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

After reading some threads where both maintaining lower speeds and aero mods were discussed, I began to think these two are mutually exclusive.
Aero mods are only effective at higher speeds and one would even question if increased skin drag at lower speed would result in negative impacts.

I may be wrong but as I understand it below about 45mph the only real relevance aero has is in cross sectional area, and outside cutting the roof off it other aero mods do not change this factor. so boat tails, side skirts and even grill blocks are probably a waste of time if you spend most of your time at lower speeds.

Aero drag is primarily the simple power:velocity relationship, which basically says double speed and power required is increased 8 fold, hence power required at 45mph is less than half of that at 60mph just to push the air out of the way.

Then there is the speed and form relationship, the greater the speed the greater the turbulence, as I understand it form becomes less relevant at lower speeds, boundary layer becomes thinner, turbulence is reduced, so the only thing you get by the nose cone, boattail, wheel covers etc is lots of strange looks.

I think many people overlook this, if it is purely a town car, then Hypermiling is the way to go, if you do a lot of HWY at speed, then get some aero on as well.

Many peope OVER estimate the speed at which aero drag becomes a significant factor in fuel consumption.

Have a look at the aero & rolling resistance calculator in the Tools section of the Garage (link is below the header, top of page). It will shed a lot of light on this issue.

E.g. the default values on that page are for my car (98 Firefly/Metro) in stock form. Notice the speed at which aero drag accounts for *half* of the fuel burned? 30 mph. Not insignificant. At 45 mph, over *70%* of fuel consumption is spent overcoming aero drag.

Drag reduction is worth pursuing unless all your driving is stop & go.

Another thought: The average speed in the Shell fuel economy competitions is probably in the neighbourhood of 20 mph... yet just look at how much effort goes to aero there.

As the example of technical information is illustrated above, my own "seat of the pants" experiences say that I can start to feel differences between 30-35 mph after application of aerodynamic enhancements.

Have you ever gone 30-35 mph on a bicycle?

It's pretty darn fast.

The fact that modern cars isolate you so well from your environment should not dissuade you from the actual affects of physics.

I had this discussion with one of my friends when we were driving on the motorway at a speed of 120 kph (75 ish mph).

I asked him to open the window and stick his hand out but first predict how much force he would need to hold it out. It was way more than he expected :)
That times 100 is how much force the car needs to overcome to maintain speed, I told him.

I've taken a small sailboat around with just a slight breeze. Aero matters at all speeds, its payoff just gets more dramatic at high speed.

Metro: 4.22 aero hp @ 45 vs 7.71 aero hp @ 55.

The percentage increase looks huge.

The actual hp increase of 3.49 hp, not so much.

This is the chart MetroMPG was talking about. These are the default values for the chart, I believe they're for a Geo Metro.

At 45 mph 70% of your power is going to overcome aero losses! You have to slow down to ~29 mph to even get down to 50% aero losses. Aerodynamics are huge even at low speeds.

http://ecomodder.com/forum/attachmen...1&d=1378999075

But since the change in power required is almost equal (proportional) to change in fuel burned, a percentage discussion makes more sense to me.

Daox, can that chart go somewhere easy to find like the efficiency mods or hypermiling tips links at the top of the page? I would like to link back to that during any mods I am doing and possibly link to it on other forums and whatnot.

Also, is there a thread that explains each column? So the %aero and %rolling are how much energy is going to overcome them? So at 35MPH, 59% of your fuel burned goes to overcoming aero?

Thanks!

You can make your own chart with your own values under tools.

Aw, which is where that exact link is! Perfect! Thanks!

As Frank said its at the top of the page under "Tools".

The values in there depend on the vehicle, I believe the default ones are for a Geo Metro. The link at the top of the calculators page explains things more:

http://ecomodder.com/forum/showthrea...ower-6341.html

I have a 3.5 foot long tail on my car, so I can speak of several benefits:

1) I drive slowly in the country side, and the tail has a very obvious benefit to those who come up behind me. They immediately know that something is going on with the car, *because* of the tail and usually do two things. 1) Pass or 2) take pictures/videos, gawk, or otherwise study the shape, then move on.

1a) People are way more inclined to promptly pass, rather than wondering why this person is driving so slowly and then hesitating to pass, if I did not have the tail on the car.

2) Ever watch the Tour deFrance and notice men with tights, elongated helmets, bikes with airfoil shaped frame tubes and so on? They are counting grams of air drag force, and all the aero shapes help to make them more efficient. Could you imagine riding a *normal* road bike at 30 mph for one hour without these benefits?

3) How about anyone with a Velomobile. Imagine being able to *cruise* at 30 mph instead of 20 mph. The airfoil on the bike allows this to happen.

4) The tail still gives me a 5% increase in fuel mileage at the slow speeds I drive at, and yes, I will gladly take this small increase. If I happen to take a road trip on the highway, which does happen on occasion, the tail is ready and waiting.

