Quote:
Originally Posted by cephraim
Is anybody interested in influencing TireRack's next LRR tire study?
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Thanks, Craig, for putting this out there. I'd like to see real rolling resistance testing on a test rig. There are too many variables involved in real world tests to be able to conclude anything from mpg road testing -- Tire Rack's results are all within the margin of error for the test method, so really have no meaning. Even on very high quality dynos, like those used for the EPA MPG figures, with accurate adjustments for for temperature and baro pressure, no wind at all, consistent test fuel, etc etc, 2% is too small a MPG difference to be measured accurately. (I've built and operated somewhat less sophisticated dynos, which I consider fairly accurate, but I had the owner of an EPA-certified facility tell me that =/- 5% is about as close as he can get with just one test.)
On the other hand, the large differences in rr can be measured on test rigs to fairly tight limits. Low rr tires range from .006 to .010 (.010 being average rr) so drag readings, with a 1000 lb load, go from 6 lb to 10 lb: figures that can be measured to within .1 lb pretty reliably.
A test rig does not necessarily provide an actual rr as would be seen on a given road, because some use a large wheel on which the the tire rolls, whereas roads are comparatively flat. Also different road surfaces produce different rr figures, and in the same way the test rig "road" surface produces a figure that relates to that surface alone. So to be comparable, the rr figures must come from the same test machine. You could not compare Bridgestone's published .006 for the 381 with a number from a different test machine.
I think Tire Rack has gone a great job -- better than any other vendor that comes to mind, but real rr numbers would further set them apart from the competition.
I am not convinced that manufacturer rr numbers are comparable from one manufacturer to the next (and for the most part, it is hard to find manufacturer's numbers). (In fact, as I think about it, why not have the rr figure as determined by standardized equipment, right on the sidewall of every tire, alongside the tread wear and traction data?)
I think the test data from the California study was all obtained from the same test equipment (I'd have to recheck) so if that info were updated every year, then Tire Rack could publish that data, perhaps instead of their MPG data.
The Tire Rack method could be interesting if they did one extensive test with a confirmed .010 tire set and a .006 tire set on the same car, in ABABAB fashion, to illustrate the results that are obtained from this increment in rr.
Getting slighly more complicated, they could redo these test for a light and boxy car (for which aero drag predominates more of the time) and a heavy and streamlined car (for which rolling drag predominates more of the time).
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