Originally Posted by slvoid
Drag = (Cd*Area*density*Speed^2)/2
Purely in terms of force needed to overcome aerodynamic drag:
For example, with completely made up numbers of Cd=.25, A=.5, density as 1, speed initial = 1.4m/s (5km/hr). Force to overcome drag is .1225
At twice the speed, drag is .49
At 4x the speed (20km/hr) drag is 1.96
At 8x the speed (40km/hr) drag is 7.84
To go from 6mph to 12mph, it's 2x faster but it takes 4x the amount of force.
So to go from 6mph to 24mph, it's 4x faster but it takes 16x the amount of force.
Compared to 6mph, to sprint to 36mph, you would need roughly 36x the amount of force.
Drag is a force, yes, but we don't really have to worry about force when we ride.
That's what gearing is for.
Instead, we're mainly concerned with power. And power is force times speed.
So, we have speed multiplied into the whole thing a third time, making the power dependent on speed cubed.
If the power required to go 5 mph is 125 (made-up, unit-less number), then you need a power of 1000 to go 10 mph. And 8000 to go 20 mph. This is the aerodynamic drag/power only.
Rolling power increases linearly with speed, so going twice as fast just means twice the power. That power is added to the aerodynamic power.
When and object moves through a fluid (air, water, et c) it affects the fluid around it.
The air within the boundary layer "sticks" to the surface of the object and is continually set in motion in the direction of the moving object. After the moving air slips off of the object, it is still in motion forward. This forward motion slowly returns to normal local air speed some distance behind the first object. Any object following behind, within this distance, will experience a lower relative air speed, and that translates to a lower drag force and less power required.
This drag is known as friction drag. Unless you have two (for the purpose) highly optimised objects, very close together, this type of drag is not lessened by the object behind.
A bluff object also has plenty of form drag, which is the difference in pressure between the front of the object and the rear of the object. The pressure by the oncoming relative wind on the front of the moving object is a force, and it acts to slow the object. If there's an equal amount of pressure from the air on the rear half of the object, the two forces cancel out, and there is no drag.
This is known as pressure recovery, and the main purpose of aerodynamic shapes is to maximise it.
Pressure drag, like friction drag, can't be eliminated.
If another object is close behind the first, its moving through the air creates a kind of "bow wave" where the air slows down (stagnation) relative to the free air far away. This slower air helps the object in front with its pressure recovery, since the lower relative wind means the negative pressure is lower.
Thus, both objects gain from being close, though not equally much.
The larger and boxier the object in front, the more you gain!