Originally Posted by fmw
You can ignore gravity. To say that additional weight makes one descend faster on a bicycle is false. It is actually quite the opposite.
Sorry fmv, but you must not get much opportunity to go down hills with riders that are heavier than you are. I'm over 200 pounds and I'm always amazed at how much affect gravity has when descending. I've tested it many times with my wife and with friends. My wife is much lighter and has less wind resisitance and when we start out side by side and coast down a hill, I leave her in the dust, and likewise, I have a friend that is 50 pounds heavier than I am with about the same wind resistance and doing the same test, he leaves me in the dust (and he and my wife and I all have the same bikes - LeMond Zurich's).
Another example is a couple years ago I was riding along on a fully loaded bike tour in New Mexico (panniers front and back with about 40 pounds of load) when I was overtaken by a Cat 3 Race (Tour of the Gila) as we were starting down a hill. I moved out to the center line as we were heading into a right hand turn so I would not interfere with the race and then we hit a steep section of this downhill. I was coasting and they were coasting (all light weight guys in a full aerodynamic tuck) and all of a sudden, I started passing them. I had to brake so the rest of them could get by me safely. I think extra weight definitely makes you descend faster.