Originally Posted by
cyccommute
Did you taken Physics 101? The reason the heavier bike is slower is because it takes more acceleration and thus more force and thus more work to move.
A heavier bike does not "take more acceleration"; it simply accelerates slower when a certain force is applied to it. If you apply the same force to two bikes of different weights, they accelerate at different rates, but the amount of work done is the same, if the force is applied for the same amount of time. If you're doing a workout (which is what the original statement was about) and you are producing a certain effort / power for a certain amount of time, bike weight only affects your speed.