If there were no air resistance or friction acting,
and if the wheels of the bike were really light, the mass of the rider
wouldn't matter and the riders would be going the same speed at the
bottom no matter what their weight was.
The reason this is the
case is the same reason that causes two objects with different weight
(for example a book and a paper-clip) to hit the ground at the same
time if you drop them from the same height: Gravity pulls harder of the
heavier object, but because its heavier its also harder to "speed up",
so in the end it all comes out the same (if you ignore air resistance).
In real life, the wheels of the bike do have some mass, and
there is also some friction and some air resistance, and the problem is
a bit more complicated. What is true, however, is that all of these
"extra" forces tend to slow the bike down a little bit. The only force
that acts to make the bike go faster is gravity, so making the rider
heavier (and thus making the force of gravity bigger) is really just
making the other forces small in comparison, and therefore making the
bike go a bit faster.
(republished on 07/30/06)