Coast down testing, 2000 Honda Insight (gen. 1)

[3-Wheeler]

Hi Metro,

For someone wanting to get some coast-down numbers, what have you determined to be a reasonable mph to start the test?

So far I've heard something like 100 kmh, which is close to 62 mph.

For data logging, I have a Garmin hand-held and can set it to record mph data every two seconds.

Also, we have to keep track of the actual baro and temperature for each run.

I'm planning on recording these numbers over the course of the winter and into spring time. That way I can build up a data base of numbers and see how consistent the coast down tests are.

I used Google Earth to find several stretches of road that are flat within a few feet over a 1/4 mile or so. The plan is to use the same stretch of road and start accumulating data. This is one of the roads I take to work.

Then when I make some runs with the under body panels off, I can get several runs and compare to the data base already accumulated. This will also give an idea on just how variable the coast down process is.

Performing this testing over the next few months will allow one to see the effects of the colder air and higher baro, and hopefully have an idea what the coast down numbers should be for various outside conditions.

With enough data, we can extrapolate to data values that we do not have, within reason.

Obviously I'd like to keep my top speed to a minimum to be legal, yet get some good numbers.

If anyone has ideas on this, please let me know.

Jim.

[MetroMPG]

Originally Posted by 3-Wheeler

Hi Metro,

For someone wanting to get some coast-down numbers, what have you determined to be a reasonable mph to start the test?

I started at 90-100 km/h for mine.

If you want to go by the book, see this thread: http://ecomodder.com/forum/showthrea...test-2992.html

Phil describes the SAE procedure and refers to the paper that details it.

Then when I make some runs with the under body panels off, I can get several runs and compare to the data base already accumulated. This will also give an idea on just how variable the coast down process is.

I think you will find it to be extremely variable, to the point of making comparisons difficult that aren't of the immediate A-B-A variety in identical conditions.

The Garmin will help quite a lot vs. the old method of relying on on the driver to watch the speedometer and call out speeds into a recorder.

[MadBill]

A couple of posts back, 3-Wheeler said:

"Performing this testing over the next few months will allow one to see the effects of the colder air and higher baro, and hopefully have an idea what the coast down numbers should be for various outside conditions."


It should be kept in mind that operating temperature has a greater percentage effect on tire rolling resistance and the viscosity of drivetrain greases and oils than on airflow. Nonetheless, valuable data..

[3-Wheeler]

Originally Posted by MadBill

...It should be kept in mind that operating temperature has a greater percentage effect on tire rolling resistance and the viscosity of drivetrain greases and oils than on airflow. Nonetheless, valuable data..

Hi Bill,

That's exactly the reason I will like to get data and quantify this effect.

There are sites out there that show how to perform coast down testing, but do not make it obvious to consider temperature and it's affect on rolling resistance (Crr).

Jim.

[Bicycle Bob]

"Obviously I'd like to keep my top speed to a minimum to be legal, yet get some good numbers.

If anyone has ideas on this, please let me know."

In many areas, the unofficial rule is that in good conditions, the speed limit goes up 10% or more. You can probably start your coast down tests at the average actual speed others go on that stretch with little risk of a ticket. You might even get more significant numbers for the aero mods while reducing the risk of a low-speed ticket.

[3-Wheeler]

Originally Posted by Daox

...What Aerohead said is true, but much more of a general rule. I assume you are talking about the rule that says something to the effect of 'A 20% reduction in Cd gives a 10% improvement in FE'. That is not going to the case at 55 mph and at 20 mph. Its just a general rule...

Hi Daox,

Yes, you are right. That's exactly what I was trying to say.

As I do coast down tests and discover just how much the air drag and rolling drag contribute to the overall vehicle drag, maybe a glimmer of light is appearing for me, on what Aerohead is alluding to.

At lower speeds, since the air drag is almost the same proportion of overall drag as Crr is, then each contributes about 50% to the overall.

As the speed increases, let's say towards 100 mph, then 90% of the overall vehicle drag is mostly air drag, and Crr is only 10%.

So if Metro lived in Germany and could drive the Autobahn at 100 mph, his decrease in CdA because of the boat tail would give him a resounding 30% decrease in drag and:

90%/100% * 30% CdA decrease = 27% decrease in fuel consumption at that speed

However, at lower speeds, let's say 20 mph where the CdA contribution and Crr contribution are each 50%, then his fuel consumption would be:

50%/100% * 30% CdA decrease = 15% decrease in fuel consumption.

If this is not the case, then there are other factors involves that dictate overall fuel consumption on the vehicle such as tuning of the engine, gearing of the vehicle and so on.

If Aerohead has some info on this, it would be nice to elaborate.

Jim.

[tasdrouille]

Originally Posted by 3-Wheeler

__________________________________________________ ________

Note that Crr does not simply imply tires, but all the moving components in the drive train and so on.

__________________________________________________ ________

AFAIK only the components on the wheel side are included in RR. That is the tires, wheel bearings, brakes and associated seals.

[3-Wheeler]

Hi Guys,

So far in my coast down testing, there are about twenty runs to crunch numbers on. The maximum speed for the runs was 60 mph.

For the record, the vehicle is a 2000 Insight, of stock weight, smoothing under-body panels, and tires pumped to 50 psi.

The twenty runs have been mostly on the way to work where there is a mile long stretch that is not too beat and rippled due to low temps.

I had the day off and spent quite a bit of time coming up with a math model to duplicate the actual coast down numbers.

*If* this was done correctly, then the Insight consumes:

About 67% of it's gas at 55 mph from air drag
And the remainder (33%) is rolling resistance due to drive train.

Aerohead may be able to substantiate this with his past experience. However the math model matches the real world data extremely well with this mix of aero drag and rolling resistance.

__________________________________________________ _________

EDIT (01-09-10 based on Aerohead's data in another post):
The coast down testing clearly shows that a single constant or Crr (coefficient of rolling resistance) is a good approach for modeling the effect of rolling resistance versus overall vehicle drag.

However, it should be noted that Crr, while being a constant, does not imply that rolling resistance is static, or unchanging. Rather it is dependent upon the velocity of the vehicle under test, and is linear in fashion. As the vehicle speed increases, the rolling drag of the vehicle increases by the same proportion.

All of the runs so far have been in winter conditions, so the effect of temperature on the Crr can not be determined until the temperatures rise.

__________________________________________________ _________

Jim.

[aerohead]

3-Wheeler,just catching your post.My CRX data are at home,I will try to remember for tomorrow.I'm trusting my memory less and less now so rather than shoot off my mouth with nonsense I'll get the written materials together for you.Catch you then.

[Christ]

Originally Posted by aerohead

3-Wheeler,just catching your post.My CRX data are at home,I will try to remember for tomorrow.I'm trusting my memory less and less now so rather than shoot off my mouth with nonsense I'll get the written materials together for you.Catch you then.

I trust your incomplete memory much better than my total lack of understanding for certain aspects of FD... you've got more time in than I do.