Originally Posted by
SlimRider
In order to attempt to maintain a constant speed while a variable headwind opposes your motion, requires that you apply a continuously variable force, as you try to maintain a constant speed. These variable applied forces translate into changes in velocity. The change in velocity, by definition is acceleration. Inertia opposes acceleration and is mass-dependent.
this is also true for terrain (it's never perfectly level) and usually also varying traffic conditions. basically, a bike is never truly traveling at a constant speed