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Old 05-13-2011, 10:24 PM   #1 (permalink)
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Minimum speed for aero advantages

Hi guys,

I've been trying to take in as much as I can from this forum over the last couple of months, but there's just so much great information that it's hard to remember everything and make sense of it all at times. I apologize in advance if this question has been asked before, but I can't seem to find an answer to it doing a few different searches:

What is the minimum speed at which aerodynamics become a factor in helping to increase MPG?

On my daily commute (Northern NJ) of approx. 10 miles, only about 3 of those miles are 55/65MPH highway driving. I drive ~55 on the 65 stretch and ~50 on the 55 stretch. But off the highway, I still have quite a bit of 35-45MPH roads with rolling hills, gentle slopes, etc. that I'm E(on)C in neutral with an automatic. Will doing basic aeromods to the FuguMobile help on those 35-45MPH coasts? I realize that the *amount* of help the mods give at that speed would be less than on the highway. But would they still help to some degree?

Guess what I'm asking is: is there a minimum speed which, if you are below it, the aero stuff just doesn't make a difference at all?

Thanks for any advice, answers for the new guy!

John

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Old 05-13-2011, 10:32 PM   #2 (permalink)
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Hi John,

35-40MPH is about the threshold, I believe in most cars. That is about the speed when aero drag is about 50% of the drag on a vehicle (steady speed on level ground), I think? Lowering drag is important on any vehicle, pretty much, as it is total loss. With weight, you can at least "get it back" to some extent by coasting. In colder temperatures, drag has a greater affect.

What are your tires inflated to? How is the alignment? Have you checked for brake drag?
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Old 05-13-2011, 10:34 PM   #3 (permalink)
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Quote:
Originally Posted by JohnS View Post
Guess what I'm asking is: is there a minimum speed which, if you are below it, the aero stuff just doesn't make a difference at all?
No. Just ask someone on a bicycle, or even roller blades. The amount of fuel spent overcoming aerodynamic drag is less at lower speeds. However, there will be non-zero aerodynamic drag at any airspeed other than zero. Check out the calculator: Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com

You won't see as large of a benefit from aeromods as someone who drives 65mph, but you would still see a benefit.
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Old 05-14-2011, 01:08 AM   #4 (permalink)
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Hi John,
I suspect the answer is around 60 Kph (35 Mph) or so as is mentioned above but it seems , from what I have been able to find , the more aero the vehicle the lower the speed at which the benefit is obtained.

I had the chance to get some info from one of the engineers working on the GM Solarcar project and his information suggested the actual benefit may cut in as low as 20 mph in some cases where the vehicles are very streamlined.

Peter.
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Old 05-14-2011, 09:04 AM   #5 (permalink)
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A commuter on a bicycle slogging along at 10mph will benefit from aeromods.

Even at slower speeds, a headwind can cause aeromods to make a big difference.
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Old 05-14-2011, 10:17 AM   #6 (permalink)
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I drive 3 miles to work and hit 40 MPH maximum. The trip mileage will vary by +/- 3 MPG depending on the wind.

Once I drove 3 miles at 35 MPH against a strong headwind and got 20 trip MPG. The truck sat long enough to completely cool off, then I drove back at the same speed with a strong tailwind and got 30 MPG.

Aerodynamic drag is important even at low speeds.
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Old 05-14-2011, 01:58 PM   #7 (permalink)
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Quote:
Originally Posted by RobertSmalls View Post
Just ask someone on a bicycle, or even roller blades.
Skated this morning. Even at 10-15 mph, we draft each other to cut down on the effort required. There is no minimum. The speeds you described are plenty fast.
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Old 05-14-2011, 03:10 PM   #8 (permalink)
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JRMichler -

Quote:
Originally Posted by JRMichler View Post
I drive 3 miles to work and hit 40 MPH maximum. The trip mileage will vary by +/- 3 MPG depending on the wind.

Once I drove 3 miles at 35 MPH against a strong headwind and got 20 trip MPG. The truck sat long enough to completely cool off, then I drove back at the same speed with a strong tailwind and got 30 MPG.

Aerodynamic drag is important even at low speeds.
I agree. What I like to say is that aero helps at any speed. At the slower speeds you may not detect it, but it adds up over time.

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Old 05-14-2011, 03:43 PM   #9 (permalink)
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Hi John,I think Peter hit on it.
If you look at the SAE protocol for coastdown tests you notice that the aero drag portion isn't out of the picture until down around 20-mph,where rolling resistance basically dominates the road load from there on down.
Since air drag varies as the square of velocity it builds geometrically as your speed increases,and as mentioned,if you get into a headwind your 'airspeed' will climb even though your speedometer is blind to it.
And since the horsepower necessary to overcome air drag varies as the cube of velocity ( at double velocity you hit twice as much air,twice as hard,twice as often ) a headwind can really mess with you,especially on that portion of your commute when your doing the higher speed.
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Old 05-14-2011, 09:21 PM   #10 (permalink)
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Thanks guys!

I appreciate your responses and all the great information...

Quote:
Originally Posted by NeilBlanchard
What are your tires inflated to? How is the alignment? Have you checked for brake drag?
Before discovering the EM site, I was at 34psi. I went up to 46psi and noticed that I consistently rolled further and faster on my regular coasts. I'm going to bump it up to 50psi on my next trip to the station. (My sidewalls are 51psi max.)

Alignment is good. Had it done in January just after buying the car.

I haven't checked for brake drag, although I've read it in other posts here. Any advice on how to do that? Until EcoModder, the only thing I knew about cars was where to put the gas and oil. Oh...and how to add the blue window washer fluid! (OK, maybe I wasn't *that* bad. But pretty close.) So I have no specialized auto tools or knowledge. Is it just a matter of jacking up the wheels one at a time, spinning them and listening for rubbing parts? Should I expect a certain number of rotations?


Quote:
Originally Posted by aerohead
I think Peter hit on it.
If you look at the SAE protocol for coastdown tests you notice that the aero drag portion isn't out of the picture until down around 20-mph,where rolling resistance basically dominates the road load from there on down.
I had guessed that 35MPH or there abouts was the speed at which the aero wouldn't be a factor. But after reading everyone's explanations about roller bladers and bikers, I can see where it still makes a difference at lower speeds. Otherwise the serious skaters and bikers wouldn't be wearing those aero-suits and aero-helmets, I guess.

And I didn't even take into account how headwinds add to the relative speed of the car to the air it's moving through.

Okay...so every little bit that can be done to help cut through the air better means that the speed where rolling resistance becomes the limiting factor is lowered. I think I've got a much better understanding of it now.

Appreciate all of you who took the time to help me. And have to add a special thanks to aerohead for the multi-part seminar that is stickied here on the site. Reading through all of that info helped me in visualizing how this stuff works.

John

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