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

[Tesla]

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.

[MetroMPG]

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.

[kach22i]

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.

[RedDevil]

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.

[Fat Charlie]

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.

[Frank Lee]

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.

[Daox]

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.

[MetroMPG]

Originally Posted by Frank Lee

The percentage increase looks huge.

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

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

[jeff88]

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!

[Frank Lee]

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