer amount of tubes and mitering in this part of the bike made it extremely tedious. The blue tape and 3d printed parts featured here were temporary just to check the fitment of all the tubes
Complete frame with front triangle, rear subframe, and kickstand. There were a lot of parts of this process that unfortunately never got pictures taken of.
Full frame after powder coat
Pictures of the "final" build post powder coat. The battery was yet to be installed and tested, but all of the mechanical work was complete. Our client was super happy with the result, and the bike/scooter is a blast to ride!