I think it's geometry?

[Kano]

See http://www.phred.org/~josh/bike/trail.html for an explanation of trail.

This was so cool! It was a very good thing that there were pictures, or I would have come back with still more questions, but I get it! I haven't gone out to play with it on my bike yet and see where mine is in relation to nothing in my frame of reference, but I want to, even though I can also look it up.

As far as flex, a longer wheelbase has more flex than a shorter wheelbase for the same frame material. More flex, less of a jarring ride on rough roads. But too much flex (especially in the chainstays) and some of your power goes into bending the frame instead of propellling the bike forward. But with new frame materials, builders are making bikes with a stiff drivetrain and a more comfortable ride. So frame geometry alone won't tell you much here without a test ride.

Makes sense -- I get some experience with that when I notice the shocks on my bike "smooshing."

You want to test ride by hammering up a hill in an almost-too-big gear to compare frame flex and power transfer.

I had to laugh here: At least on my current bike, there's no such thing as a gear that's not too big for a hill -- hammer up a hill? If I ever get on a bike and manage to "hammer" up a hill....

"So, am I understanding that perhaps a bike can be "amended" to make it work as I evolve as a rider, at least to a point?"

Exactly. You'll hear some riders here talk about "flipping the stem" to go from an upward slanting stem to a flat stem that positions the handlebars lower. Or you can replace the stem with one of a different angle and length. But if the bike is designed with a high head tube relative to the saddle position (which some bikes with slanted top tubes are) then you will be limited.

This is very exciting! It seems to mean that while I already feel limited on my current bike after just a few months, a well-fitted road bike would give me much more "grow room," yes?

I'm not sure I believe the marketing hype for compact frames. The frame itself becomes stiffer if the seat tube is shorter, but then you sit on a longer seat post which has no cross-bracing. It should be stiffer for out-of-saddle sprints, I'm not convinced it's helpful for in-saddle riding. So I don't think you should go looking for a compact frame, but there's nothing wrong with one either unless that gives you a ridiculously long seat post. Just check the height difference between your saddle position and the top of the head tube so you don't limit your handlebar positions. You want a range of between level and at least 2 inches, maybe as much as 4 inches, handlebars below saddle.

I think I've heard the term "compact frame" but I'm not sure I understand this -- There seem to be "generic" frames (man-bikes, as we've been calling them), WSD frames, and are these "compact" frames something else? Are they specifically related to the sloping top tube? Do I understand that this "compact" thing, sloping top tube thing, and the WSD thing mostly affect how my upper body reaches the handlebars?

Now -- that handlebars/saddle thing -- is this the flat top part of the bar, the drops, or the hoods?

This is fun! So far, I don't have a clue what the numbers mean, but it's fun to look at the different models, compare numbers and see if I can spot the differences in the pictures! I don't think I'll worry about the numbers, but it's interesting to compare the bikes and start seeing what's different about them. One of these days, I might start poking around looking at materials...

Thanks again for your time Deb!