Re: Do heavier bicyclists go faster downhill than lighter ones?

Just like most things in science when you hear someone say "all things fall 
at the same rate" or "heavy things fall faster" you need to determine all 
the conditions under which the statement applies.

Both of the above statements can be true, but under different conditions.  
The big difference here is whether we are talking about falling objects in 
a fluid or a vacuum.

In a vacuum all objects fall at the same rate under the influence of a 
gravitational field.  This is not true for things falling in a fluid (like 
our atmosphere).  Objects falling in a fluid have drag and this effects how 
they fall.

Obviously different things fall at different rates in our atmosphere (a 
feather falls much slower than a hammer).  On the moon which has almost no 
atmosphere all things fall at the same rate.  This was demostrated by an 
Apollo astronaught who dropped a feather and hammer at the same time on 
the moon and they fell at the same rate.

Now lets think about drag.  Drag is a force and it opposes motion through a 
fluid.  Drag is primarily a function of the shape and size of the object 
(right down to its surface texture) and the speed the object is moving 
through the fluid.  The force making you fall comes from gravity, this 
force is a function of your mass.

So things with the same shape have the same drag, and things with the same 
mass have the same force downwards from gravity.

So get two feathers exactly the same size, shape, texture etc (so same 
drag) but one is made of feather and the other is made of lead.  The lead 
feather will fall faster.

This leads us to terminal velocity.  This is the fastest velocity an object 
will fall through a fluid under the influence of gravity.  A you go faster 
you drag increases.  The force oppossing your motion through the fluid 
increases as you go faster.  The force pulling you through the fluid stays 
the same (gravity and your mass).  When the force of drag equals the force 
from gravity you stop accelerating and you have reached terminal velocity. 
 This is obviously low for things with high drag and low mass (feather) and 
higher for things with lower drag and bigger mass (hammer).

So now back to your original question.  In general a heavier bicycle will 
have a higher terminal velocity and a higher acceleration to terminal 
velocity than a lighter bicycle IF they have the same drag, however heavier 
bicycles may also have higher drag from the air (a wider person sitting on 
them), and also may have more drag from their wheels on the ground or from 
their bearings etc.  So to be sure you either need to test the bicycle, or 
to know all the factors like mass and drag.

In things like bicycles, bob sleds and billy carts it generally pays to be 
heavier rather than lighter for going downhill if you have an atmosphere.