In the C&V days, racers could get a neutral support wheel. (Is this still done anywhere? I'm out of touch, but I doubt it).
For someone else's wheel to shift properly, the freewheel spacing had to be standardized. For USA racers, what I'm most familiar with, but also probably elsewhere, Campy's standard was THE standard. Maybe that was different in France? In the '70s and earlier, French hubs often (usually) had more space there than Campy, just another reason not to use French stuff for racing. I bet the pro teams that used French hubs respaced them to the Campy dimension, but that's just a guess.
The spec I'm talking about is what's labeled as dimension "A" in this diagram from Sutherland's:
The Campy spec, from the shoulder at the end of the freewheel thread to the outer face of the cone locknut, was
30 mm for 5-sp, and 35 mm for 6-sp. Some brands of freewheel moved the cogs right or left a little, relative to that shoulder on the hub, compared to other brands, so complete compatibility was not assured. But it was good to stick to those numbers. The equivalent number for French hubs was 34 mm for a 5-sp, lots of extra dish for no good reason. Campy rules, Normandy drools.
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