Ergo comparison
Can an average man ride a smaller frame comfortably? As it turns out, yes. After a few adjustments, I’ve settled into (what is apparently) a 50 cm frame quite well. Fixing and riding this old steelie has been a real pleasure, but I wondered how a 5’10” man could fit so well on a bike designed for a shorter person. So I made a few measurements. Some of the results surprised me.

The bikes: A 2009 56 cm Tarmac all-carbon Comp, standard crank, 17 lbs
vs.
1980 50 cm Nishiki steel frame Sport. 27 lbs.

[Disclaimer: Although I ride every day, neither Jane nor I race. The following measurements are skewed toward comfort. All dimensions are in centimeters (cm).]

Floor to top/seat: Tarmac = 99. Nishiki = 97

Floor to handlebar: Tarmac = 97. Nishiki = 93

Pedal ground clearance: Tarmac = 9. Nishiki = 12.

BB to top/seat: Tarmac = 75. Nishiki = 73.

Maximum extension, top of seat to bottom of downstroke: Tarmac = 91. Nishiki = 88

Horz. Offset, crank to bottom of seatpost: Tarmac = 16. Nishiki = 16.

Horz. Offset, crank to top of seatpost: Tarmac = 20. Nishiki = 20.

Top/seatpost to handlebar: Tarmac = 66. Nishiki = 62
[This is one difference I can feel. DB]

Top/seatpost to back/hoods: Tarmac = 79. Nishiki = 75.

Wheelbase: Tarmac = 99. Nishiki = 103. [!! DB]

Top tube length: Tarmac = 56. Nishiki = 56

Seat tube length (BB to top/tube): Tarmac = 55. Nishiki = 50.

Downtube length: Tarmac = 64. Nishiki = 61

Axle height: Tarmac = 34. Nishiki = 35.

Handlebar width: Tarmac = 42. Nishiki = 40.

Unlike the Tarmac, the stem and stem extension on the Nishiki are a single piece, as they are on all these older bikes, apparently. If I can find such a piece with a slightly longer extension, I may like it even more. DB

Completely unrelated: my hometown of Red Wing, MN has had a Behrens Auto Parts on Main Street since the beginning of time.

Small world. I once knew a monseigneur who called himself RPK79 and did weekends with a local rap band. Later he quit the monastery and built a pizza empire. But I think he changed his name to Howard.

RPK79 is far more common than you think.

What you need to realize here is that the size number of the bike is just that, just a number. It does not tell us anything about anything and is only an extremely wide approximation.
I can easily ride my 61 spesh crux but I believe I would have a tough time riding a 61 tarmac and it would be completely impossible for me to ride the Kona major jake.

This is the reason test rides are important. however the bike size does tell us something. A smaller size normally has a shorter head tube. For some this is desirable, for others not so much. Secondly the angles may be different but that normally only happens when the bike actually is smaller in size and not just in number.
Most of the other things can be adjusted. Then again a 140mm stem on a small frame will make the bike relatively front heavy.

Are you talking about quill stems? They come in different lengths and heights. I have always had to cut down the height on quill stems for 48-50cm frames, otherwise the frames fit identically to 52-54cm frames except with slightly lower standover.

Please don't shoot me. I'm only the piano player.

Yep, quill stems. Sorry, but thanks. And I guess I'd like to source a quill stem with a slightly longer extension ("stem length" in the modern lingo). Advice appreciated and thanks again.

I get my quill stems from the junk bin, but Origin 8 makes some cheap ones, and Nitto makes some premium ones. AVA makes killer stems but my AVA is my personal favorite. Vintage ones can be 25.0mm, 25.4mm, 25.8mm, 26.0mm, 26.4mm clamp diameters. Modern ones come in just 26.0mm and the very rare 31.8mm. There's also two different post diameters, 7/8" for 1" steerers, and 1" for 1 1/8" threaded steerers which were used on some threaded headset MTBs. You can also pick up a quill to threadless adapter if you already have a box of threadless stems.

You will notice a lot of vintage small frames come with short stems, 60mm, 70mm and 80mm which are short compared to the modern sped 90mm, 100mm and 110mm. That's because the top tube length didn't vary much at all between sizes. When it did, the majority of the difference usually came from seat tube angle and had little effect on reach. Reach was adjusted using very short stems. I don't find it surprising that you're able to make a vintage small frame fit. Many of them are just medium frames with shorter seat tubes, shorter head tubes and shorter stems. Below 52cm shorter head tubes don't do anything except keep the top tube horizontal with the shortened seat tube unless you cut down the stem height.

Excellent information. Thanks so much. DB