Tapeworm said:

But what would hit first, a ball-bearing or a slightly bigger ball-bearing?

A heavy bike is only faster if its aerodynamics are equal or better than a lighter bike?

A light bike that had better aerodynamics would be as fast or faster than the heavier bike?

If two riders being the same weight and build but riding bikes with say a 1kg difference, how much difference are we talking? Greater difference depending on the slope?

Good questions. Let me take a shot at some quick answers:

1. If the slightly bigger ball bearing was made of the same density material, with the same surface finish, it would reach a higher terminal velocity and hit sooner. This is because of the properties of the equations for weight and air resistance. The weight driving the ball down is due to the volume of the sphere which relates to the cube of the diameter. However, the air drag is a function of the frontal area, which is a function of the square. These same equations are why the heavier rider has the advantage downhill over the lighter one, assuming he's in the same relative aero position.

2. To a point, yes. But it's the total weight and drag of the bike and rider that determine top speed, and the bike really doesn't matter much since we riders have much more of each. Aero positioning on the bike is a huge factor (sitting up vs down in TT position with elbows and knees tight. I've read that we can gain more aero drag reduction from wearing an aero helmet, taking off the gloves and wearing a tight skinsuit than a wheelset can provide.

3. Same answer; weight and aero of the rider is more important.

4. Using the calculator spreadsheet, looks like 1 kg more (on bike or rider's body) increases speed on a 10% slope from 40.1 to 40.3 mph. On a 5% downgrade, the numbers are 31.1 and 31.2 mph. These figures used "hands on drops, elbows locked" drag estimates. Substituting the cdA value for "full racing tuck" takes the speed up to 36.9 mph...which is huge. Most of us have experienced this on a long descent: tuck in well, and a few seconds later you're in the passing zone.