Originally Posted by
Kontact
Than a freewheel, and probably because the freewheel has a back plate that retains the threaded sprockets. Cassette sprockets aren't threaded so they don't require something strong to resist the tightening caused by chain tension. Cassettes locate the low cog closer to the spokes.
Sounds logical, but I see quite a bit of clearance for existing freewheel's back plate to move more than 2mm further inward while maintaining the same thickness.
But I guess that when you no longer have hub and freewheel possibly coming from two different companies or even countries, it should be possible to move the largest cog more accurately into optimal position wrt the spoke flange.
As for resistance to the idea of going from 5 to 6 cogs using standard spacing, I recall extremes of chainline angle being mentioned at the time, something that Ultra-6 addressed. And with the pioneering (bushingless) Sedisport narrow chain becoming available at the same time, with it's added flexibility, going to 7 narrow-spaced cogs thus became more logical in terms of any frictional issue having to do with chainline.
My '79 Fuji Professional still has it's original Ultra-7 freewheel, and with modern 8s chain I detect no issues having to do with chainline.
When 130mm road hub spacing became standard, 7s still was in use and (as miamijim already mentioned) they just added 4mm spacing to the left end of the axle to make everything work (and build a stronger wheel at the same time).