Quote:
Originally Posted by Captain Insano
Wouldn't overall vehicle and rotating weight play a large role in the propsed test, not just drag? If the wheels had a decent amount of weight difference, I woudl think even over 1 lb per wheel/tire, that might be affecting the overall time it takes vehicle to slow down during the test. So I'm saying you have at least two variables at play in this test. And really more, wind speed changes that occur in real life, even just from minute to minute. Things like that.
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http://aspeckt.unitbv.ro/jspui/bitst...4030-Paper.pdf is a reasonable starting place.
You are obviously correct about other things impacting the slowing of the vehicle. Those would include rotating inertia, mechanical drag, and aerodynamic forces. those would all be then further impacted by other parameters such as sloped roads, road crown, wind, etc. The procedure however can still be broken down into a characterizable phenomenon.
Mechanical drag - generally very linear and constant. dependant on the total mechanical friction of the drivetrain.
Rotating inertia - agian, very consistant, and not really all that variable. The impact is easily calculated.
outside forces - can be adjusted by running repeated tests in both directions on a given piece of road. This will cancel out wind and slope.
aero forces - follow a power rule and will look very different from the other forces at varying speeds.
One would need to perform a fairly large number of coast down tests on a known circuit at a variety of speeds in both directions. Then, a regression analysis could be performed which should make it possible to separate out the aerodynamic forces mathematically from other forces. I should note, that unless you have an instrumented suspension, you will afaik only be able to extract the force of drag. Extracting downforce would require knowing and measuring suspension defflection. Once a baseline analysis was performed it would be fairly simple to perform further tests with modifications and basic data collection.
This type of testing is so common/helpful in motorsports that Chip Ganassi actually bought an old tunnel in Pennsylvania
(
http://www.racecar-engineering.com/a...f-laurel-hill/). These tests are highly useful to establish correlations between CFD, wind tunnel, and real world testing which is a serious problem.
EDIT: SAE J2084 lays out a procedure for coastdown testing. if anyone wants to spend $80
http://engineers.ihs.com/document/ab...RXCAAAAAAAAAAA