mnichols wrote:Which would mean that the reason bikes descend at different speeds would be rolling resistance, aero dynamics, and mechanics (hubs, etc) but not weight
If you take an item, and double it in linear size, keeping the shape and everything else identical, the weight increases by a factor of 8 (2x2x2), but the frontal area, and hence aerodynamic drag force, only increases by a factor of 4 (2x2).
The terminal speed is where the accelerating force due to weight is balanced by the retarding force of aerodynamic drag. At the terminal speed of the small version, the accelerating force on the large object would be double the retarding force, so its speed increases until aerodynamic drag has increased enough to balance the weight again.
(That's for a falling object - a bike is more complicated due to rolling resistance etc, but the same principle applies)