Emily - I agree with Ted, post of the week award. There are innumerable changes that describe the evolution of the bicycle over the last 10-20-40 years, although it's interesting to note that the basic design - 2 inline wheels, frame and fork, upright riding position, etc - is unchanged since the advent of the "safety" in the 1890s.
I think the evolution at least in the period of interest to Classic Rendezvous could be described mostly in the realm of the perfection / practicalization of multi-speed power transmission systems, braking systems that could stand up to alpine passes, and the beginning of the materials science revolution that brought us from gas pipe to butted tubes to casting and forging of aluminium, and finally to titanium alloys (and later, off topic) to the space age materials including carbon composites and ceramics that are nearly ubiquitous in the bicycle market of the early 21st century.
You could add to that materials evolution the changes in tire casing materials (cotton to silk to nylon corded to Kevlar beads) and also the movement from wool clothing to synthetics.
Lastly, the FIT of the bike has evolved. No longer is selecting a frame size based on your leg length a guarantee of a proper fit. Now one can consider the crank length, compact geometry, fork rake, etc. in order to arrive at a proper bio-mechanic integration of the human and the machine.
Great question!
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Mark Petry
mark@petry.org
206 618 9642
Bainbridge Island, WA USA
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