Charlie Pendejo

I want to mount a friction drive motor from QiRoll on my Bike Friday tandem, for occasional help with the bigger / steeper hills the kiddo and I encounter touring.

The main mounting option clamps to split chain or seat stay, which this bike lacks. They tossed in an adapter which will instead clamp around a circular ~44 mm tube on a Strida. Using a shim to widen my ~26.5 mm tube seems like the most straightforward way of making this happen.

Looks simple enough to have a couple half-cylinders 3-D printed, using any of a variety of materials (glass-reinforced nylon, maybe?) - any concerns or alternate suggestions?

I suppose an entire new bracket with the correct inner diameter for my bike's tube could be 3-D printed - thought with no prior 3-D design experience, that seems forbiddingly complicated. Or on the other end of the spectrum, simplest thing might be to simply wrap the tube (with tape, metal foil, ...?) to widen a section, but that seems sloppy.

Thanks for any suggestions.

Sonofamechanic 

26 to 44 so a ~9mm thick shim?—you will want something that doesn’t compresses
too much after that much build up—so tape is likely out and metal foil wrapped almost 1cm thick will be difficult to wind on tightly for the full extent and then bind down on the seat tube properly. But, in addition to your excellent idea of 3D printing a nylon-type shim, also think about thick rubber (e.g. car tire sidewall) cut into strips and arranged parallel to the seat tube and under the clamp (at least easier to source). Since its a friction drive you don’t have to be as concerned with some (slight) movement in the mount (and throwing a
chain). Best of luck!

Andrew R Stewart 

I like metal and my hammer is machining. So I would look at turning out an Al sleeve with the correct O and I diameters then slice it in half to make a pair of clamping shims. 9mm is a huge amount for a shim though.

Have you tried to contact Bike Friday yet? IIRC they have offered an assisted power option on some bikes. Perhaps they have a nicer solution.

I will say that over the years dealing with a number of various friction drive aftermarket devices on repairs I have see a lot of downsides and issues not well thought out. I hope your experience is different. Andy

Kontact

Agree with Andrew - these kind of add-ons seem to work poorly. They never seem to tighten down in a way that effectively deals with the torque on the system.

Given how thick this shim is going to need to be, I would make it out of resin and fiberglass or cotton. Find something 26.5mm, put some sort of release on it and roll the wet cloth around it until you get to 44mm. Use electrical tape to compress it while the resin cures. Slot one side with a saw and flex it over your bike's tube. This will be stronger than printed plastic and less marring than metal.

Charlie Pendejo

Thanks, I may try your rubber strip idea as at least a first pass, before deciding if a custom shim is warranted.

I had an earlier go with metal tape, figuring that should avoid the compression issue, and found that it I pulled it tight enough to avoid crinkling, it was fragile enough to break. And in the end, discovered I had it in the wrong location, oops.

In case it's of use to a future reader: I found it pretty simple to crank out a design file at tinkercad.com and upload it to craftcloud3d.com which then quotes me prices and delivery time estimates from a variety of 3D printing facilities, with a downright bewildering array of material choices. Just the first two sites I found, I'm sure there's other choices.

Charlie Pendejo

Bike Friday offers a hub drive, which is certainly a neater solution. I acknowledge friction drive's downsides, but it has some key advantages:

• far lighter weight will probably still allow me to haul the already-heavy steel tandem up & down stairs at train/subway stations and such
• easily disengaged for zero drag under human power, which I expect to continue using most of the time
• replacing the front wheel with BF's hub drive would mean losing my drum brake, which is handy for downhills on the tandem; an aftermarket rear wheel would mean losing the dual-drive hub which functionally replaces a front derailleur

I figured it was the "least worst" option for occasional assist. FWIW Santana offers a similar unit for their tandems, so at least it's not just me.