" Aerodynamics doesn't begin to have an effect until 60 MPH "
 

I keep seeing this comment over and over.
I remember a comment that Darin made about how important that aero is even at 45 MPH.

Can someone suggest a direct link to some info on this ?
It gets frustrating.

I am not sure of any links (haven't looked), but when I drive my C-max PHEV in all electric, the windnoise gets louder around 45-50MPH

There is a calculator under the "tools" in "garage&tools" that you can use to calculate the hp/kwh to move the car at speed.

Depending on the car/SUV it generally is just as hard to push it from rolling resistance to push through the air between 30-40 mph. After that speed the aero quickly increases while rolling resistance is mostly unchanged

No reference, but from memory rolling resistance is linear and aerodynamic resistance is exponential.

There is a long tail on the lower end, IIRC aerohead has said 25 MPH is where the streamlines begin to order themsleves. At some point (45 MPH?) aero drag exceeds rolling resistance. The situation is complicated by the Reynolds Number and crosswinds.

This term is commonly misused. Aerodynamic resistance is a power function (an independent variable, in this case v, raised to a constant, 2), not an exponential function (where the independent variable is an exponent).

That squared term in aerodynamic force equations is derived from the dynamic pressure of the flow, which is proportional to its energy. If you remember back to physics, kinetic energy is proportional to the squared velocity of a body since it is an integral of its momentum (P = mv --> K = [1/2]mv^2).* For a fluid of constant density, like the airflow around cars, dividing both sides by a reference volume gives

dynamic pressure = (1/2)(density)v^2

Multiplied by a reference area gives aerodynamic force. Dividing the reference area by a convenient/arbitrary fixed area, such as the cross-sectional area of a car or the plan area of a plane, gives a dimensionless drag coefficient.

*In reality, K = (1/2)mv^2 is an approximation of a more exact quantum equation. That wasn't known until the 20th century; the macro approximation was discovered by a Frenchwoman in the 1700s. Prior to her experiments it was thought that kinetic energy varied linearly with velocity.

It's not a very useful comment; it depends on the size, mass, and drag of the car, and terrain, and road surface, and....

Say you have a car driving on a flat asphalt road that has equal rolling and aerodynamic drag at 40 mph, like a large SUV or something. If you change something and reduce the aero drag by 10%, is that not "hav[ing] an effect"?

I stand with the common man. :)

One edge case would be salt. High resistance and low traction.

Let me make a list of all the scientific research that proves that aerodynamics don't have an effect until 60mph:

List
End of list

Wait, if we modify the statement a little there is some evidence that at speeds below 60mpg aerodynamics don't have any effect... if you drive on the moon of course. And above 60mph they don't have an effect there either.

Sorry, no offense. I just had to make a joke. :D

I do get the general idea of what people are getting at. Aerodynamics don't have as much of an effect at slower speeds as they do at higher speeds (in Earth's atmosphere or in any other fluid). But on the other hand, the extent to which they have a noticeable effect and whether that effect is worthwhile or not depends on both the vehicle and driver in question.

I believe the process is like the torque vs hp curve on an ICE. Max efficiency is supposed to be wheere the lines cross.

Otoh, I have a couple of toy planes that fly about 20 mph.

Gears & Gasoline recently made a video where they cut the fuel economy of their Insight in half, by ruining the aero (among other things). They were not driving at high speeds.

Next time I see somebody say "aero doesn't make a difference below highway speeds" I'm gonna send them a link.

BTW here's the video:
https://www.youtube.com/watch?v=eLmLG52La58

This is spot on. People tend to exaggerate when they retell a story. So the facts get blown all out of proportion.

Most of us in this community know that mechanical drag is a linear function, whereas aero drag is exponential. There is a point where the aero drag exceeds the mechanical drag. Where that point is depends on the vehicle.

Below that point, the aero drag will be lesser than the mechanical drag. But there will still be some aero drag. So it can still benefit from aerodynamic improvements. Likewise, above that point there is still some mechanical drag. So even at high speeds, the car can still benefit from reduced rolling resistance. So it's not as cut and dried as people like to think.

That said, it would be wise to prioritize based on your situation. Pick the low-hanging fruit first, then move up the branches if you're still hungry.