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Well... Posted by PGAero [Email]  (#1143) [Profile/Gallery]
(more from PGAero) on Sat, 9 Oct 2010 01:38:07
In Reply to: Do the math! But is it right??, Noel, Fri, 8 Oct 2010 06:27:25 Members do not see ads below this line. - Help Keep This Site Online - Signup |
Your math is right, but assuming a rigid tire is, to me, a tenuous assumption.
The tire interacts with the road surface differently based on tire pressure, tread depth, etc. etc... I would not be surprised to learn that the rolling diameter/circumference is less than the theoretically calculated circumference of the tire based on tire size or an unloaded diameter measurement. The thing doesn't appreciably change is the diameter and circumference of the steel belts in the tire. The rubber, on the other hand, is designed to be pliable.
And, to consider a different angle, if a new tire has a tread depth of 11/32", and an old/worn tire has a tread depth of 2/32", then the diameter changes from 25.3 to 24.7" (Using the same tire as you did.) This affects the number of revolutions by nearly as much the discrepancy we're trying to figure out. A worn tire would result in a theoretical 815 rev/mile. This change during the course of wearing a tire out makes a bigger difference than the discrepancy between 823 and 815.
Most car's aren't even accurate enough in their odometer/speedometer readings to render this change noticeable by their drivers. (Maybe more accurately, only people like me and the original poster notice this kind of thing... because I do, only because I know it's mathematically happening down there)
Speedometer check sections on highways typically go for 5 miles. If your trip/odometer reads to 0.1 miles, then you can check to within 2% accuracy you odometer (0.1/5.0 = 0.02). We've now seen that the rolling radius/circumference of our tires change by about 2% during the course of their life.
My guess is, once again, that the theoretical circumference is different (by 3%?) than the actual rolling circumference. I have to admit that I don't ever remember seeing the figure of 823 rev/mile for tires in the past. (Then again, I haven't paid close attention to that spec.)
Cheers,
~Peter
posted by 212.2.234...
_______________________________________ Current: '03 9-5 Aero Wagon, 5spd, Polar/Black Past: '06 9-5 Combi, AT, Polar/Black '04 9-5 Aero Wagon, AT, Nocturne/Granite '03 9-5 Aero Wagon, AT, Steel/Charcoal '00 9-3 Viggen, 5D, Silver/Black '93 9000 Aero, 5MT, Cirrus/Black (Owned this one twice) '86 900 SPG, 5MT, Edwardian/Buffalo Grey
, Sat, 9 Oct 2010 21:05:16, Sun, 10 Oct 2010 03:28:11, Thu, 7 Oct 2010 19:57:16, Sat, 9 Oct 2010 01:38:07 <-- Viewing This Message, Fri, 8 Oct 2010 07:36:56, Sat, 9 Oct 2010 18:28:22
, Thu, 7 Oct 2010 05:42:38
, Fri, 8 Oct 2010 16:54:56
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