I think I need to wade into this since I worked at a shop for three years where a large portion of our sales were re-selling used classic road bikes. What Dale and others wrote about the way bikes "used to be" sized is 100% right one but they haven't looked at the repurcussions of such a sizing change. While working at The Bikesmith in Seattle I bought, consigned, and sold hundreds of classic road bikes and in around 70% of the cases they were selling the bike because it was "too big." What had happened was someone buys a nice bike and rides it for years, then buys a new bike and with the "new" sizing. After taking it home they look at their old bike compared and it just looks "too big."
Now a quick bit on sizing if you look at the industry numbers. This is a topic I discussed many time with my co-worker at the time Bob Zumwalt who had been in the industry for 50+ years. In the early-80's, and before, most road bikes sold were in the 58-60cm size. During the mid-80's this started to change with the belief that smaller frames were stiffer and lighter. This made a change to a 54-56cm as the "average" frame size. Thus many 58cm riders bought new 56cm frames making the older frme "too big."
I think the whole debate is silly since standover height, which most people think of as "seattube lenght" can vary dramaticly depending BB height and tire size. The standover height of a 56cm cyclocross bike would be similar to a 60cm touring bike with smaller tires. I think of all bikes in relation to toptube length and own bikes from 52 to 57cm and can ride them all fine. Also saying you need to have a "taller" bike to make the stem come even close to the seat is 100% bunk. With the wide variety of stems and seat posts on the market the argument for a "taller" bike is totally false; as proof look at many of the compact frames made and where riders actually put the bars. Bikes with the bars 3+" below the saddle is really from the 80's and people making the trasition from MTB's. If you like the looks of a "traditional" stem say so, it's the fashion not the size you're concered about don't hide behind the "it's too small." If it has the same toptube length it will be the same fit in the end. Sorry I got kinda on a rant there. . . anyway the reason you see so many 58-60cm bikes at swaps and on ebay is because the owners now believe the old bikes are too big.
enjoy, Brandon Ives
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