A common mis-conception is to think about it like the air is BLOWING AT THE VEHICLE...
But that is not true at all.

The reason I point this out is because an AIR MASS with ANGULAR VELOCITY would include KINETIC ENERGY and also INERTIA that is different from how it really works on a vehicle passing THROUGH that mass of air as it is (relatively) standing still.

If you have lately been hit from the side by a gust of wind while driving, the difference should seem familiar.

Think of a pool table. The rack of balls at the foot end, instead of standing still - all move together towards the cue ball...
What happens when they hit?

Then compare to how it really works - you shoot the cue ball down the table at them....


This is just one way I can think of to explain it - think of each of the pool table balls as molecules of AIR

It is very different which direction they all bounce off to, isn't it?




So what I conceive of as happening is that when a vehicle at high speed travels through a volume of air - it creates a disturbance in the air mass WHICH IS A LOCALIZED EVENT that requires a great deal of energy.

This happens continuously all along the path of the vehicle, so the lead car in a pack of cars on a race track might be viewed in some ways like a snow-plow. But air is thinner than snow, and fills back in quickly.


Now think about the difference between "WAD CUTTER" and more stream-lined bullets:
A Wad Cutter is thought to have primo short range aero characteristics because the flat face forms a round air cushion. But the price is range.

A sniper round has a long taper to cut through the air between itself and the target, it also rotates in flight for balance.

Videos of bullets and their ballistics may be more useful at figuring least drag on race cars than anything else - because it isn't the drag of a non-moving body through a flow of air (except a relative one) that matters, it is the initial penetration and scattering of a predictable and NON MOVING mass in front of the vehicle that should be the main focus of reduction.

As in how to reduce the energy required to hammer a spike through that wall of air...


And that is the difference between conventional thinking on it, and mine


If you really think about it - it could change the whole way you think of aerodynamics as they apply to cars and trucks.

For example: The front end of anything pays the price of slamming open the hole in the air, if the rest of the vehicle is long by comparison, the "price of it's passage" so to speak, has already been paid.


Does it make any difference to taper down the disturbance at the rear?

I doubt it. Doing so would pull or cost more energy. The true picture tells us the air molecules or mass have already been chucked out of the way and will find their way back to the least pressure they find.

The KEY FACTOR here seems to be that the air disturbance that results in energy expense occurs only along the length of the vehicle. What is behind is resultant, and nothing can be done about.

If you could, why would you? It fills itself in.

*Caveat: At certain speeds, such as cruising, if the collapse of the air disturbance is known it could lead to enlightened tapering of the vehicle(s). Speed would be a factor


So it seems to me that the cost of driving a vehicle at whatever speed or whatever weight is directly proportional to the shape and overall frontal area of whatever vehicle is being driven and modified by the shape of it regarding how efficiently air is deflected not only to the side, but in whatever other ways it may be used such as cooling, or "Through Body" flows

Total price in MPG would be based on maximum cross sectional area


But THIS thing changes the entire model, my friends:

It IS NOT wind blowing AT the machine

It is the MACHINE going down the road through the air....

I hope I made that key point. You have to think of it as a wall, even if a thin one
"a wall made out of blocks of air"

~Wolf out


PS: This has me wondering if there is an ideal shape for the front of a vehicle that would "PUNCH" an air pressure wave that would create an envelope the rest of the vehicle could slide through until it closed at the back...

STOP THINKING ABOUT SPHYNCTERS! But yeah....

So long as the body of the vehicle was in it, the penetration fees are paid


But that points to LOOOOOOOONG cars and trucks.

We've already been there, I can probably find Connie Mark VII's and '72 Thunderbirds all over the place

 

I've been noticing "triple" rigs on US roads now too, almost a road train - how much longer than a tandem semi-trailer rig it is I don't know at the moment, but the idea seems to be same as above.

The shape of big truck cabs seems to have nearly stabilized at a shape that gives them around 11 MPG under load, which is what my old '82 F100 got with the 'Swampers that were on it, but big rigs run on diesel. Trucks began trying out roof top airfoils back in the seventies, and now even short haul trucks often use some kind of aero device.

Even modern "Conventional" cabs take some account of airflow around the tractor, and sleepers are designed to fill the gap between cab and van.

 

And they are skirting and putting tail fins on the freight trailers, as well.

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It is said that this Tesla Semi truck is more aerodynamic than a Bugatti Chiron!


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I believe all those skirts and tail fins are necessary to go to California. They have wheel covers as can be seen on the trucks above. There are some that run those Super Singles which also save a little in fuel.

 

They been skirtin' wheels for a long time...



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Yes they have! Those have class! But what I was referring to is more like a hubcap. Just a flat disc that covers the side of the rim.

 

From what I've heard, the trade off to all these skirts is hotter tires and shorter tire life. For the driver, sitting on the side of the road waiting on a tire truck is worse than losing a slight bit in MPG. For the book keeper of the big fleet, the skirts make sense from his air conditioned office that has a set time to clock out.

 

The tires will catch up in technology but, to your point...at what cost?

 

Had super singles on my dump trailer rig
Not a lot of fuel savings, but helped in bridge weight laws.
Useless in the sand quarrys, truck stayed stuck. Loader operators hated seeing that truck coming