Originally Posted by
jeirvine
Many gradual changes, but 1923 was key:
"In 1923 the rules were changed to allow riders to swap parts instead of repairing them. This is a monumental change in the character of the Tour de France. Before, bad luck reigned as the supreme arbiter of the race. Now, perhaps the race might be more of a test of the athletic prowess of the riders."
http://www.bikeraceinfo.com/tdf/tdfhistory.html
Unfortunately it seems now we are into how can they can cheat with Performance Enhancing Drugs and not get caught