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Old 12-22-2011, 11:56 PM   #21 (permalink)
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Originally Posted by drmiller100 View Post
I did a bunch of calcs, and rolling resistance is ALWAYS the least significant of the three items I listed. At low speeds we assume city traffic with start/stop, and in that case the energy to accelerate the car is by far the biggest contributor.

At high speed (over 40 mph) aero is the biggest issue.

If you have a heavy car at low speed, then acceleration of the car is the biggest issue. If you have a light car at low speed, then acceleration of the car is still the biggest issue.

Your thoughts?
What con
Hi Mr Miller,

Can you list your calculations?

I would like to know how Crr is always less than Cd?

At the speeds I typically drive, Crr is actually larger than Cd (38 mph).

Jim.

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Old 12-23-2011, 12:06 AM   #22 (permalink)
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Originally Posted by 3-Wheeler View Post
Hi Mr Miller,

Can you list your calculations?

I would like to know how Crr is always less than Cd?

At the speeds I typically drive, Crr is actually larger than Cd (38 mph).

Jim.
ok, at low speeds acceleration of the car from a stop is the biggest issue. then comes rolling, and then aero.

I mis"spoke".
But still, rolling resistance is not nearly as important as the energy to start and stop the vehicle in typical city traffic.

Interesting that you put most of your miles at under 38 mph. Do you feel safe at those speeds on the freeway?
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Old 12-23-2011, 12:10 AM   #23 (permalink)
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Originally Posted by jimepting View Post
Jim,

I've been studying your data and I have one suggestion as to a possible error source.

We all know how very sensitive the Insight fuel economy measurement is to tire pressure. Looking over the part of the year you were testing, I'm wondering if you may have had significant tire pressure variations. Tire temperature changes tire pressure. On sunny warmer days, the road would have been warmer, and on cold overcast days the road surface would have been colder. Cold stiff tires have higher rolling resistance.

I have a slight hill in front of my house which runs for .4 miles. When I give my Insight a little push, I gain speed to 8 MPH at the first intersection in the summer. In the winter, I can barely make 5 MPH at that point. You say that the car is thoroughly warmed up, but perhaps the tire temperatures are still varying significantly on different days.

From my racing I also know this. We use a bit more pressure on cold days to put the traction into the sweet spot.
Hi Jim,

You could very well be correct about this.

I simply do not have enough data or experience with coast down testing to say that tire pressure is an issue or not, but I would think that this is indeed a major factor in Crr.

There are several bicycle sites that I have investigated, and rolling resistance seems primarily a "road/tire" function. Tire carcass deflection is a large factor.

I typically keep tire pressure from 55 to 60 psi, but still ambient temperature, road conditions, dust and so on *could* be a big factor and I may not even be aware of the many factors.

All this testing and data collecting is just part of our aero "research and development" process here at EcoModder.

Jim.
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Old 12-23-2011, 12:10 AM   #24 (permalink)
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Originally Posted by 3-Wheeler View Post
Hi Neil,

Obviously at slower speeds, the Crr predominates.

Jim.
what kind of mileage do you typically get?

Does this correlate directly to your Crr???? For instance, for a 2500 pound car, with a Crr of about .008, at 35mph, my math comes back with about 2 horsepower rolling resistance.

which by my math says you should get about 90 mpg if your drag is identical.

Do you really get 90 mpg?
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Old 12-23-2011, 12:16 AM   #25 (permalink)
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Originally Posted by drmiller100 View Post
ok, at low speeds acceleration of the car from a stop is the biggest issue. then comes rolling, and then aero.

I mis"spoke".
But still, rolling resistance is not nearly as important as the energy to start and stop the vehicle in typical city traffic.

Interesting that you put most of your miles at under 38 mph. Do you feel safe at those speeds on the freeway?
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Originally Posted by drmiller100 View Post
ok, at low speeds acceleration of the car from a stop is the biggest issue. then comes rolling, and then aero.
This statement needs to be qualified.

In a stop/go situation, then yes, acceleration can be a large factor. However coasting in the same situation can nullify this effect and then some.

