Probably the easiest way to understand why hills alter cycling efficiency is this:
All the energy you "store" by climbing a hill and overcoming gravity is "released" when you go down a hill to the same altitude at which you started climbing.
However, while you were climbing you also had to produce energy to move through "air". The only way a hill could begin to give back the same amount of energy downhill as when you climb would be by "giving" you a tailwind above the speed at which you descend. If you slow down 10mph on the hill, then you need at least a 10mph tailwind ABOVE YOUR CURRENT SPEED going downhill just to break even!
The "big deal", is that the ability to climb at just 1 or 2 mph difference returns an incredible amount of distance or speed on the downhill. Anyone who has ridden a hilly route with good riders, knows just how this works out - when amplified over 100 mile ride. Those guys are already gone and at home showering while you're 15 miles from the finish.
Last edited by Richard Cranium; 08-28-06 at 07:17 AM.