Dave ,
I think you are forgetting that there is plenty of room once you get to / have a larger inside diameter of such a freewheel/spider , once you clear the Bottom Bracket Shell on the bike. You will see there is plenty of room.
Note : on my Mountain Bike there is 28 mm of space space between the frame ( downtube ) and the outer chain ring/chain . so in reality up to 25 mm of space to work with,
On my Road Bike there is about 17 mm of clearance.
Or to make it all fit, design so that the outer chainring, the one connected directly to the RC motor, is dished out a little . there is room to do that, look at where the pedal is connected and where the outer chain ring is on a regular crankset.
there is quite a bit of room there, by just designing a spider with dish on it it is doable .
A chain / bash guard would be helpful when doing this to save your pants/legs.
Perhaps take the thin-est freewheel on the market , grind down the teeth, bolt up the custom made spiders to one side ( different spiders for different BCD's ) and then bolt up the inner chain ring that drives the chain to the cogset on the rear, and you are good to go. not sure if a 4 bolt or 5 bolt pattern on the small freewheel will be good enough, testing will tell the answer.
and
Of course there are many smart people here on E.S. surly someone can easily ( for them ) design a thin freewheel that bolt up a chainring on one side and spider arms to bolt onto the crank on the other side, or even use the same bolts for both the chain ring and spider, so that one set of bolt holes are needed to bolt something onto each side of the freewheel.
tangentdave said:
A freewheel is not an easy thing to make, I'm definitely not going to attempt one. However, if someone were to design a way to convert existing cranksets like these to use a freewheel, I'd jump all over it.
I think you'd need a dual flanged freewheel, one flange being the center/input, the second on the opposite/outside side of the ratchet. Doubtful this arrangement would be as thin as two chainrings, meaning interference issues between freewheel and frame. Still need extra spindle length to accommodate the extra part (freewheel).
-dave