Joined: Jan 2010
Posts: 9,818
Likes: 1,790
From: Northern California
Bikes: Cheltenham-Pedersen racer, Boulder F/S Paris-Roubaix, Varsity racer, '52 Christophe, '62 Continental, '92 Merckx, '75 Limongi, '76 Presto, '72 Gitane SC, '71 Schwinn SS, etc.
13-28t 7s was a popular choice, as were 13-26t and 12-28t, all in 7s (narrower-spaced) format.
7s spacing requires use of "narrow" chain, 7.3mm max.
The older (standard-spaced) 6s freewheels used wider chain, roughly 8mm wide.
The 7s freehub adjusts all of the dimensions to accomodate the wider 7s cassette, and can still (with the 7s cassette) easily be narrowed to 124mm just by removing a 1mm washer from each end of the axle assembly. The resulting 124mm hub fits without much effort into older 120 and 121mm frames (my old 5/10s Peugeots measure 121mm).