You people are all waaayyy too light. You need to increase your Pukka Pie and Doughnut intake for a while, if you really want to court Mr Gravity and his Accelerating ways 
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Didn't that Italian chap disprove the "eat more pies to descend faster" theory of gravity?
No, not really.
Only if you are on the moon with no air resistance.
http://www.youtube.com/watch?v=5C5_dOEyAfkNewton applies: F=MA
A body remains stationary or at motion in a straight line untill a force is applied to it.
It accelerates in proportion to the force applied, but ininverse proportion to the mass of the object.
You need to consider the forces.
Gravity exerts a force proportional to the mass of the object.
So a 1Kg mass exerts a force of 1 * 9.81 Newtons on Earth.
We call this downward force the 'weight'.
The same object will have the same mass on the moon ( there is no change in the amount of material in the object ); but because the gravitation attraction is less, it exerts a smaller force. So less weight.
The weight changes according to the gravity, but the mass does not.
Now let's look at acceleration and speed.
A car has an engine, which produces power.
This produces a force in the forward direction.
This force causes acceleration, according to F=MA.
The car accelerates.
However, an opposing force appears.
Wind resistance. We are not on the moon.
The force of wind resistance increases with the square of speed ( subs: pls check ).
So as the car accelerates, the opposing wind resistance builds rapidly.
There comes a point where the force of the wind resistance equals the force produced by the engine, and there is no further nett force pushing forward.
At this point, the forces are in ballance and the car has reached it's "maximum speed, grommit".
Same applies to bikes.
Consider 2 otherwise identical bikes at the start of a descent, 1 has a mass of 100kg, the other 200kg.
What forces are at work?
In both cases, gravity.
Assuming the road is not vertical, the force of gravity will be reduced in relation to the gradient. It's simple enough to calculate the horizontal and vertical components.
But this applies equally to both bikes. Even if we had a vertical road ( freefall ), let's look at it...
Bike 1 (100kg ) :
Downward force = Cyclist's effort + MG.
So (Effort +100*9.81)
Upward force = function of V^2.
Ballance is reached when these are equal ( terminal velocity ).
Bike 2( 200Kg ):
Downward force = Cyclist's effort + MG.
So (Effort +200*9.81)
Upward force = function of V^2.
Ballance is reached when these are equal ( terminal velocity ).
Bike 2 will reach a higher velocity before the air resistance ballances out the downward force caused by MG.
Same applies when we reduce the gradient to more normal levels.
Max speed on the descent is the terminal velocity determined by the ballance of forces.
Forces (+) are M*g * trig function of gradient;
Forces ( - ) are speed and x-section.
We don't even need to get to terminal velocity for the more massive bike to have advantage.
From the get-go, it's downward force is <gradient adjustment>*MG which at 200*9.81 is double the lighter bike's 100*9.81.
It will accelerate faster, due to the greater force.
Both bikes will reach equilibrium, but the more massive will reach a higher equilibrium.
That's rambling.
Remind me not to do schoolboy physics on a Friday evening after several glasses of grog.
--
R
