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Wednesday, August 26, 2009

Pedal Freewheel Mk2

I discovered that I don't have the right parts or tools to do this as I had thought I could at the end of the last Pedal Freewheel post. All the screws, bolts, or threaded rods I have that are long enough and thin enough to be welded to the inside face of the crank arm's core don't have any nuts that will fit! Anything I have nuts for is either too short or too large a diameter.

Worse, however, is that I forgot about the face of the BB bearing cup/nut, which sticks out far enough that despite having that small clearance, only has it because it is smaller diameter than the inside face of the freewheel/etc. So I could fit the plate, but not the boltheads or nuts, thus the idea is unworkable with anything I have on hand.

All is not lost, however, because I had a fortuitous accidental discovery. As I was dejectedly spinning the freewheel and hub piece around on the BB spindle first one way and then the other, the threads on the freewheel engaged the threads on the BB bearing cup!

They are the same diameter, threading, etc!

I pulled out the only "spare" BB bearing cup I have, and it happens to be the same diameter, and it is wide(deep) enough to give full-width support to the freewheel (unlike the hub, which only engages about half of the FW's threaded width). The bearing cup is 30mm inside diameter, just a bit smaller than the outside of the crank's core, so if I lathe the outside of the crank to match that, it'd slip right over and everything would fit perfectly. But if I lathe it down, it'll just be a slip-fit (even if it's a tight one), and would almost certainly come off or just spin in place once I really lean into the cranks on hills and whatnot if I ever have to.

Since I don't want to weld the cup to the crank, I can use splines to transfer the power. If I either just file away the unwanted diameter of the crank in places I don't want splines to be, and then file corresponding notches in the cup's flat cylindrical surface (the part that doesn't actually get used inside the BB) to the ridges on the crank, then it should transfer power well enough without further attachment. Given the hardness of the metal on both pieces, I'll probably have to use the Dremel for the majority of the work, and then file to get squared edges/corners later, or it'll take weeks to make. :)

The only catch is that the bearing cup has no outer retaining ring built into it, and I don't think the locking ringnut that normally goes onto it on the BB will fit here, unless I grind off the hexnut outer portion of it used to tighten it into the BB. I'd rather not modify it in any way I can't reverse, since I may need to use this part on the next version of this bike, what with not having been able to get any more scrapped bikes for a long time now.

So I *could* weld a couple of tabs across the flat outside of the hexnut area, and later grind them off if necessary to reuse it for it's normal application.

Or I could drill a hole laterally thru a couple of the hex nut's faces, then press fit a pin into that that would protrude just enough to prevent the freewheel from unthreading itself beyond the edge of that face. I have several pins that are just long enough for that, removed from the wheelchair motors' power-brake-lock that I am not using on the bike. This is probably what I will do, assuming any of my drill bits are hard enough to make it thru the bearing cup's hardened metal (probably not).

So I think I have a solution, and will ponder it overnight in case something else crops up that kills the idea or I get a better one.

EDIT: I've found that (naturally) what I discovered is already known to others, as exampled in this thread on Endless Sphere, and this thread too.

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