Ranko Kohime

What is involved in converting an IGH into a mid-drive? I've seen many examples, which usually have the sprocket in the area where a spoke flange would be.

Are there special sprockets that bolt to the flange, or, are only certain hubs available with shells designed for use as mid-drives?

Dan Burkhart 

I've seen cogs that bolt to the flange, but I don't know off hand where to get them. You can buy Sturmey Archer hubs already set up as gear boxes, complete with reverse.
https://www.sturmey-archer.com/products/hubs/cid/6/id/46

tcs

You can make or purchase cogs that have the standard disc brake (a.k.a. 'disk break') six-bolt mount. With one, any std. disc IGH can be used as a mid-drive if the chain line can be arranged.

fietsbob

pretty much how it works , primary chain goes to the hub's cog,
a secondary chain is attached to the hub shell..

Rohloff uses a 4 bolt pattern for the disc mount, and there are chainrings for that pattern in common,
then the second chain goes to the left side of a lefty hub.

Another German company, Pinion, has further developed a mid drive to have frames built around it..
It internally resembles a small motorcycle transmission, 18 ratios..
1 builder:https://www.en.tout-terrain.de/bicycles/silkroad-xplore/

the Sturmey (and Shimano) 3 speed hubs with a Cog on the hub shell
are used on adult upright tricycle builds .

***

Wabac in the JFK early 60's father of a friend , used a similar thing
though it needed to be fabricated in the shop, [he made prosthetic limbs as a business]

On a regular DF bike the hub went up at the top of the seatstay,
second chain went to another 3 speed in the hub.

Jeff Wills

They are known as "Tomicogs": https://tomicog.blogspot.com/

Contact United Bicycle Supply or Bike Tools Etc. about availability of the trike/quad hubs:
https://www.unitedbicyclesupply.com/i...s&tc=Quad_Hubs
https://www.biketoolsetc.com/index.cg...s&tc=Quad-Hubs
I think they're the same company- UBS is wholesale, Bike Tools Etc. is retail.

Dan Burkhart 

UBS shows out of stock. They are low on most hub models, waiting on a container they know not the whereabouts of last I spoke to them.

Jeff Wills

Yeah. When I last talked to them, they said they often don't know what's coming until they open the container.

Ranko Kohime

Umm... DO WANT!

But seriously. Out of my price range. But I still want it.

tcs

Actually, a bottom bracket transmission rather than a mid-drive (that is, as the term has been used since the 1880s).

tcs

That's not what Boone, Phil, or Velo Solo call theirs, but whatever.

xenologer

when I bought mom a khs trike, the mid drive was an ordinary nexus 3spd hub with a cog welded to one flange. Got a tig welder and some aluminum cogs?

fietsbob

Or Know a company owning a CNC laser cutter..
Ive seen some where there were a bunch of holes made in the cog,
to pop-rivet to the hubshell's spoke holes.

Monster Pete

I'd just buy an off the shelf Sturmey Archer gearbox. They're mostly intended for use on delta trikes so can be found with a reverse gear feature- when you pedal backwards, you move backwards in low gear.