4a) Summer road construction has forced me to take a different route, and these can include highway driving, so the tail is definitely nice to have.

Jim.

Sure, but those have motive power sources in the fractional HP range.

That 3+ hp on a Metro from 45 to 55 represents what- less than 7% of available power.

On something with a bigger engine to car ratio (most anything) the percentage is even less.

aero
 

when you consider that some 95-sq-ft, 80,000-pound tractor trailer rigs can achieve higher fuel economy than some 36-sq-ft pickup trucks driving around Denton,Texas, you begin to appreciate the implications of low drag.
If you intend never to leave the urban landscape,a Smart Car,Scion IQ,Fiat 500,etc.is a pragmatic solution.EVs would be my choice.

Take a piece of plywood and cut it to size to duplicate the frontal area of your car. Bolt it to the front of a bicycle and see how fast you can pedal, compared to not having the piece of plywood bolted to the bike, or just run with it (the plywood). 90% of a bicycles power requirement at 30 MPH is aero drag.

regards
Mech

^Why? The Cd would be all wrong.

plywood
 

Cd 1.11 X approx. 6-sq-ft Af = 6.6 sq-ft CdA.
Ouch! :eek:

Getting nowhere fast?

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...

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.

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!

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.

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.

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.

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?

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.


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.

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

Measurements are what we use to confirm or refute perceptions. The FE curve from Chilton's old mileage improvement book showed peak mpg for a 70's Camaro at 20-30 mph. With my bicycle front fairing I get measurable improvement at 10 mph.

So your improvement with bicycle fairing then provides you with a basis for a hypothesis that you can test a similar improvement on your car.
The ongoing bicycle analogies do not provide any evidence for improved aero at those speeds on a motor vehicle, they merely suggest a corrolation which forms the basis of a hypothesis that needs to be tested on said vehicle.

I formed a hypothesis on the basis of information gathered, much from this site, that the addition of wheel covers, air dam and a few other mods I should see close to 10% improvement in my FE, well it didn't show zip.

Does this mean that aero is a waste of time, no I don't think so, but what it does suggest to me is that it is a lot more complex and just because it improves FE in one persons situation doesn't mean it will do the same for everyone else.
I don't have all the equipment at my disposal to determine how these mods are modifying flow around my vehicle I can only propose ideas to the best of my knowledge and then test them with the rough cut tools at my disposal.
In some cases it may merely be that my testing is not sensitive enough in others it may well be that an improvement in one area may be negated by altered flow elswhere.

As said earlier, I'm not fully convinced that it's as simple as CdA when one gets into the lower speed ranges, but I'm still actively searching and reading to try to understand better what is really going on.

In all this, I always have Aerohead in the back of my mind saying fix the rear then look at the rest, wish I could but a boattail or even Kammback is not really feasable ATM for my vehicle.

Start with mechanical mods if you think they will make a bigger difference.

The torque converter lockup was my biggest improvement followed by the alternator delete. Is yours auto or stick?

Aero will help at your speeds but you need to log you MPG to tell what works best on your particular vehicle. Every vehicle design responds differently.

I went 19 to 33mpg and I felt like you when I started.

Never underestimate the power of incremental ecomods!

Cheers!

HALF the load on the drivetrain comes from aero drag at just 28-30MPH. Ignore aero drag and you will be stuck with incremental gains. Improve aero drag and make substantial gains; and if you want to add the incremental stuff too, then great!

YMMV

Regarding the rest:

"Big words make my head hurt"
--Colonel Cameron Mitchel, "Stargate: SG-1"

:D

Road Load Curve
 

On GOOGLE,at 'Development of aerodynamics for a solar race car', by Ozawa et al.,HONDA Research & Development Co.,Ltd.,they offer a Road Load power curve for both their solar racer and a 'conventional' car (read Honda Accord).
I enlarged the table to 11" X 17".At any velocity you can just pick off the aero or rolling resistance road load component.
It's an easy way to see what aero is doing at the lower velocities.And it's prima facie evidence.No Unicorn-Corral stuff.Worth a look.

The link: Development of aerodynamics for a solar race car

Bicycles were mentioned in this thread, thought this chart might be appreciated here.

Squoval 3 - Cervélo
http://www.cervelo.com//Media/images...-1-600x320.JPG
http://www.cervelo.com/media/images/...-0-960x480.jpg
http://www.cervelo.com//Media/images...-2-200x205.JPGhttp://www.cervelo.com//Media/images...-1-300x288.JPG

Too bad they didn't test that tube profile the other (better) way around.

It probably wouldn't sell, too much ignorance of aerodynamics out there.

If the customers were educated about aerodynamics, they would have to get past the sales people providing misinformation.

If the sales people were educated about aerodynamics, they would have a hard sell with the customers.

The lowest common denominator seems to rule most things in life, which is why we celebrate those who manage to pop their heads above it, if even for a short while.