Mini Cargo Bike


Motivation

I wanted to build a cargo bike to serve as a commuter and grocery getter, but didn't want to invest in the custom fixturing required for a long-john style cargo bike. I really loved the look of the Omnium Mini, and the (relatively) standard bike frame design meant that I could build the bike in my school's standard Anvil frame jig.

Design

This was the first bike I designed, so I relied heavily on the geometry provided by Omnium on their website. I began with an overall 2D sketch of the bike geometry, and then filled in the details to create a full 3D model

Unique Features

ChainStay Yoke

A large portion of the design and machining work in this build went into the chainstay-bottom bracket yoke. These steel parts allow for chainring and crank clearance and a large rear tire without the need for a complex bend or dimpled tubes. My yoke is loosely based on the paragon machine works yoke, but specific to my bike geometry. 

There are two symmetrical halves of the yoke which are hollowed out on the inside to reduce weight. The halves are then fusion welded together (no filler) to create a smooth outer surface.  

Yoke with chainstays, bottom bracket, and rear tire

Isolated view of the yoke

Top view of yoke profile


Internal pocketing of yoke

Stock for the two halves and fixture plate

Completed 1st Operation

Machining the Yoke

Machining these parts really helped me grow as a machinist, since they involved custom fixturing, small internal pockets, surfacing and of course the parts were made of steel which I had never machined before (on the CNC).


Each half began its life as a half inch thick steel plate. The first operation involves machining the inside of the yoke, including the pockets and outer profile. 7 M4 holes were also drilled and tapped to allow holding the part on the second op.


IMG_2561.MOV

Video of 1st Op Machining

Fixture plate Op 1

Fixture plate at the end of Op 2



The aluminum fixture plate was then machined, first by facing the backside and adding a locating bore. The plate was then flipped and the profile of the yoke was added, along with 7 M4 clearance holes.

Yoke 1st Op attached to fixture plate

Yoke mounted on fixture plate in vice, ready for Op 2

Second Op after roughing pass

Second Op after the finishing pass

Pie Slice Downtube "Bend"

A large part of the Omnium Mini look is the continuous downtube with a bend in the middle. I wanted to use a 1.5in tube here for added stiffness to the cantilever front rack. However, my school didn't have the facilities to bend a thin 1.5in tube to a 40° angle. I ended up deciding to instead make a series of angled "pie slices" to create the effect of a bend.

In order to get the pie slices to align correctly, I made a custom jig to hold the 4 tubes involved. It features 4 tube holders attached to a base plate, and uses V-Block welding clamps to hold the tubes in place. 

I ended up tacking the slices while holding the tubes with zipties, mainly because I realized once welded the V-Block screw clamps would be stuck on the frame (oops). Zipties also gave me better clearance to weld. Since I was only tacking, no ziptie melting occured.

Front Rack

I wanted the front rack to be removable in case it ever got bent or I needed a custom rack for specialty cargo. To do this, I designed weld-on rack mounts that create a flat surface on the 1.5in tube on the front of the bike.

Rack Mount Top

 

Rack Mount Bottom. Pocketing reduces weight

1st Op after Roughing

1st Op after Surfacing

Completed 2nd Op

Front and back of the two rack mounts

I experimented with some engravings on these parts for fun. I was unsure how they would look after powder coat, but they're still visible. Check out the glam pics! Ultimately these engravings are covered by the front rack, so they're just little easter eggs for myself.

The rack itself is constructed out of steel box tubing

Getting nice with the welds

A little rainbow is always nice

Waterjetted mount tabs were added to the rack to create holes for mounting baskets/crates, as well as serve as hook mount points for things like bungies or straps.

Studs were added by turning the head off a stainless M8 bolt, which were tacked into place using the rack mounts themselves as a guide

A flange left on the stud allowed for easy fusion welding.

Front Triangle

Test Fitting the head tube, seat tube, and front tube in the Anvil Frame Jig

A collection of fusion welded bottle bosses (threaded inserts) for mounting things like bottle cages and cable management

All front triangle tubes tacked in place

Front Tube - Head tube joint weld

Bottom Bracket joint weld

Seat Tube - Top Tube joint weld

I added a spare bottom bracket shell to the front tube to allow for threaded inserts (custom flashlight?) to be added in the future.

Tig brazed Top Tube - Head Tube gusset

Rear Triangle

The miters and welds on the rear triangle were much more difficult than the front, but the use of my yoke definitely reduced the hassle. The paragon hooded dropouts also helped make mitering the rear a little easier.

Mitering the back of the seat stays was easy thanks to the Cobra Stay Slayer

Tacking the chainstays and yoke

Tacking the seatstays

Starting to really look like a bike!

Brake braces help distribute the moment created by the disk brake. Some zip tie fixturing helps get the job done

A seat stay bridge helps stiffen the rear triangle and allows for mounting a rear fender

Rear rack mounts were silver brazed onto the seat stays

Rear wheel and rear rack test fit

Fork

To help finish this bike in a timely manner, I decided to modify an existing fork rather than build one from scratch. I picked up a bmx for for a 20in wheel, sandblasted it, extended its steerer, and added a disk brake mount.

A little bit of microflat welding to create a concentric, reduced shank steerer extension

Using a piece of extrusion to align the fork steerer and the steerer extension.

Steerer extension complete!

Test Rides!

First build up! Note the many parts borrowed off of other bikes including brakes, rear tire, and front wheel. The rear hub is internally shifting, so I get the beauty of a single speed chainline plus 8 gears of shifting (even when stopped!).

First test ride!

Rear rack, Front rack, and crate added. The rear tire got an upgrade as well.

First grocery run!

Colin's flex stay bike and my cargo bike headed to powder coat!

Built up and in the wild!