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

[jeff88]

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

[Daox]

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

[3-Wheeler]

Originally Posted by 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 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.

[Frank Lee]

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.

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.

[aerohead]

Originally Posted by 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.

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.

[user removed]

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

[Frank Lee]

^Why? The Cd would be all wrong.

[aerohead]

Originally Posted by Old Mechanic

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

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

[Xist]

Originally Posted by Old Mechanic

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

Originally Posted by aerohead

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

Getting nowhere fast?

[gone-ot]

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