Note: imi is the OP
When a thread goes 7pages, sometimes it's best to just focus on the first post...
Gravity is the same for any mass falling in a vacuum (right?),
Correct
but there are many other factors and more physics going on in a downhill bike coast than gravity in a vacuum... acceleration, momentum, air resistance, road friction, etc...
Also Correct, there are more factors than just gravity.(this seems to be the majority of the thread's consternation)
Anyone with the physics knowledge can sort this one out? Basically two riders on exactly the same rigs, all conditions being equal coasting down a hill, the only difference being that one weighs twice as much as the other with comparable extra girth.
"All conditions being equal" versus "extra girth" are a contradiction, since girth is going to effect air resistance; and I think you are primarily interested in the weight issue right? Please clarify exactly what you meant by this so we can clear up some of the confusion and arguments.
May I suggest modifying the original query to put both riders in recumbents with full aeroshells big enough to hold the larger of the two?
Starting from zero mph who will get off to the best start? Will the other one catch up or overtake somewhere down the hill? Who will get further up the next hill by just coasting?
cheers!
Uhh yeah... I'm not touching this point till the OP clears up some of the hypothetical setup to remove ambiguity.