Hello,

Because of disability I am in process of heavily modifying a bike. (Trike, actually. It is a Pfiff Primo. It suits my requirements in ways I won't get into here; I'll just say I want to stick with it.) I would like to convert it from an internally geared hub with a coaster brake, to a rear derailleur and freewheel type of gearing. This trike already has a good rim brake on the rear wheel, so losing the coaster brake is fine, don't worry about that.

The problem I am struggling with is the derailleur hanger. There are a zillion derailleur hangers out there but every one of them is designed for a specific frame, and none are for this bike since this bike didn't come with a derailleur.

This bike does not have any bosses for a hanger. The dropouts are angled down and forward.

I have a friend who is a precision machinist, who would help me make something. But she's not a bicyclist, so I need to figure out what the end result should be, so I can explain it to her. I am thinking we can make a part that is held in by the axle nuts but also fits around the shape of the stays, so it won't move. Maybe use the rack mounting point as part of where we attach it. I'm sure we can figure something out. And I visited the Park Tool page here: https://www.parktool.com/blog/repair-help/rear-derailleur-hanger-alignment

I will buy or borrow that tool!

Probably go with the familiar Shimano Deore, though I am open to other suggestions.

My question is this: According to the Park Tool site, we have to get the threaded hole the derailleur attaches to, in just the correct plane to line up with the sprockets. But it is silent about all the other dimensions. Is it OK to just wing it, pick something that seems right, like, by comparing to other bikes and get the same general relationship between the axle and the derailleur attachment point? At which point we just adjust the limit screws and index screw to put it through its paces? Or is there something else I need to do to get it right?

Or, does anyone know of a pre-made, generic hanger that might work for me, without having to machine a custom part?

Thanks!