" 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.

60-mph
 

* There may have been a specific case in which the comment had merit. Perhaps a Bonneville streamliner class vehicle, or the 1993 EV1 land speed record car at Ft. Stockton, Texas. Without the context, we'd be at a loss.
* We know that aerodynamic drag varies as the square of the velocity.
* And we know that the power to overcome aerodynamic drag varies as the cube of the velocity.
* Here's an aero power profile for the 2019 Chevy BOLT ( using Cd 0.31 )
35- mph = 2.3316-hp
45- mph = 4.9557-hp
55- mph = 9.048- hp
60- mph = 11.7468-hp
65- mph = 14.9351-hp
75- mph = 22.943- hp
85- mph = 33.3983- hp
95- mph = 46.6271- hp ( 1-mph over rated top speed )
* One thought, which could explain the comment, might have had to do with the 'cross-over point' velocity, at which aerodynamic drag just began to exceed that of the rolling resistance on a road load power curve for this particular vehicle. For this to be true for the BOLT would require Cd 0.142.
Honk if you have a small one!

Vman455 posted Permalink#5 in vain.

mechanical drag
 

* Road Load Horsepower leaves 'mechanical' drag outside, dealing solely with rolling resistance and aerodynamic drag, by definition.
* To go from the Road Load Horsepower to Brake Horsepower, you would introduce all the transmitted power inefficiencies between the tire/road interface, and engine's flywheel, including any engine-driven accessories ( differential, wheel bearings, CV-Joint shafts, propeller shaft, transmission/ transaxle, power steering pump, water pump, alternator, AC compressor, etc. ).
* EV power electronics, motor losses, transmission losses, heat pump load, cooling system pump, etc. would have to be factored in to compute an EV's 'brake' kilowatts and BSFC-e, on top of its Road Load Horsepower/ Kilowatts.

So we're nitpicking terminology?

Common speak and math jargon are two different things. And I'm not a mathematician. So you'll have to excuse me for using the common definition of the word "exponential."

But I repeat myself.

My mistake. Totally misunderstood your post.

I assume the speed that aero matters depends on whether you're talking about drag or lift/downforce, right?

@ everyone : thanks for the replies !

I get that a lot. :)

I was empathising with Vmann455 without necessarily agreeing. Or covering for my error.

It's hard to shepherd the hive mind. There was a major kerfluffle over the Template recently, people got banned.

My effort was to get people to say first approximation instead of rule of thumb. You can imagine how that went.

Sounds exciting. :p

I'm sure that template has its uses. But let's be realistic; it's almost a century old. Our knowledge of aerodynamics has come a long way since then.

I think it's easier to generate drag at low speed than lift/downforce. So yeah.

But then you have the people who say "you won't make any downforce under 100 miles an hour", or something to that effect. That's not entirely true, either.

That might be an important part of the "seat of the pants" experience most likely responsible for someone's off-hand comment claiming 60mph is where aerodynamics takes affect.

On the aerodynamic experiments I conducted on my S10 pickup truck, some of the modifications could be felt starting in the 30-35 mph range.

This is the same range as feeling "a push" on my hand sticking out the window.

Below 30 mph hand out the window is perhaps a breeze, but above 30-35 it feels more like a "force".

Again, just "seat of the pants" subjective jargon where my 30mph is someone else's 60mph before they take notice.

I have to agree that in this context the wind noise issue, that is the flow coming into the vehicle at 60mph getting someone's unwanted attention might be a common experience, common enough to allow the 60mph comment to gain traction.

I mean how many people do you see driving down the road with their hand out the window measuring aerodynamic force using their hand as a diving and or lifting plane? Myself and maybe a couple of curious children not glued to their cell phones or TV's built into the back of their parents headrests.

If low speed airflow had no effect then you wouldn't ever notice a light cross wind having an effect and the suspension would not move while parked on a pleasantly breezy day.

There's probably someone out there that thinks aerodynamics aren't important until you hit 120mph.

"Aerodynamics are for people who can't build engines." - Enzo Ferrari

drag, lift, downforce
 

* Since all-three would occur concurrently on any given vehicle, all three effects would be tracking simultaneously, along with identical air velocity, regardless of the indicated velocity.
* I agree completely that, individual priorities would dominate the narrative, and conclusions.
* Relativistic.

