Speed & Aero or why drag a boattail around at 45mph?

[Xist]

Originally Posted by Old Tele man

Air has viscosity, just like water. Try "running" in water and just see how fast you can't move!

How about swimming in syrup?

Or running in syrup...

[Tesla]

Yes I do understand basic aero stuff, bikes water & viscosity etc.
I'm trying to quantify effect vs velocity, I did look at that chart and the numbers seem to indicate a more significant effect than what I thought.
Just going by seat of pants accelerator position and coast downs at various speed, my personal experience does not seem to corrolate the same, when I pull the foot off the throttle to coast, deceleration seems to decrease at an exponential rate, once down to about 40mph it's like the only thing that slows her down is an incline, otherwise she just keeps rolling.
The thread below probably posed the question better:
http://ecomodder.com/forum/showthrea...ant-24813.html

Indications are that Cd does change with speed, i.e it should improve with speed, still counter to my perceptions, so am still trying to work through it and also wondering how the different types of aero drag interact at lower speeds.

Another thing I just thought of is BSFC maps and drivetrain mating to stock vehicle, the drivetrain is optimised for typically expected use patterns, so they are generally tuned for moderate to high speeds, therefore if you tend to use vehicle at low load only, it will be running less efficiently and hence any aero benefits at lower speeds may well be negated by reduced drivetrain efficiency, so unless vehicle intake and exhaust are tuned down for a lower rpm range there may be no improvement at the pump.

[MetroMPG]

On your last point, you're partially correct: the best situation would see gearing changed to account for lower engine load due to improved aerodynamics. (Why aeromodding a car with a CVT might show better results than a conventional automatic or manual.)

But having said that, I've never seen a vehicle that doesn't use less fuel as a result of less throttle input!

[Gasoline Fumes]

Originally Posted by Tesla

Just going by seat of pants accelerator position and coast downs at various speed, my personal experience does not seem to corrolate the same, when I pull the foot off the throttle to coast, deceleration seems to decrease at an exponential rate, once down to about 40mph it's like the only thing that slows her down is an incline, otherwise she just keeps rolling.

I assuming you're coasting in neutral? My lightweight brick of a car will coast forever at 20 MPH, but loses speed pretty quickly at even 30 MPH. A heavier vehicle is going to store a lot more energy.

[3-Wheeler]

Originally Posted by Tesla

....... when I pull the foot off the throttle to coast, deceleration seems to decrease at an exponential rate, once down to about 40mph it's like the only thing that slows her down is an incline, otherwise she just keeps rolling.......

I have two direct comparisons for coast down testing from 30 mph:

1) The Honda Insight with tail and smooth under-tray
2) 1985 VF500 Honda with custom full size fairing

Both vehicles are light-weight examples of each class, i.e. single-track and dual-track.

However, when EOC coasting, the Insight will *easily* coast a 1/4 mile on a flat section of road, while the motorcycle coasts only about 1/8 mile.

Same conditions, same section of road, similar outside temperature. Same road used to get to work for the past 20 odd years, with plenty of chances to compare the two.

The car keeps going, while the motorcycle has way more air drag for it's weight than the car does, and slows down much quicker.

Big difference in air drag at only 30 mph.

Jim.

[3-Wheeler]

Originally Posted by Tesla

....Indications are that Cd does change with speed, i.e it should improve with speed....

This is called "wishful thinking".

I actually run into a guy at work like this.

He claims "my car gets the best mileage at 85 mph on the freeway". And continues "when I slow down, my mileage drops!".

There is no convincing him otherwise, so I just nod my head and go on my way.

Jim.

[3-Wheeler]

Originally Posted by MetroMPG

.....But having said that, I've never seen a vehicle that doesn't use less fuel as a result of less throttle input!

Darin,

You already said it above.

And that is exactly the reason why one of our summer college interns, who is the driver in a super high mileage vehicle, was telling me his driving strategy.

Did he tell me he accelerates hard to highway speeds, then shutoff to run in the "reduced Cd Zone"? No.

He starts the engine, runs close to wide-open-throttle at low rpm, gets to his target speed, which varies due to course shape, shuts down, and .....
..... coasts......

He indicated that, again depending on course shape, his average speed is maybe 20 mph. That's it.

His class of vehicle gets about 600 mpg. Not too shabby.

The car has a long tail too.

Jim.

[mikeyjd]

Originally Posted by 3-Wheeler

Darin,

You already said it above.

And that is exactly the reason why one of our summer college interns, who is the driver in a super high mileage vehicle, was telling me his driving strategy.

Did he tell me he accelerates hard to highway speeds, then shutoff to run in the "reduced Cd Zone"? No.

He starts the engine, runs close to wide-open-throttle at low rpm, gets to his target speed, which varies due to course shape, shuts down, and .....
..... coasts......

He indicated that, again depending on course shape, his average speed is maybe 20 mph. That's it.

His class of vehicle gets about 600 mpg. Not too shabby.

The car has a long tail too.

Jim.

Pics?

[Tesla]

Originally Posted by MetroMPG

But having said that, I've never seen a vehicle that doesn't use less fuel as a result of less throttle input!

My understanding was that for most cars FE is a U type curve, so too slow or too fast and FE goes out the window.
Are you suggesting it is an inverse relationship?, i.e. if we look the throttle at idle speed the vehicle will achieve it's best fuel economy.

Originally Posted by Gasoline Fumes

I assuming you're coasting in neutral? My lightweight brick of a car will coast forever at 20 MPH, but loses speed pretty quickly at even 30 MPH. A heavier vehicle is going to store a lot more energy.

Yes, coasting in neutral and yes mine is quite heavy, so there may be some effect from momentum that I am seeing in my experience.

[Tesla]

Originally Posted by 3-Wheeler

This is called "wishful thinking".

I actually run into a guy at work like this.

He claims "my car gets the best mileage at 85 mph on the freeway". And continues "when I slow down, my mileage drops!".

There is no convincing him otherwise, so I just nod my head and go on my way.

Jim.

Maybe you should read the thread linked earlier, and there is another link in that thread to the full conversation.
The impression was that these people had direct experience in the area, so I am just relaying what was infered to be fact.

Also if you noted the context in which I wrote it,

Indications are that Cd does change with speed, i.e it should improve with speed, still counter to my perceptions, so am still trying to work through it and also wondering how the different types of aero drag interact at lower speeds.

Why would I propose a wishful thinking proposition that was counter to my perception and argument?