What's the bike, and from when?
Such narrow tooth spacing is useful for a half-step gearing pattern. They would be used with 5 speed rear clusters with fairly wide jumps between the cogs. Together with the narrow chainset and by alternating from big ring to little ring, you could get 10 evenly spaced jumps between gears, over a surprisingly wide range, with no duplicates or holes. Only problems are that half the shifts were double shifts, and that the total range is not what we would want today. I don't know if it was really common in racing, but I know a lot of bikes were so equipped back in the day. Typically the cogset would span 14-28 or more for this setup.
Ann Arbor, MI
> I've seen a Magistroni Sr. crank for sale that may be
\r?\n> period correct for my project, with 101 mm spacing.
\r?\n> My question concerns the chainrings. What is the
\r?\n> smallest permissible size for the low ring? I'm
\r?\n> seeing a tooth gap of 3-5 teeth between low and high
\r?\n> on these rings that are presented for sale and I'm
\r?\n> asking myself what's the use? As much as I'm trying
\r?\n> to get the right stuff back on the frameset I want
\r?\n> something that will get up the hill. I'm never going
\r?\n> to submit this bike for a concours review, and I
\r?\n> intend on riding it regularly. My first impressions
\r?\n> may be inappropriate, but I can't see the utility of a
\r?\n> 3-5 tooth gap on the chainrings. If, on the other
\r?\n> hand, a 40-42 tooth lower ring can be had for this 101
\r?\n> mm, 3 arm setup, then I'd be happy with the
\r?\n> Magistroni, whether or not the chainrings were quite
\r?\n> correct. If anyone knows of Magistroni Sr. cranksets
\r?\n> with 40/50 ring sets or so I'd greatly appreciate a
\r?\n> confirmation. Otherwise, I'll put as old a Record
\r?\n> crank as I can find on the bike and say to myself that
\r?\n> any sane person in 1958 would do the same once they
\r?\n> saw Campagnolo's new crankset.
\r?\n> Scott Baxter
\r?\n> St. Louis, MO