A bicycle (or a soapbox) can be made to have minimal rolling resistance - but the biggest factor in determining its ultimate freewheeling speed is air resistance, or drag. Drag increases with speed, and is dependent on many factors such as surface area and smoothness.

On a plane inclined at an angle alpha to the horizontal, the force (down the slope) due to gravity is given by Mass x g x sin(alpha). Let us say the drag is a function D of the velocity. So the combined force is given by:

 F = Mass x g x sin(alpha) - D(vel)

Newton's second law states that Force=Mass x Acceleration, or equivalently:

 Acceleration = Force/Mass

So for our freewheeling bicycle:

 Acceleration =  g x sin(alpha) - D(vel)/Mass

The first term, g x sin(alpha), is the same regardless of mass. If we ignore drag then this tells us that all bodies will accelerate at the same rate down a frictionless inclined plane. However the second term, D(vel)/Mass, says that the acceleration will be reduced due to drag, by an amount depending on the velocity and inversely proportional to the mass. In other words, the larger the mass, the smaller the effect of drag at any given velocity.

So the heavier boys will accelerate faster, and therefore have an advantage in the downhill freewheeling race.

This seems to indicate that in the situation presented by the original question, a sensible strategy might have been to put a few rocks in the bags to increase the acceleration of the bicycle. This assumes the tyres were well inflated, to avoid energy loss due to additional deformation caused by the extra weight.

I realise this doesn't answer the original question - all I can say about that is that after I have played squash I have to wait a while before rehydrating, otherwise water tends to come out through my pores almost as fast as I can drink it!