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Thread: Bored

  1. #1
    Registered Member
    Join Date
    Nov 2015

    Member #:2764


    Happened to check out an rpm ratio calculator on this site and it's the first one I've seen which compensates for the loaded height of the tire, which I always thought was correct. But instead of guessing, you can measure from the floor to the center of the wheel then double that number for your tire diameter for calculating ratio's.

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  2. #2
    Registered Member enigma57's Avatar
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    Mar 2016

    Member #:2941
    Galt's Gulch
    And just to make it even more interesting...... I have yet to see a calculator that allows for tire 'growth' at higher speeds. Centrifugal force causes this and in fairness, there is no universal mathematical factor that can be used to calculate tire growth, as it is not only dynamic relative to revolutions of the tire per minute, but varies with weight of tire, weight of vehicle, construction of tire, size, aspect ratio, sidewall height, temperature and air pressure for each tire.

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  3. #3
    Registered Member chevynut's Avatar
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    Nov 2011

    Member #:115
    Fort Collins, CO
    The differences are pretty small. For something like a 275/55-17 tire the loaded radius is about 3% smaller than the free radius, or about 1/2". The "effective radius" which includes the dynamic effects increases the tire diameter on the order of .5-1% or something like .1". So the dynamic effect makes the loaded radius closer to the free radius. Tire pressure is a bigger influence than the dynamic effects. In the end, you're talking about a difference of 2% or so, or maybe 50 RPM out of 2500 RPM. The variation in the diameter in different brands of tires of the "same size" is almost more than that. And what if your speedo is off by 1-2%?

    The calculators give exact numbers, but they're only as good as the data you put in them. If you really want to know the exact RPM for a given setup you need to input the exact dynamic radius of the tire at the conditions you're interested in.
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  4. #4
    Registered Member
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    Apr 2012

    Member #:571
    What Cnut said. The only tires that "grow" are race tires. Otherwise go by simple calculations for tire diameter based on section width, aspect ratio, and wheel diameter. They are good for comparisons between tire sizes if you treat them all the same.


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