I measure differences in coast down tests for bicycle fairings at 13-15 mph.

Where did you hear that? The lines cross at the same RPM, always! (Exactly what RPM depends on what units you use for power.)

In SAE units, HP is defined as (Torque * RPM / 5250). So those lines will always cross at 5250 RPM, assuming the engine was measured both above and below that RPM.

5200+ RPM is almost never the most efficient speed to spin your engine.

When I was a lot into road cycling, my topspeed used to be about 60 km/h.
That was in an all out sprint, I do not have a power meter but the Kreuzotter calculator says 1249 W of power are required for that on my roadbike in my sprinting position.
If I went with my most agressive TT position (Superman), I'd need only 570W for the same speed.
With a fully faired recumbent I'd be at a sustainable 180W.

Aerodynamics matter a lot even at that kind of speed.
And I love to show it to road cyclists by passing them on my rather comfortable and aerodynamic recumbent.

RPM and efficiency
 

Each engine BSFC map will show an island of highest thermal efficiency, spanning some rpm range.
If memory serves me, it has to do with mean effective pressure developed and coincident pumping losses.

1259-Watts
 

Somewhere, maybe some book on human factors, they mentioned that top athletes can momentarily generate around 2- horsepower ( 1,491- Watts ), so your numbers certainly reflect the real world! :thumbup:

Given my build and exact position I suppose the calculator estimate is a little on the high end.
And also given my height and gender I probably will never reach pro athlete levels.
But to be fair, olympic track sprinters can reach a lot more than 1500W in an all out sprint.
They often reach in excess of 2000W

Aerodynamics matter at all speeds. Be it 10-20-30 and so on. Ask any soap box derby racer if aerodynamics matter during his or her coasting race? BTW with just gravity they can hit speeds up to 21 mph on a 950 ft track.

The best fuel mileage will always be attained by running in the highest gear at the lowest possible engine speed RPM's with out lugging the engine.

Most vehicles have a sweet spot for fuel mileage between 40 and 45 mph. That is the point where in most cases wind resistance becomes a factor with the power needed to over come it starts to accelerate quickly as your speeds increase.

Here is a white paper from Cummings Diesel that goes over speed and aerodynamics vs fuel mileage. Yes it involves big rig trucks and trailers but the principles they talk about still apply across all classes of road vehicles. BTW it is a PDF when you get to the page.

Cummins_Secrets_of_Better_Fuel_Economy.pdf

Oh well and good, but it seems to me Aerodynamics does not really matter all that much unless your drive a tiny car with a TINY motor and trying for as much MPG as possible,

I drive a 93 Star Craft full sized G20 Van.

Forever I keep hearing IT IS A BOX, your never get good MPG out of a box.

YET Chevy in 2019 made a full sized Express Van, long wheel base 3500 155" with a 4.30V6 Direct injected, 8 speed:

AND it is officially on the window sticker listed as its MPG of 19 City and 29 Highway.

It is a brick sitting high on its wheels and get 29 highway.

There is no real Aerodynamics on it.

Rich

That's pretty bad fuel efficiency compared to let's say a Mercedes Sprinter, long and tall version.
You can drive them with 6-7L/100 km when going slow because of the rather efficient engine and super tall gearing.
However these high cda's realy bite you at speed, when I set cruise to 160 km/h I usualy see ~18L/100km :eek:

Vans are/can be, aerodynamic in their own sort of way.

As a vehicle increases in size, it's volume increases faster than it's external area. Because of that you can haul more stuff or people with less aerodynamic drag per person or quantity of cargo.

Bigger vehicles also end up becoming longer in proportion to their height and width. That is also an aerodynamic improvement. For an example, trains are very aerodynamic because of their length.

Not that curves can't help. But a curvy vehicle body isn't everything.

Can you translate that to Miles and MPH and MPG??


Please.

Rich

That is another reason I believe my 93 will do well:

6-7/~18 ≤ 3

Hope this helps.

My Type IIs had better Cd than any of my Beetles, just more A.

among the Miata racers, the old view was that aero wasn't worthwhile
 

That's been proven to be very wrong over the last decade or so...down force and drag can be produced and changed in very obvious quantities well below 60 mph.

My current project is an electric bike with only 750 watts of power, it's getting a vintage moped race fairing, and both top speed and watt consumption will be compared before and after fitting the fairing.