Isn't it always the smallest things that take the longest time? Having spent a couple of 12 hour days going over the CAD model, checking every part for interference, correctly sized holes, bend radii and correct fit to the adjoining parts; the assembly model looks pretty much the same as when I started! Not that there haven't been any changes, just that they are all so small that there is no sense of progression…
Some parts I am really pleased with though, like the completed rear dropouts (I only redrew these 5 times yesterday!):
Having said all that, this week has left me feeling most like an engineer; with the detailed refinement of the parts, and adjusting them so they can be drafted for manufacture. Much as I enjoy the imagining up all the parts at the beginning of a project, I'm really excited to see them cut. I'm finishing up the drawings, so they should go for cutting this week.
Until then, I've had to settle for buying shiny bits! All of the driveline parts are off-the-shelf bicycle bits, so I've got wheels and cranks and tyres and brakes and pedals in a nice, mechanical pile. Most special of which is the Sturmey-Archer gearbox. <!-- more -->
In the end, I had to settle for the three speed QS-RC3. I did want to get the 5-speed QS-RC5(W) (with it's huge 139mm over locknut distance [OLD]), but I'll have to wait for another shipment from Taiwan; 6-weeks will be a little late for Maker Faire, and three gears is still better than one!
The QS-RC3 is designed as a centre-mounted gearbox, instead of a gear hub, so it has an output sprocket screwed onto the drive side spoke flange, through half the spoke holes. The only reason I can think of why this gearbox isn't available as a hub is the extra width of the coaster brake/reverse unit on the left side; at 127mm OLD, it's likely to be too wide for most bike units.
That output sprocket is only bolted in place though, so I'm off to see if it can be built into a wheel. I think that an aluminium shim inside the wider, threaded holes will be enough to reduce the likelihood of spoke fracture because of the concentrated loads, but I'm going back to the wheel builder to get an expert opinion on that. Otherwise, It'll be mid-mounted, through to a fixed-gear rear wheel.
I have some 20mm thru axle hubs with disc mounts for the front wheels, and one of the advantages of having wheels built to order is making little adjustments to suit the application. For these, I've had the rim centre brought a couple of spoke turns towards the inside (disc) side to reduce the bending moment on the stub axle as much as possible. For a single sided mount, the rim doesn't need to be central to the axle like it is on a bike.
This also means I can remove the outboard bearing cover/axle spacer from both hubs, and bolt the axle up to the bearing; this will leave a much smaller stub on the outside of the wheel, and should mean that the tyre is the outermost part of the vehicle.