Drag vs speed curves available for modern cars 2

[2dye4]

Due to this gas thing i would like very much to see a curve of the total rolling resistance vs speed for my two cars. Rav4 and Sienna.

I originally set out to measure these myself by finding a lone flat stretch of highway on a windless day and run er up to 75 mph and record the coast down. Fit this to a curve and differentiate and get a rolling resistance proportionality.

I searched Google but had no luck locating this type of data. I don't want the values calculated using ideal formulas and drag coefficients, as the real thing is bound to be better. Does anyone know of a site with data like this.??

Thanks

 

[FEMdude]

I am afraid the data exist, but is not available to public (I have tried to locate them myself with no luck).
So you may need to measure the drag by yourself by performing several measurements in both directions and taking the average.
One possibility could be to tow your vehicle with a very long rope, and measure the force by a suitable scale/force sensor (don't try this on public road, or on road with tight corners .
btw; why topics related to fuel consumption mysteriously dissappear from the forum after short perioid of time?

 

[GregLocock]

B) because they are not work related, for many engineers.

A) you have to be many car lengths behind a towing vehicle before you will get accurate figures.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[SomptingGuy]

Some search terms:

J2263
ABCD
Buckley

- Steve

 

[Ataloss]

Car and Driver magazine (I think that was the one) used to and may still include this in the 'specs' page of their road test reviews. Try looking in there. I'm not sure where they got this number from, whether it was from their own testing or from the OEMs.

Bob

 

[GregLocock]

I doubt anyone would do this testing and publish the results except for money. It would be interesting to see the error bounds on any published data. In my experience it takes about half a days testing on a closed track to get results that make much sense. Wind and gradients have an enormous influence.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[carnage1]

car and driver publishes aerodynamic data on cars (frontal area and Cd), however it does not include the rolling resistance.
Power consumption due to rolling resistance is linear at any legal speeds
Power consumption due to aerodynamic drag is a cubic function of speed at the majority of speeds cars travel (at VERY low speeds it is a squared function)

 

[evelrod]

The things one does to amuse when retired from meaningful labor---I doubt this has anything to do 'with the price of rice in China'...I was amused at the time, though.

In the late 90's just after I retired, I did a little test, just for my own amusement. I had been driving my Model A everyday as is my habit. On the way home I go down a long hill and I always used 'Oklahoma overdrive' with the A only reaching around 30 to 35 mph. One day I was in the wife's Lincoln and tried the same thing. Wow. Speed was in the 60's. This led to the one day I was bored to tears and decided on 'The Big Test' ;o)

Standing start at top of hill and coast to bottom, braking before stop sign. Certainly not scientific, it was never intended to be.
1930 stock Model A Ford Std. Coupe---35mph
1937 stock Buick Roadmaster---45mph
1948 Norton ES-2---25mph
2001 Lincoln LS---55+ not max because of need to brake early.
I ran out of interest at this point and did not do my Dodge truck or the motor home. Now I'm curious about them, but the hill, while it's still there, is in a housing tract.

Rod

 

[carnage1]

wow, I'd have never guessed the motorcycle to be the slowest in that test.

 

[patprimmer]

Have you ever looked at the aero of a motor cycle and rider, especially a vintage motor cycle. Not pretty, even compared to an A model.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[2dye4]

Thanks all

I will check the Car and Driver source
I think i might just do some testing of my own though. Maybe build a little setup to record the data automatically to a laptop.

 

[Artisi]

evelrod has the right idea, measure each unit on a down hill run - preferable without a stop sign at th bottom. I would take 2 readings 1. measure the time from start to a set point along the downhill - you can convert this to average speed if you want to. 2.check the difference in distance between vehicles from start to a complete rolling stop.
This will give you a good indicator of the all resistances not just the wind resistance.

Make sure trye pressure are to spec and brakes are not dragging.

 

[Bribyk]

A downhill coast comparison will work for vehicles of a similar size and type but won't work comparing a motorcycle and a Lincoln, for example. The mass part doesn't cancel out of your "potential = kinetic + friction + drag" energy balance once you add in your drag and friction (as they're not dependent on mass). Your rolling friction is relatively fixed (assuming your not comparing a 4X4 to a 2WD) and not mass-dependent. The motorcycle had a poor top speed because its mass/(friction+drag) ratio is low.

 

[thundair]

This may be a place to start

http://www.landracing.com/formula/coastdown.htm

Cheers

I don't know anything but the people that do.

 

[Artisi]

" Bribyk (Mechanical) 19 Jun 08 10:35
A downhill coast comparison will work for vehicles of a similar size and type but won't work comparing a motorcycle and a Lincoln, for example.------"

The original posting was to compare 2 similar vehicles, hence the suggestion of a downhill comparison.

 

[cibachrome]

Since the coastdown test is used to set the EPA dynamometer settings in a US emissions test, Shouldn't these values be public information? My guess is that there is an avenue to get this information.

 

[GregLocock]

There is an NHTSA or DOE database of vehicle characteristics, I can't remember where it is, or whether it includes CdA.

If not then you are right, they must be in the certification somewhere.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[2dye4]

My take is to find the flattest stretch of road and "when the state police are nowhere to be found" run er up to 75 and monitor the coast down.
So this in both directions preferable from the same starting and end point.
I think it should be possible to calculate out the road terrain given a run in both directions.

cibachrome I also think so.

By the way i spent the last tank of gas in my sienna hyper-miling (driving for efficiency) and got 27 mpg for my efforts.
This compared to 23 normally.

 

[evelrod]

2dye4......Was it worth the effort? For safety sake, better to keep that type of driving away from Socal freeways as these folks haven't figured out that gas is nearly five bucks a gallon. The average speed on the I-15 out of San Diego is still ~80 mph!

Rod

 

[Rob45]

That was Road & Track Magazine, not C&D, that used to do the coastdown testing, and Sports Car Graphic that described the testing as a means of determining rear-wheel horsepower and for determining optimum shift points.

Still, a coast-dwon test is pretty easily done with no more than a stopwatch if you have fairly good resolution on the speedometer. Coast from e.g. 45mph to 35 mph, use that as the data point for 30 mph, etc.
Don't forget to make an allowance for wheel inertias.