Originally Posted by
CrankyFranky
Are the hubs identical? Do they both have stock spacers/locknuts/knurled nuts on ‘em?
Assume that the freewheels are the same? I'm all for measuring both.
Are the small cogs the same tooth ##?
This might be too much more than you’d want to do, but (say) if you removed both freewheels and measured the distances between the hub freewheel shoulder and the knurled nut (DO face), are the two measurements identical?
If you have a dishing tool, are the FW-side measurements the same for both wheels?
Are the seat stays on the Medici slimmer than on the Davidson at the DO end?
This is a great question!
In early 1980s, one could find both 120 and 126. My ‘ 82 Stan Pike was 120, perhaps because he was a small custom builder at the end of his carreer, too busy to keep up with emerging trends, and I was too busy at Uni to be aware of six speeds either, never mind that I ordered it in mid 1981.
This is good advice. Measurements matter more than "standards." And the 120 and 126 "standards" are pretty loose, as we've recently discussed elsewhere.
So the first thing I'd do is measure the actual hub OLN widths. There's a chance your hub that's been in the 120mm frame is a bit more than 120.
"Ideally" the hub spacing should follow the freewheel width and desired chainline, but that's a lot of measuring and math, so the tendency is for 5s wheels to be 120 and 6s wheels to be 126 for convenience sake. There's no reason you can't run a 5s freewheel on a 126 hub but a 6s standard FW on a 120 rear hub is pushing it; your wheel will have quite a lot of dish and axle overhang and the problems that often result from those conditions. A 6s Ultra freewheel will be a little wider than most 5s standard FWs but usually fits a 5s hub without major axle respacing if any.
If you really want to swap wheels from one frame to another with their different rear triangle widths, you could just split the difference on the rear hub spacing and set them both at 123mm OLN, which should be just ok with the 6s freewheels and won't cause long-term problems for your frames. I've seen this sort of "compromise" spacing on frames too during that 5 to 6 transition period. But if the freewheels are different widths you'll have to readjust the limit screws when switching from one freewheel to another. And you might end up having to swap an axle or two to get the length right. Might be more trouble than it's worth in the end.