Originally Posted by
choddo
Of course mass isn’t the cause of a heavier rider rolling downhill faster (doh!), but the lower drag force relative to the graviational force as per the comment above about not being twice the frontal area.
It depends how you look at it.
If both riders had the same mass then any difference in speed would be due entirely to differences in aerodynamic drag and rolling resistance.
But if one rider was significantly heavier they would roll faster even despite a potential increase in aero drag. Mass is the dominant factor in the equation. Changes in mass affect rolling speed in a cubic function, while aerodynamic drag affects speed in a square function.
So really it is mass which makes fatter riders roll faster on descents unless they have a parachute on the bike!