Quote:
Originally Posted by 3-Wheeler
Thanks for posting that information Aerohead,
Take a look at your data below. The data is curve fit with a second order polynomial so we can fill in more points.
The data shows several interesting things:
As expected, the aero portion of the drag is not linear, but rises sharply since it is a cube function (velocity ^3).
The road drag is linear and is directly related to the velocity!! This means that my initial hunch about rolling friction of moving components in the drive train was correct. Apparently how this relates to my coast down testing was misinterpreted and needs to be looked at further.
With this in mind, I will reword the apology about the Instructables posting regarding Crr in an earlier post.
Notice also the total drag at 55 mph. See that the aero portion consumes about 70% of the total drag. This very close to the data generated for the Insight, and essentially collaborates my coast down numbers. Apparently that old GPS unit doesn't do too bad of job for this application.
This is also good news for anyone making aero mods to their vehicles, as it shows that the vast majority of drag is due to the air portion of the overall drag.
Thanks again Phil !! 
Jim. |
Jim,you're scary-fast with data! That's great!
From your curve,at any slope-intercept,you can just "read" your power consumption.The beauty of curves!
I re-visited my SAE J1263 Recommended Practice for the coastdown.I get cross-eyed looking at it.
They're very specific in their methodology:
* Vehicle weight per axle,within 10-lb accuracy.
* Moment of inertia for wheel,tire,brake rotor or drum ( they do have an estimator for it).
* Drag coefficient dependence on yaw angle.
* Temp coefficient of rolling resistance.
* Vehicle test mass
* Effective mass of wheel,tire,brake components
* Total equivalent mass of drivetrain components
* Total effective vehicle mass
* Tire rolling radius
* Time,within 0.1% of coastdown interval
* Time,resolution 0.1 seconds
* Speed,accuracy plus or minus 0.25 mph(0.4 km/h)
* Speed,resolution 0.1 mph ( 0.2 km/h )
* Temp,resolution 2-degrees F or 1-degree C
* Temp,accuracy plus or minus 2-degree F or 1-degree C ( shielded from radiant sources)
* Atmospheric press. Baro with plus or minus 0.2"Hg or 0.7 kPa
* Wind: speed,direction,continuously monitored with plus or minus 1-mph or 1.6 km/h accuracy.
* Tire press plus or minus 0.5 psi or 4 kPa
TEST CONDITIONS:
* Temp 30 F( -1C ) and 90 F ( 32 C )
* Fog No!
* Wind: 10-mph max ( 16 km/h ) or 12.3 mph peak ( 20 km/h )
* Ave crosswind vector max 5 mph ( 8 km/h )
Road: dry,vlean,straight,must not exceed 0.5% grade,and if there is grade, grade must be constant.Concrete or rolled asphalt.
COASTDOWN SPEED RANGE
* 60mph-25 mph ( 100-40-km/h )
Frontal area can be estimated @
0.80 X H" X W"
--------------
144 " SQ per Ft-sq
WARM-UP
* 30 minutes of continuous driving @ 50 mph immediately prior to test.
Runs: 10-runs minimum in alternate directions
Procedure: Accelerate to 65 mph,start recording equip.,de-clutch,shift to neutral,re-engage clutch,coast.Repeat in opposite direction.
A root-mean-square deviation of 0.25 mph disqualifies the data and it must be discarded along with it's reverse direction paired run.
At least 3-sets must qualify or coastdown is invalid.
Al's closing at 4:00 so I'm about out of time.Hope these tidbits are of use.Keep us apprised of your monster numbers-crunching! See ya,Phil.