Anyone who gets let's say, 150 mpg during hypermiling contests, is using this same technique to great effect.

I can get higher mileage in town under the proper conditions, than out on a deserted country road.

Quote:
Originally Posted by drmiller100 View Post
Interesting that you put most of your miles at under 38 mph. Do you feel safe at those speeds on the freeway?
It is for that reason that I do NOT drive on the highway, but instead go out of my way to take backroads to/from work. This gives me the freedom to drive at much slower speeds where there is relatively lower traffic.

In this situation, the effects acceleration is nil and Cd next, with Crr being the largest drag factor.

Jim.
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Old 12-23-2011, 12:21 AM   #26 (permalink)
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T

I can get higher mileage in town under the proper conditions, than out on a deserted country road.


Jim.
How???? What makes the city better then country?
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Old 12-23-2011, 12:25 AM   #27 (permalink)
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Originally Posted by drmiller100 View Post
what kind of mileage do you typically get?

Does this correlate directly to your Crr???? For instance, for a 2500 pound car, with a Crr of about .008, at 35mph, my math comes back with about 2 horsepower rolling resistance.

which by my math says you should get about 90 mpg if your drag is identical.

Do you really get 90 mpg?
Actually, yes I get quite a bit more than 90mpg during the warmer months.

Lot's of coasting is involved to get these high numbers, but does show what is possible. In these conditions, Crr predominates.



Jim.
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Last edited by 3-Wheeler; 12-23-2011 at 12:15 PM.. Reason: changed wording slightly.
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Old 12-23-2011, 12:31 AM   #28 (permalink)
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How???? What makes the city better then country?
When driving in the country, let's say my average speed is 38 mph. My sustained effective gas consumption is equivalent to 120mpg while utilizing lean burn of the Honda 3-cylinder engine.

This is the maximum MPG's of this engine design on hot summer days (90F).

To get higher than this, coasting is involved.

The choice now becomes:
1) Coast from 35 mph to 20 mph on a country road.
2) Do the same thing in town in an appropriate speed zone.

(Note: I am not practicing pulse and glide, but simply utilizing the mass of the car to coast to the next stop or change in direction of the road).

The country road coasting is obviously more dangerous, since the posted speed limit there is 55 mph.

So doing the same thing in town is easier if the traffic conditions allow this to happen, since there is less speed differential than the country road approach.

I'm starting to like town driving more than country in certain conditions.

Jim.

Last edited by 3-Wheeler; 12-23-2011 at 12:18 PM.. Reason: Note about pulse and glide
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Old 12-23-2011, 11:55 AM   #29 (permalink)
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Wow.

My apologies.

You are doing something completely different then I thought you were doing.

Your mileage is spectacular!
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Old 12-23-2011, 12:12 PM   #30 (permalink)
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Wow.

My apologies.

You are doing something completely different then I thought you were doing.

Your mileage is spectacular!
Thanks!

Actually, most of my driving on back country roads is very typical of most other motorists, except that I drive at a slower speed to keep losses to a minimum.

On the way to work there are at least 10 sections of road that go from the posted 55 mph limit to a lower speed such as 35 or even 25 mph.

When this transition happens, I shut off the injectors to the motor and stop the ICE, and then coast all the way to the lower speed limit sign.

Sometimes I restart the ICE to maintain a 25 mph speed. Other times I coast down to 10 mph or so towards a stop sign.

I am constantly tailoring my driving to accommodate current conditions, such as traffic or colder/hotter weather conditions and so on.

The ICE engine in the Honda is amazingly versatile, as I can routinely drive all the way down to 28 mph in fifth gear with a very light throttle, and using HV battery assist when needed, and even in the colder conditions. I've had the car three years now, and it's inspiring how the car has taught me how to ecodrive. And that is the main reason why the gas records have continued to climb. The aero mods are only part of the equation.

Driving the way I am right now, I would have to venture that the brake pads would last the life of the vehicle, since they are rarely used to slow down on the drive to work.

Obviously, city driving at rush hour is a whole other matter, of which I am spared due to the nature of my current drive route.

Jim.

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