[EarlQHan]

Originally Posted by mobybrz

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

Originally Posted by Captain Insano

I am not saying it would be useless, only that with all those variables simultaneously in play you might draw erroneous conclusions. Besides the external elements, if the car did appear to have more drag but the wheel tire combo was lighter than oem, was it more drag of the 9" wheel or the weight change affecting inertia? On the flip side, if wheel tire combo had what appeared to be less drag and it was heavier was it due to weight or drag. See what I mean? And you might have in reality more drag on that wider wheel, but it actually appears to have less overall drag because the weight is heavier and having a more dramatic affect on the test (inertia versus drag) on proposed coast down from speed test.

Your article about the tunnel actually supports what I am saying. It is a very controlled environment.

The original person that asked about this was not talking about a static car. u/Josh was asking if the proposed road test would be ok to determine drag difference of using a wider wheel.

To be honest, I think you guys are overthinking a very simple matter. These are low-powered, production-based, closed-wheel cars. The aero effect is more than likely negligible for a 7.5" wide wheel vs. a 9" wide wheel. The tire is the important part, and assuming you make a similar increase in tire width, the additional grip will overcome the drag and inertial forces in lap times. And that's the thing that really matters here; sometimes it's really easy to lose sight of that.

IndyCar reference isn't relevant in this case, now more than ever, because it's a spec series. They try to make gains by reducing drivetrain loss. I know because my old roommate is an IndyCar engineer and they'd do things like make brake dynos and diff dynos and all other sorts of ridiculous things to gain .01 s.

Edit: Simplest solution I can think of is stretching the OEM tire on a wider wheel. Some wheels, like the RPF-1 might be as light as stock, making the weight similar. I imagine the inertia isn't too different. Do a coast down with each. Not exact, but it can be used as a reference if you want to know how a wider wheel alone will affect the car.