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Old 03-10-2018, 01:33 PM   #34 (permalink)
gumby79
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little jona - '91 Dodge D 250 first gen cummins LE
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90 day: 23.4 mpg (US)

Little Jona airo modded - '91 Dodge RAM 3/4 TON D 250 2×4 AUTO
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Contact patch vs load vs PSI

Quote:
Originally Posted by RedDevil View Post
Hi CapriRacer, thanks for that.
In the meantime I edited my post and added another measurement of the circumference, for reasons mentioned in the post, and that gave a smaller circumference and hence a much smaller difference; less than 1.7%

It is safe to assume that the tread diameter went down as the lugs wear. The reduction of the difference between rolling and actual diameter for my worn tires suggests the rolling diameter has not changed much.

But, as I wrote in my first post on this topic, the rolling distance has actually changed over time; my commute became about 0.6% shorter by the odometer, give or take 0.2% but definitely shorter even though the route has not changed one bit.

So the rolling distance did increase over time, probably due to gentle stretching and settling of the belts.

Googling and reading into rolling circumference, everyone seems to agree that it varies with load and inflation pressure. Tread wear does not get mentioned, at least not by the experts.

This puzzles me.
I can't see how load influences rolling distance.
The contact patch would always be flat unless the road surface gives way (mud, sand, snow). Even if it does deform, as long as it approximates flatness it will still be very near constant as the difference reduces quadratically.
Pressure may stretch the belts a bit, but every tire has some pressure so they are already tight.
the answer to the puzzle is :
The change of load does not change the flatness of the contact patch only the size of the contact patch if there's 4000 pounds on a tire and the tire has 40 pounds of air in it each square inch of the contact patch has 40 lb of pressure applied. So if load÷psi= ~ size of contact patch. 4000÷40= ~100sq in contact patch. This equation only works for approximation as it does not account for sidewall stiffness tread block stiffness steel belt stiffness or any of the other load bearing traits of a tire. My understanding is the difference between a low-rolling-resistance tire and a conventional Tire is how much of an effect these other traits( tread/ sidewall stiffness) have on the footprint size.



Capri.
Is a run-flat the same as a low rolling-resistance if not which one would actually have the better rolling resistance?
Y Rated Tire (186mph)vs Q rated Tire(100mph) which one, all other conditions being the same, would have better rolling resistance , and would any gains be consumed by the extra mass of the reinforcement requiring more horsepower to make it rotate?
__________________
1st gen cummins 91.5 dodge d250 ,HX35W/12/6 QSV
ehxsost manafulld wrap, Aero Tonto
best tank: distance 649gps mi 24.04 mpg 27.011usg
Best mpg : 31.32mpg 100mi 3.193 USG 5/2/20


Former
'83 GMC S-15 Jimmy 2door 2wd O/D auto 3.73R&P
'79 Chevy K20 4X4 350ci 400hp msd custom th400 /np205. 7.5-new 14mpg modded befor modding was a thing
87' Hyundai Excel
83 ranger w/87 2.9 L FI2wd auto 18mpg on the floor
04 Mitsubishi Gallant 2.4L auto 26mpg
06 Subaru Forrester XT(WRX PACKAGE) MT AWD Turbocharged 18 plying dirty best of 26mpg@70mph
95Chevy Blazer 4x4 auto 14-18mpg
04 Chevy Blazer 4x4 auto 16-22mpg


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